U.S. patent application number 13/676437 was filed with the patent office on 2013-11-28 for inflated tubular solar concentrators.
This patent application is currently assigned to Cool Earth Solar Inc.. The applicant listed for this patent is Cool Earth Solar Inc.. Invention is credited to Jacques Belanger, Paul Dentinger, Robert L. Lamkin, John Liptac, Gregory Meess, James Page.
Application Number | 20130314774 13/676437 |
Document ID | / |
Family ID | 48430144 |
Filed Date | 2013-11-28 |
United States Patent
Application |
20130314774 |
Kind Code |
A1 |
Page; James ; et
al. |
November 28, 2013 |
INFLATED TUBULAR SOLAR CONCENTRATORS
Abstract
A solar collector utilizes an inflated tubular film which
concentrates sunlight onto a solar receiver. The film incorporates
refractive elements in a pattern which focuses light in one or two
dimensions to create foci in the form of lines, spots, or other
shapes. The film may be replaceable. The film may include layers of
material to optimize optical, structural, thermal, and durability
characteristics.
Inventors: |
Page; James; (Oakland,
CA) ; Lamkin; Robert L.; (Pleasanton, CA) ;
Liptac; John; (Livermore, CA) ; Meess; Gregory;
(Oakland, CA) ; Dentinger; Paul; (Sunol, CA)
; Belanger; Jacques; (Livermore, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cool Earth Solar Inc.; |
|
|
US |
|
|
Assignee: |
Cool Earth Solar Inc.
Livermore
CA
|
Family ID: |
48430144 |
Appl. No.: |
13/676437 |
Filed: |
November 14, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61560547 |
Nov 16, 2011 |
|
|
|
61652114 |
May 25, 2012 |
|
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Current U.S.
Class: |
359/361 ;
136/246; 136/259; 359/837 |
Current CPC
Class: |
F24S 2030/145 20180501;
G02B 19/0042 20130101; G02B 19/0004 20130101; F24S 23/00 20180501;
F24S 2030/14 20180501; F24S 30/452 20180501; H01L 31/0543 20141201;
Y02E 10/47 20130101; G01S 3/7861 20130101; H02S 20/00 20130101;
F24S 23/31 20180501; G02B 5/045 20130101; F24S 23/745 20180501;
Y02E 10/44 20130101; H01L 31/0547 20141201; G02B 5/04 20130101;
G02B 27/0972 20130101; H02S 40/00 20130101; F24S 30/425 20180501;
Y02E 10/52 20130101; G02B 7/183 20130101 |
Class at
Publication: |
359/361 ;
136/246; 136/259; 359/837 |
International
Class: |
H01L 31/052 20060101
H01L031/052; G02B 5/04 20060101 G02B005/04 |
Claims
1. A solar concentrator comprising: a film configured to refract
sunlight, wherein the film forms a tubular shape enclosing a
cavity, wherein a first portion of the film includes a refractive
region configured to direct the incident sunlight towards one or
more focus regions within the cavity; an inflation gas at least
partially filling the cavity, wherein the inflation gas is
configured to maintain the tubular shape of the film; and a
receiver configured to capture the energy in the sunlight refracted
from the film, the receiver being located inside the cavity and at
one of the one or more focus regions.
2. The solar concentrator of claim 1 wherein each focus region
comprises one or more focal points.
3. The solar concentrator of claim 1 wherein the solar concentrator
further comprises circuitry to convert the captured energy in the
sunlight to electrical energy.
4. The solar concentrator of claim 1 wherein the solar concentrator
further comprises a thermal energy receiver.
5. The solar concentrator of claim 1 wherein the refractive region
comprises a plurality of prisms configured to refract the
sunlight.
6. The solar concentrator of claim 5 wherein the plurality of
prisms are arranged in two or more groups of prisms.
7. The solar concentrator of claim 6 wherein the two or more groups
of prisms are arranged symmetrically within the refractive
region.
8. The solar concentrator of claim 5 wherein each prism comprises
one or more grooves, each groove having a depth.
9. The solar concentrator of claim 1 wherein the refractive region
is about 30% to 50% of a total surface area of the tubular
shape.
10. The solar concentrator of claim 1 wherein the refractive region
comprises a plurality of tiles, each tile having an area and
including one or more prism grooves.
11. The solar concentrator of claim 10 wherein the plurality of
tiles includes a first tile having a first area and a second tile
having a second area, and wherein the first area is larger than the
second area.
12. The solar concentrator of claim 11 wherein the first tile is
located at a first distance from a center of the refractive region
and the second tile is located at a second distance from the center
of the refractive region, and wherein the first distance is less
than the second distance.
13. The solar concentrator of claim 1 wherein the film is comprises
one or more of PET, acrylic, Polyolefins, Ionomer, or Fluorinated
polymers.
14. The solar concentrator of claim 1 wherein a portion of the film
comprises a metal.
15. The solar concentrator of claim 1 wherein the tubular shape has
an axis and a diameter and wherein the receiver comprises an active
element having a width, wherein the active element is disposed
perpendicular to the axis and the width of the active element is
less than 15% of the diameter of the tubular shape.
16. A solar concentrator system comprising: one or more tubular
solar concentrators configured to receive sunlight and direct the
sunlight to a receiver located within each of the one or more
tubular solar concentrators; and a support structure configured to
hold the one or more tubular solar concentrators, wherein the
support structure further comprises: a base frame configured to
rotate about an azimuth rotation axis; an upper frame configured to
rotate about an elevation rotation axis; and a tracking mechanism
configured to continually track the position of the Sun and to
position the base frame and the upper frame to follow a path of the
Sun.
17. The solar concentrator system of claim 16 wherein the upper
frame comprises one or more rollers, sliders, or linkage arms
configured to move the support structure about the elevation
rotation axis.
18. The solar concentrator system of claim 17 wherein the one or
more rollers, sliders, or linkage arms are disposed substantially
farther from the elevation rotation axis.
19. A solar concentrator comprising: a transparent film configured
to refract incident sunlight; wherein the transparent film is part
of a cylinder structure and the transparent film refracts the
incident sunlight to concentrate it in more than one direction.
20. The solar concentrator of claim 19 wherein the cylinder
structure has a longitudinal cylinder axis and at least one
direction in which the incident sunlight is concentrated is not
perpendicular to the longitudinal cylinder axis.
21. The solar concentrator of claim 19 wherein a direction of
refracted sunlight varies at each point on a surface of the
cylinder structure that refracts the incident sunlight.
22. The solar concentrator of claim 19 further comprising one or
more focal regions disposed within the cylinder structure, wherein
an average illumination level at the one or more focal regions is
about 500 to 2000 times more than an illumination level of the
incident sunlight.
23. The solar concentrator of claim 19 wherein a substantial amount
of the refracted sunlight is focused at one or more points within
the cylinder.
24. The solar concentrator of claim 19 wherein the transparent film
comprises Polyethylene terephthalate (PET).
25. The solar concentrator of claim 19 wherein the transparent film
has a front surface and an opposing back surface, and wherein a
plurality of grooves are disposed on the front surface, or the back
surface, or both the front and back surfaces.
26. The solar concentrator of claim 25 further comprising one or
more tiles disposed on the front surface, or the back surface, or
both the front and back surfaces, each of the one or more tiles
including a set of grooves from the plurality of grooves.
27. A solar concentrator comprising: a film structure configured to
receive and refract incident light, wherein the film structure
comprises two or more layers, wherein a first layer in the film
structure comprises Polyethylene terephthalate (PET), and wherein
the film structure is configured in a shape of a cylinder enclosing
an inflation space, the inflation space being occupied by a
gas.
28. The solar concentrator of claim 27 wherein the film structure
comprises a second layer disposed below the first layer, wherein
the second layer comprises acrylic, fluorinated acrylic, ionomer,
or other fluorinated polymer.
29. The solar concentrator of claim 28 wherein a thickness of the
second layer ranges between 0.001 mm and 0.1 mm.
30. The solar concentrator of claim 28 further comprising a
plurality of grooves formed in the second layer.
31. The solar concentrator of claim 27 wherein the film structure
comprises a second layer disposed on top of the first layer and
wherein the second layer comprises fluorinated polymer or
silicone.
32. The solar concentrator of claim 31 wherein the second layer
further comprises an ultraviolet (UV) radiation absorbing
material.
33. The solar concentrator of claim 32 wherein the UV absorbing
material comprises fluorinated acrylic.
34. The solar concentrator of claim 31 wherein the film structure
further comprises a third layer disposed between the second layer
and the first layer, the third layer configured to block
ultraviolet radiation from reaching the first layer.
35. The solar concentrator of claim 31 wherein the film structure
further comprises a third layer disposed between the second layer
and the first layer, the third layer configured to prevent the
migration of chemical compounds between the first layer and the
second layer.
36. The solar concentrator of claim 27 wherein the film structure
comprises a second layer disposed on top of the first layer and
wherein the second layer comprises a material resistant to (a)
temperatures in the range of -40.degree. C. to 80.degree. C., (b)
humidity in the range of 0-100% relative humidity, and (c)
ultraviolet (UV) exposure.
37. The solar concentrator of claim 27 wherein the PET comprises an
ultraviolet light absorber material as an additive.
38. A solar concentrator comprising: a first film configured to be
exposed to incident sunlight; a second film attached to the first
film and configured to provide structural support, wherein the
first film and the second film together form a tubular structure
having an enclosed inflation space; and a receiver detachably
connected to the second film and configured to receive refracted
sunlight from the first film.
39. The solar concentrator of claim 38 wherein the first film
refracts incident sunlight to create one or more areas of
concentrated solar energy.
40. The solar concentrator of claim 38 wherein the second film
comprises a metal.
41. A solar concentrator system comprising: a film having a tubular
shape and attached to an elongated chassis; one or more heat sink
elements connected to a first surface of the elongated chassis
along the length of the elongated chassis, wherein each of the one
or more heat sink elements comprises one or more fin structures,
and wherein the one or more heat sink elements are connected to the
first surface using a material having a thermal conductivity
between 0.005 W/m-k and 180 W/m-k; one or more photovoltaic cells
coupled to a second surface of the one or more heat sink elements;
and one or more optical elements configured to direct incident
sunlight onto the one or more photovoltaic cells, wherein the one
or more heat sink elements is configured to dissipate heat
generated at the one or more photovoltaic cells.
42. The solar concentrator system of claim 41 wherein the film
comprises one or more refractive prism elements configured to
concentrate incident sunlight.
43. The solar concentrator system of claim 42 wherein the film
encloses an inflation space and wherein the one or more
photovoltaic cells are located within the inflation space.
44. The solar concentrator system of claim 43 wherein the inflation
space comprises a gas including one of: air, helium, carbon
dioxide, nitrogen, argon, hydrogen, oxygen, or water vapor.
45. The solar concentrator system of claim 42 wherein the film
encloses an inflation space and wherein the one or more
photovoltaic cells are located at a surface of the tubular
structure.
46. The solar concentrator system of claim 42 wherein the tubular
structure encloses an inflation space and wherein the one or more
photovoltaic cells are located outside a perimeter defined by the
tubular shape.
47. A solar concentrator system comprising: one or more tubular
solar concentrators configured to capture and concentrate sunlight,
wherein each tubular solar concentrator has a longitudinal axis and
first rotation axis parallel to the longitudinal axis and wherein
the tubular concentrator is configured to rotate about the first
rotation axis; and one or more receivers, each receiver coupled to
at least one of the one or more tubular solar concentrators, the
receiver configured to capture energy from the concentrated
sunlight and translate in a direction perpendicular to the first
rotation axis.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority under 35 USC .sctn.119(e)
to (a) U.S. Provisional Patent Application No. 61/560,547 filed on
Nov. 16, 2011 and (b) U.S. Provisional Patent Application No.
61/652,114, filed on May 25, 2012, the disclosures of both these
applications are incorporated by reference herein in their entirety
for all purposes.
BACKGROUND
[0002] Solar radiation is the most abundant energy source on earth.
However, attempts to harness solar power on large scales have so
far failed to be economically competitive with most fossil-fuel
energy sources.
[0003] One reason for the lack of adoption of solar energy sources
on a large scale is that fossil-fuel energy sources have the
advantage of economic externalities, such as low-cost or cost-free
pollution and emission. Another reason for the lack of adoption of
solar energy sources on a large scale is that the solar flux is not
intense enough for direct conversion at one solar flux to be cost
effective.
[0004] Solar energy concentrator technology has sought to address
this issue. For example, solar radiation energy is easily
manipulated and concentrated using refraction, diffraction, or
reflection to produce solar radiation energy having many thousands
of times the initial flux. This can be done using only modest
materials such as refractors, diffractors and reflectors.
[0005] Specifically, solar radiation is one of the most easy energy
forms to manipulate and concentrate. It can be refracted,
diffracted, or reflected to many thousands of times the initial
flux utilizing only modest materials.
[0006] With so many possible approaches, there have been a
multitude of previous attempts to implement low cost solar energy
concentrators. So far, however, solar concentrator systems cost too
much to compete unsubsidized with fossil fuels, in part because of
large amounts of material and large areas that that solar
collectors occupy. The large amounts of materials used to make
solar concentration systems and the large areas that are occupied
by solar concentration systems render solar concentration systems
unsuitable for large-scale solar farming.
[0007] Accordingly, there is a need in the art for improved
apparatuses and methods for the collection of solar energy.
SUMMARY
[0008] A tubular solar concentrator provides high levels of solar
energy concentration/capture with improved conversion efficiency
and lower cost. The collector may be assembled from
readily-available materials such as clear and metalized polymer
films. A thermal receiver or a concentrated photovoltaic receiver
may be positioned within, outside of, or at a surface of, a chamber
of the concentrator. In some embodiments, the collector may employ
single-axis tracking. The solar concentrator includes a film that
is configured to refract sunlight. The film forms a tubular shape
enclosing a cavity. A first portion of the film may include a
refractive region configured to direct the incident sunlight
towards one or more focus regions within the cavity. The solar
concentrator may further include an inflation gas at least
partially fills the cavity and helps to maintain the tubular shape
of the film. The solar concentrator may further include a receiver
that can capture the energy in the sunlight refracted from the
film. The receiver may be located inside the cavity and at one of
the one or more focus areas.
[0009] In some embodiments, the refractive region comprises a
plurality of prisms configured to refract the sunlight. In a
particular embodiment, the refractive region is about 30% to 50% of
a total surface area of the tube. In some embodiments, the tubular
shape has an axis and a diameter and the receiver may include an
active element having a width. The active element is disposed
perpendicular to the axis and the width of the active element is
less than 50% of the diameter of the tubular shape.
[0010] Some embodiments of the present invention provide a solar
concentrator system that includes one or more tubular solar
concentrators that are configured to receive sunlight and direct
the sunlight to a receiver located within each of the one or more
tubular solar concentrators. The system further includes a support
structure configured to hold the one or more tubular solar
concentrators. The support structure may further include a base
frame configured to rotate about an azimuth rotation axis, an upper
frame configured to rotate about an elevation rotation axis, and a
tracking mechanism configured to continually track the position of
the Sun and may be used to position the base frame and the upper
frame to follow a path of the Sun.
[0011] Another embodiment of a solar concentrator system provides
receivers for capturing concentrated sunlight which maintain
optimal position and orientation via a simplified tracking system.
In this embodiment, the tracking system has a first rotational
motion of the tubular concentrator(s) about a longitudinal rotation
axis parallel to the tube axis and a second motion of each receiver
relative to its corresponding tubular concentrator, so that each
receiver may maintain an appropriate position with respect to the
focal region of concentrated light created by the tubular
concentrator.
[0012] A particular embodiment of the present invention provide a
solar concentrator that includes a transparent film that is
configured to refract incident sunlight. The transparent film is
part of a cylinder structure and the transparent film refracts the
incident sunlight to concentrate it in more than one direction. The
solar concentrator further includes one or more focal regions that
are disposed within the cylinder structure. An average illumination
level at the one or more focal regions is about 500 to 2000 times
more than an illumination level of the incident sunlight.
[0013] Another embodiment of the present invention provides A solar
concentrator that includes a film structure that can receive and
refract incident light. The film structure may include two or more
layers. In some embodiments, a first structural layer in the film
structure may include Polyethylene terephthalate (PET). The film
structure may be in the shape of a cylinder that encloses an
inflation space that may be occupied by a gas. The solar film
structure of the solar concentrator may include an inner optical
layer disposed below the first layer. The inner optical layer may
include acrylic, fluorinated acrylic, ionomer, or other fluorinated
polymer. In some embodiments, the thickness of the inner optical
layer may range between 0.001 mm and 0.1 mm. In some embodiments,
the solar concentrator may also include a plurality of grooves
formed in the inner optical layer. In some embodiments, the film
structure may further include an outer layer disposed on top of the
first layer and the outer layer may include fluorinated polymer or
silicone. In some embodiments, the outer layer may further include
an ultraviolet (UV) radiation absorbing material, e.g., fluorinated
acrylic.
[0014] In some embodiments, the solar concentrator film structure
may further include an intermediate layer disposed between the
outer layer and the first structural layer. The intermediate layer
may be designed to block ultraviolet radiation from reaching the
first structural layer. In some embodiments the intermediate layer
may be configured to prevent the migration of chemical compounds
between the adjoining layers. In some embodiments, the film
structure may include an outer layer disposed on top of the first
structural layer. In this instance, the outer layer may include a
material that is resistant (a) temperatures in the range of
-40.degree. C. to 80.degree. C., (b) humidity in the range of
0-100% relative humidity, and (c) ultraviolet (UV) exposure. In
some embodiments, the PET material of the solar concentrator may
include an ultraviolet light absorber material as an additive.
[0015] Some embodiments of the present invention provide a solar
concentrator that includes a first film that is configured to be
exposed to incident sunlight, a second film that is attached to the
first film and is configured to provide structural support. The
first film and the second film together form a tubular structure
that encloses an inflation space. The solar concentrator further
includes a receiver detachably connected to the second film which
is configured to receive refracted sunlight from the first film.
The first film of the solar concentrator refracts incident sunlight
to create one or more areas of concentrated solar energy. In some
embodiments, the second film may include a metal such as
aluminum.
[0016] A particular embodiment of the present invention provides a
solar concentrator system that includes a film having a tubular
shape which is attached to an elongated chassis, and one or more
heat sink elements connected to a first surface of the elongated
chassis along the length of the elongated chassis. Each of the one
or more heat sink elements may include one or more fin structures.
The one or more heat sink elements are connected to the first
surface using a material having a thermal conductivity between
0.005 W/m-k and 180 W/m-k. The system may further include one or
more photovoltaic cells coupled to a second surface of the
elongated chassis and one or more optical elements coupled to the
elongated chassis and that configured to direct incident sunlight
onto the one or more photovoltaic cells. The one or more heat sink
elements are designed to dissipate heat generated at the one or
more photovoltaic cells. In some embodiments, the film comprises
one or more refractive prism elements that are configured to
concentrate incident sunlight. The film of the solar concentrator
system may enclose an inflation space and the one or more
photovoltaic cells may be located within the inflation space. In
some embodiments, the inflation space may be filled fully or
partially with a gas such as air, helium, carbon dioxide, nitrogen,
argon, hydrogen, oxygen, or water vapor.
[0017] In some embodiments, the one or more photovoltaic cells may
be located at a surface of the tubular structure or may be located
outside a perimeter defined by the tubular shape of the film.
[0018] These and other embodiments of the present invention, as
well as its features and some potential advantages are described in
more detail in conjunction with the text below and attached
figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIGS. 1 and 1A illustrate simplified perspective and
cross-sectional views respectively, of an inflatable concentrator
according to an embodiment of the present invention.
[0020] FIGS. 2 and 2A illustrate simplified perspective and
cross-sectional views respectively, of an inflatable concentrator
according to another embodiment of the present invention.
[0021] FIGS. 3 and 3A illustrate simplified perspective and
cross-sectional views respectively, of an inflatable concentrator
according to still another embodiment of the present invention.
[0022] FIGS. 4 and 4A illustrate simplified perspective and
cross-sectional views respectively, of an inflatable concentrator
according to yet another embodiment of the present invention.
[0023] FIGS. 5-5B illustrate simplified views of an embodiment of a
concentrated photovoltaic (CPV) receiver according to an embodiment
of the present invention.
[0024] FIG. 5C illustrates a simplified side view illustrating a
CPV receiver according to another embodiment of the present
invention.
[0025] FIGS. 5E-5G illustrate a three dimensional interconnect
scheme for a receiver system according to an embodiment of the
present invention.
[0026] FIG. 5D illustrates a simplified side view illustrating a
CPV receiver according to still another embodiment of the present
invention.
[0027] FIGS. 6A-6C illustrate simplified cross-sectional views of
various solar collector embodiments.
[0028] FIG. 7 illustrates a simplified view of a collector together
with a tracking apparatus according to an embodiment of the present
invention.
[0029] FIG. 8 illustrates a simplified view of plurality of
collectors in a stacked configuration according to an embodiment of
the present invention.
[0030] FIG. 9A illustrates a simplified isometric view of a
film-based solar collector trough structure according to an
embodiment of the present invention.
[0031] FIGS. 9B-9E illustrate various other views of the collector
structure of FIG. 9A according to an embodiment of the present
invention.
[0032] FIG. 10A illustrates an overview of an inflatable trough
solar energy collection system according to an embodiment of the
present invention.
[0033] FIG. 10B illustrates a closer isometric view of a solar
collector assembly according to an embodiment of the present
invention.
[0034] FIG. 10C illustrates a side view of a collector assembly
according to an embodiment of the present invention.
[0035] FIG. 10D illustrates an enlarged section view of a collector
assembly with a frame and sub-frame removed for clarity according
to an embodiment of the present invention.
[0036] FIG. 10E illustrates a cutaway isometric view of an
embodiment of a trough array according to an embodiment of the
present invention.
[0037] FIG. 10F illustrates another view of a frame and a sub-frame
according to an embodiment of the present invention.
[0038] FIG. 10G illustrates an enlarged detail view of the area A
denoted in FIG. 10F according to an embodiment of the present
invention.
[0039] FIG. 10H illustrates a schematic cross section view of the
behavior of light for the system in FIGS. 10A-G according to an
embodiment of the present invention.
[0040] FIG. 10I illustrates an enlarged view of a receiver location
and region of focused light according to an embodiment of the
present invention.
[0041] FIG. 11 illustrates an inflated trough solar collection
system according to an embodiment of the present invention.
[0042] FIG. 12 illustrates an inflated trough solar collection
system according to another embodiment of the present
invention.
[0043] FIG. 13 illustrates an embodiment of a solar collector
trough array that is shaped by a negative pressure differential
according to an embodiment of the present invention.
[0044] FIGS. 14A and 14B illustrate a solar concentration trough
system with a different frame according to an embodiment of the
present invention according to an embodiment of the present
invention.
[0045] FIG. 15A illustrates an isometric view of an embodiment of a
film-based solar collector trough according to an embodiment of the
present invention.
[0046] FIG. 15B illustrates an end view of the structure of FIG.
15A according to an embodiment of the present invention.
[0047] FIG. 16A illustrates a concentrator system with a modified
film that can create performance enhancements according to an
embodiment of the present invention.
[0048] FIG. 16B illustrates an enlarged view of a small portion of
a film from FIG. 16A with exaggerated surface shapes according to
an embodiment of the present invention.
[0049] FIG. 17A illustrates a film-based solar concentration system
that employs a transparent film with a modified shape according to
an embodiment of the present invention.
[0050] FIG. 17B illustrates details of a surface of the film based
concentration system of FIG. 17A according to an embodiment of the
present invention.
[0051] FIG. 17C1 illustrates another transparent film system with
refractive prism shapes on its concave side according to an
embodiment of the present invention.
[0052] FIG. 17C2 illustrates a partial close-up view of the film
and prisms of FIG. 17C1 according to an embodiment of the present
invention.
[0053] FIG. 17D illustrates an inflated film based tubular
refractive concentrator according to an embodiment of the present
invention.
[0054] FIG. 17E shows a film optic with a refractive prism pattern
that causes incident sunlight to be concentrated in two directions
with resulting regions of focus having linear shapes but separated
by some distance according to an embodiment of the present
invention.
[0055] FIG. 17F shows a film optic with a refractive prism pattern
that causes incident sunlight to be concentrated in one dimension
to form an intermediate continuous linear region of focus, and
secondary optical elements that further concentrate light in two
dimensions to create discrete spot regions of focus according to an
embodiment of the present invention.
[0056] FIGS. 17G1-17G6 show a prism design for a cylindrical optic
which creates 2-D concentration according to an embodiment of the
present invention.
[0057] FIG. 17H shows a prism design for a cylindrical optic which
creates 2-D concentration with tiles of axisymetric prisms
according to an embodiment of the present invention.
[0058] FIGS. 17I1-17I8 show details of a 2-D concentrating prism
design method for cylindrical optic using discretized calculation
process according to an embodiment of the present invention.
[0059] FIG. 17J1 illustrates a top view of a refractive prism
pattern that may be wrapped to form a cylindrical or tubular
concentrating optic according to an embodiment of the present
invention.
[0060] FIG. 17J2 illustrates an isometric view the pattern of FIG.
17J1 as wrapped onto a tubular optic according to an embodiment of
the present invention.
[0061] FIG. 17K1 illustrates a tiled composite refractive prism
pattern that may be wrapped to form a cylindrical or tubular
concentrating optic according to an embodiment of the present
invention.
[0062] FIG. 17K2 illustrates an isometric view the pattern of FIG.
17K1 as wrapped onto a tubular optic according to an embodiment of
the present invention.
[0063] FIG. 17L1 illustrates a top view of a mapping pattern for a
tubular refractive optic and target focus regions according to an
embodiment of the present invention.
[0064] FIG. 17L2 illustrates an isometric view of the mapping
pattern of FIG. 17L1 according to an embodiment of the present
invention.
[0065] FIG. 17M1 illustrates a top view of another mapping pattern
for a tubular refractive optic and target focus regions according
to an embodiment of the present invention.
[0066] FIG. 17M2 illustrates an isometric view of the mapping
pattern of FIG. 17M1 according to an embodiment of the present
invention.
[0067] FIG. 17N1 Illustrates a top view of another mapping pattern
for a tubular refractive optic and target focus regions according
to an embodiment of the present invention.
[0068] FIG. 17N2 illustrates an isometric view of the mapping
pattern of FIG. 17N1 according to an embodiment of the present
invention.
[0069] FIG. 17O1 Illustrates a top view of another mapping pattern
for a tubular refractive optic and target focus regions according
to an embodiment of the present invention.
[0070] FIG. 17O2 illustrates an isometric view of the mapping
pattern of FIG. 17O1 according to an embodiment of the present
invention.
[0071] FIG. 17P1 Illustrates a top view of another mapping pattern
for a tubular refractive optic and target focus regions according
to an embodiment of the present invention.
[0072] FIG. 17P2 illustrates an isometric view of the mapping
pattern of FIG. 17P1 according to an embodiment of the present
invention.
[0073] FIG. 17Q1 illustrates a top view of another mapping pattern
for a tubular refractive optic and target focus regions according
to an embodiment of the present invention.
[0074] FIG. 17Q2 illustrates an isometric view of the mapping
pattern of FIG. 17Q1 according to an embodiment of the present
invention.
[0075] FIG. 17R1 illustrates a top view of a flattened film with
specific tiles of prism patterns according to an embodiment of the
present invention.
[0076] FIG. 17R2 illustrates the film of FIG. 17R1 as formed into a
cylinder shape according to an embodiment of the present
invention.
[0077] FIGS. 17R3A-17R3C illustrate an irradiance map of the spot
of light created by a simulated ray trace through one of the
patches of prisms in FIGS. 17R1 and 17R2 according to an embodiment
of the present invention.
[0078] FIGS. 17R4A-17R4C illustrate an irradiance map of the spot
of light created by a simulated ray trace through another one of
the patches of prisms in FIGS. 17R1 and 17R2 according to an
embodiment of the present invention.
[0079] FIGS. 17R5A-17R5C illustrate an irradiance map of the spot
of light created by a simulated ray trace through one of the
patches of prisms in FIGS. 17R1 and 17R2 according to an embodiment
of the present invention.
[0080] FIGS. 17R6A-17R6C illustrate an irradiance map of the spot
of light created by a simulated ray trace through one of the
patches of prisms in FIGS. 17R1 and 17R2 according to an embodiment
of the present invention.
[0081] FIGS. 17R7A-17R7C illustrate an irradiance map of the spot
of light created by a simulated ray trace through one of the
patches of prisms in FIGS. 17R1 and 17R2 according to an embodiment
of the present invention.
[0082] FIGS. 17R8A-17R8C illustrate an irradiance map of the spot
of light created by a simulated ray trace through all of the
patches of prisms in FIGS. 17R1 and 17R2 simultaneously according
to an embodiment of the present invention.
[0083] FIGS. 17S1-17S9 illustrate various surfaces, points and
vectors used in the mathematical representations and calculations
of shapes for 2D concentrating cylindrical Fresnel prisms according
to an embodiment of the present invention.
[0084] FIGS. 17T1A-17T2C illustrate irradiance maps of the spot of
light created by a simulated ray trace through a section of
idealized, continuous groove 2D concentrating cylindrical Fresnel
lens according to an embodiment of the present invention.
[0085] FIG. 18 illustrates a film based solar concentration trough
system with a stationary trough array and moveable receivers
according to an embodiment of the present invention.
[0086] FIG. 18A illustrates a side view of the embodiment of the
system of FIG. 18 according to an embodiment of the present
invention.
[0087] FIG. 19 illustrates a solar concentration trough system that
uses inflation air and a membrane to eliminate the need for a rigid
frame according to an embodiment of the present invention.
[0088] FIG. 19A illustrates multiple systems of FIG. 19 configured
to track the sun together according to an embodiment of the present
invention.
[0089] FIGS. 20A and 20B illustrate a solar concentration system
that uses a film without inflation pressure according to an
embodiment of the present invention.
[0090] FIG. 20C illustrates a system of FIG. 20A including a
roll-to-roll film replacement system according to an embodiment of
the present invention.
[0091] FIG. 21 illustrates a solar concentration trough system that
uses inflation air and a membrane to eliminate a need for a rigid
frame according to an embodiment of the present invention.
[0092] FIGS. 22A-22D illustrate different secondary optics for use
with a receiver according to an embodiment of the present invention
according to an embodiment of the present invention.
[0093] FIGS. 23A-23C Show a different embodiment of inflated
concentrators mounted on a tracking system according to an
embodiment of the present invention.
[0094] FIGS. 24A and 24B show a different embodiment of inflated
concentrators mounted on a different tracking system according to
another embodiment of the present invention.
[0095] FIG. 25 shows a method of attaching and removing a film from
a film holder which may be employed to allow changing of film-based
inflated optics according to an embodiment of the present
invention.
[0096] FIGS. 26A-26D show another method of attaching and removing
a film from a film holder which may be employed to allow changing
of film-based inflated optics according to an embodiment of the
present invention.
[0097] FIGS. 27A-27C show variations of film anchor features and
sealing materials according to an embodiment of the present
invention.
[0098] FIGS. 28A-28D show variations of film anchor construction
according to an embodiment of the present invention.
[0099] FIGS. 29A-29C show additional variations of film anchor and
seal materials according to an embodiment of the present
invention.
[0100] FIGS. 30A and 30B show variations of film anchors and
related structures to which film is secured according to an
embodiment of the present invention.
[0101] FIG. 31A shows a view of another method of attaching a film
to a solar receiver or heat sink according to an embodiment of the
present invention.
[0102] FIG. 31B shows a view of another method of attaching a film
to a solar receiver or heat sink according to an embodiment of the
present invention.
[0103] FIGS. 32A and 32B show a method of sealing the end of a
tubular inflated solar concentrator according to an embodiment of
the present invention.
[0104] FIGS. 33A and 33B show another method of sealing the end of
a tubular inflated solar concentrator according to an embodiment of
the present invention.
[0105] FIGS. 34A and 34B show another method of sealing the end of
a tubular inflated solar concentrator according to an embodiment of
the present invention.
[0106] FIG. 35 shows another method of sealing the end of a tubular
inflated solar concentrator according to an embodiment of the
present invention.
[0107] FIGS. 36A and 36B show isometric cutaway and partial section
views respectively of another method of sealing the end of a
tubular inflated solar concentrator according to an embodiment of
the present invention.
[0108] FIGS. 37A and 37B show isometric cutaway and partial section
views respectively of yet another method of sealing the end of a
tubular inflated solar concentrator according to an embodiment of
the present invention.
[0109] FIGS. 38A and 38B show isometric cutaway and partial section
views respectively of another method of sealing the end of a
tubular inflated solar concentrator according to an embodiment of
the present invention.
[0110] FIGS. 39A and 39B show isometric cutaway and partial section
views respectively of another method of sealing the end of a
tubular inflated solar concentrator according to an embodiment of
the present invention.
[0111] FIGS. 40A-40E show various views of a method of sealing a
tubular solar inflated concentrator to a receiver and sealing its
ends, according to an embodiment of the present invention.
[0112] FIGS. 41A and 41B show another method of sealing the ends of
an inflatable concentrator according to an embodiment of the
present invention.
[0113] FIGS. 42A-42D show another method of sealing the ends of an
inflatable solar concentrator according to an embodiment of the
present invention.
[0114] FIGS. 43A-43F show another method of attaching an inflatable
tubular concentrator to a receiver heat sink and sealing its ends
according to an embodiment of the present invention.
[0115] FIG. 43G shows construction of a film in flat form related
to the method of FIGS. 43A-43F according to an embodiment of the
present invention.
[0116] FIG. 43H shows a close-up view of the construction of an
anchor feature for the film and attachment method of FIGS. 43A-43G
according to an embodiment of the present invention.
[0117] FIG. 43I shows the film of FIG. 43H when curved into a
tubular configuration and before installation according to an
embodiment of the present invention.
[0118] FIG. 43J shows the shape that the film of FIGS. 43H-43I
takes when it is installed according to an embodiment of the
present invention.
[0119] FIGS. 44A-44E show another method of inflatable tubular
concentrator construction according to an embodiment of the present
invention.
[0120] FIG. 45 shows an inflated tubular solar concentrator with
endcap seals and internal cables according to an embodiment of the
present invention.
[0121] FIG. 46A shows a solar receiver with heat sink and secondary
optics according to an embodiment of the present invention.
[0122] FIG. 46B shows a close-up cutaway view of a secondary optic,
holder, and cell for the receiver of FIG. 46A.
[0123] FIGS. 47A-47C show another embodiment of a solar receiver
with secondary optics and heat sinks according to an embodiment of
the present invention.
[0124] FIGS. 48A-48D show an embodiment of a solar receiver chassis
thermally insulated from heat sinks according to an embodiment of
the present invention.
[0125] FIGS. 49A-49D show partial cross section views of variations
of another solar concentrator and tracking system according to an
embodiment of the present invention.
[0126] FIGS. 49E and 49F show isometric views with different
tracking positions of the solar concentrator system of FIGS.
49A-49D.
[0127] FIG. 50A-50E illustrate another method of attaching an
inflatable tubular concentrator to a receiver heat sink and sealing
its ends according to an embodiment of the present invention.
[0128] FIGS. 51A and 51B show a method for creating a heat sink
with transverse fins according to an embodiment of the present
invention.
[0129] FIG. 52A-52C show several options for materials,
construction and geometric features of films for use in refractive
optical elements according to an embodiment of the present
invention.
[0130] FIGS. 53A-53C illustrate a solar concentrator system
according to yet another embodiment of the present invention.
DETAILED DESCRIPTION
[0131] Solar radiation is a relatively easy form of energy to
manipulate and concentrate. It can be refracted, diffracted, or
reflected, to achieve concentrations of up to thousands of times
the initial flux, utilizing only modest materials.
[0132] Conventionally, however, the costs associated with a solar
concentrator system has proven prohibitive for competing with
unsubsidized with fossil fuels, in part because of excessive
material costs and large areas that conventional solar collectors
occupy. These excessive materials costs and the large areas that
are occupied by solar concentration systems may render them
unsuitable for large-scale solar power generation projects.
[0133] One possible approach to reducing cost is to reduce the cost
associated with manufacturing of major structures of a solar power
plant. This may be done by exploiting spontaneous and natural
tendencies of materials and by use of more efficient manufacturing
techniques.
[0134] In one instance the tendency of a thin, flat film to assume
a consistent tubular shape when rolled and inflated may be used to
create an inexpensive solar concentrator. Specifically in a
particular embodiment, small prisms may be formed in a clear film
to create a desired focus or foci when the film is inflated in a
tubular configuration.
[0135] In another instance, the tendency of a flat reflective film
to assume a smooth concave shape under the influence of a pressure
differential may be used to fabricate a solar concentrator.
Specifically, in a particular embodiment, inflation air may be used
to impart a curved profile to a reflective component of a
concentrator for a solar collector structure.
[0136] Such inflatable solar concentrators may offer certain
benefits over conventional concentrator designs that employ more
common structural elements. For example, an inflatable concentrator
uses air as a structural member, and may employ thin plastic
membranes (herein referred to as films) as a primary optic. This
can yield significant weight advantages in a system deployed in the
field. The weight advantages in the concentrator itself can in turn
reduce the amount and complexity of the structures of the mounting
and tracking systems used with the solar concentrator. This will
help to reduce the overall mass and cost of the solar collector
system.
[0137] According to certain embodiments, a solar collector may
utilize an inflated refractive concentrator having a tube-like
shape and including refractive prism elements in order to achieve
one or more focus areas of concentrated refracted light on a
receiver. The collector may be assembled from inexpensive,
lightweight, and readily-available materials such as polymer films.
As described below, depending upon the particular embodiment, a
thermal or concentrated photovoltaic (CPV) receiver may be disposed
within, outside of, or at a surface of, the inflated
concentrator.
[0138] According to certain other embodiments, a solar collector
may utilize an inflated reflecting concentrator having a tube-like
shape in order to achieve focus of concentrated reflected light
along a line on a receiver. The collector may be assembled from
inexpensive, lightweight, and readily-available materials such as
aluminized polymer film (exhibiting reflecting properties) and
polyester film (exhibiting optically transparent properties). As
described below, depending upon the particular embodiment, a
thermal or concentrated photovoltaic (CPV) receiver may be disposed
within, outside of, or at a surface of, the inflated concentrator.
In addition as described herein (for example in connection with
FIGS. 7-8), by virtue of its operation to gather and focus light in
one dimension, single-axis tracking of such a trough-type collector
may be sufficient.
[0139] Certain embodiments may seek to reduce the levelized cost of
energy of a solar power plant, and to maximize the scale at which
such plants can be deployed. Embodiments of solar collector devices
and methods may be utilized in conjunction with power plants having
one or more of the attributes described in that patent
application.
[0140] The objectives of reduced levelized cost and maximized scale
of a solar power plant, can be achieved through the use of elements
employing minimal materials and low-cost materials that are able to
be mass produced. Potentially desirable attributes of various
elements of such a solar power plant, include simple, rapid, and
accurate installation and assembly, ease of maintenance,
robustness, favorable performance at and/or below certain
environmental conditions such as a design wind speed, and
survivability at and below a higher maximum wind speed.
[0141] In particular embodiments, inflation air may be used to
impart a concave profile to a reflective component of a
concentrator for a solar collector structure. Specifically, a
reflective surface in the form a metalized film shaped by inflation
pressure, may be used to create an elongated inflated tubular
concentrator defining a reflective trough for communicating
concentrated solar energy to a receiver, such as a thermal or
photovoltaic receiver.
[0142] FIGS. 1 and 1A show simplified perspective and
cross-sectional views, respectively, of one embodiment of an
inflated concentrator. Concentrator 100 comprises a clear film 102
joined to a reflective film 104 (here Aluminized) by a film seal
106. According to certain embodiments, the films may be directly
sealed to each other. According to other embodiments, the film seal
can be formed by having the films attached to separate sealing
member(s).
[0143] In certain embodiments, the films may define a tubular shape
in which the cross-section of the concave reflective film is
half-circular. The inclusion of circular end pieces 108, may define
an internal inflation space 110 having a substantially circular
profile. Alternately, in certain embodiments end(s) of the films
may be self-sealed, pinched like a sausage, or sealed together in
the same plane as the other linear edge seals. Such approaches may
allow for lower cost manufacturing. While some light from the ends
may be lost, or the "spot" may not extend all the way along the
tube, the resulting cost benefit could be favorable.
[0144] In certain embodiments clear film 102 may comprise a
polymer. Many different types of polymers are candidates for clear
film 102. One form of polymer which may be suitable is a polyester,
examples of which include but are not limited to polyethylene
terephthalate (PET) and similar or derivative polyesters such as
polyethylene napthalate (PEN), or polyesters made from isophthalic
acid, or other diols such as but not limited to butyl, 2,2,4,4
tetramethylcyclobutyl or cyclohexane.
[0145] According to certain embodiments clear film 102 may be
formed from poly(meth methacrylate) (PMMA) and co-, ter-, tetra-,
or other multimonomeric polymers of methacrylates or acrylates
including but not limited to monomers of ethyl, propyl and butyl
acrylate and methacrylates. Other examples of polymers forming the
upper transparent film include but are not limited to polycarbonate
(PC), polymethylpentane (TPX), cyclic olefin derived polymers such
as Cyclic olefin co-polymers (COC), cyclic olefin polymer (COP),
ionomer, fluorinated polymers such as polyvinilidene fluoride and
difluoride (PVF and PVDF), ethylene tetrafluoroethylene (ETFE),
ethylene chlorotrifluoroethylene (ECTFE), fluorinated ethylene
propylene (FEP), THV and derivatives of fluorinated polymers, and
co-extruded, coated, adhered, or laminated species of the above.
Examples of thicknesses of layers of such materials may include
from about 0.012 mm to 20 mm, depending on the strength of the
material and the size of the collector. In some embodiments, film
102 may comprise two or more layers. Each layer can be chosen from
any of the materials listed above.
[0146] Incident optical energy 111 may pass through the clear film
102, and be reflected by reflective film 104 to concentrate light
along an elongated focus region 112. Provision of a receiver in
this elongated focus region, may allow conversion of the reflected
solar energy into other forms (including but not limited to thermal
energy or electrical energy).
[0147] In some embodiments, a full half circle cross section for a
reflector (half-cylinder) reflects only a portion of the incident
rays 111 back in a direction where they can be captured by a
receiver. Another portion of the incident rays 111 may reflect in a
direction such that they bounce off the reflective surface again,
from a different location, sometimes multiple times, without
converging at a feasible receiver location 112.
[0148] FIGS. 2 and 2A show simplified perspective and
cross-sectional views, respectively, of an alternative embodiment
of an inflated concentrator. Concentrator 200 comprises a clear
film 202 joined to a reflective film 204 by a film seal 206.
[0149] In this particular embodiment, the concentrator 200 further
comprises a batten structure 220. If films 200 and 202 do not form
a substantially circular cross section, battens 220 may apply
force(s) to films 200 and 202 to maintain their boundary locations
under the influence of a pressure differential. If, however, films
200 and 202 together form a substantially circular cross section,
then batten(s) 220 may not be necessary or may have minimal weight
and strength. This is because the battens may not need to apply
forces to films 200 and 202 to maintain their boundary locations.
In that case, battens 220 may need only apply forces to maintain
the concentrator position under the influence of gravity, wind and
other environmental loads.
[0150] In certain embodiments, batten 220 may provide for film
attachment and/or film sealing. For example batten 220 may comprise
a solid or hollow member such as a rod, to which one or more of the
films may be attached as part of the film seal. A detailed
discussion of film sealing is found in the U.S. patent application
Ser. No. 13/015,339 filed on Jan. 27, 2011, which is incorporated
by reference herein for all purposes.
[0151] While the particular embodiment of FIG. 2 shows batten 220
as forming part of the film seal allowing for the creation of an
internal inflation space 210, this is not required. According to
alternative embodiments, batten 220 may be separate from the film
seal.
[0152] According to certain embodiments, trough-type concentrators
may be aligned with the sun utilizing single-axis tracking. In some
embodiments, the single-axis tracking may be achieved by rotation
about the long axis of the concentrators. Single-axis tracking is
possible for any angle of the long axis relative to horizontal,
including vertical.
[0153] The nature of the tracking can depend upon the orientation
of the trough-type concentrator. For a North-South trough
orientation, single-axis tracking may involve nearly a 180.degree.
range of motion. An East-West trough orientation may involve
tracking through a wide range of motion every day, but the motion
may be slow in the middle of the day and fast at the beginning and
end of the day.
[0154] Trough-type collectors according to embodiments of the
present invention may be oriented East-West, North-South, or at any
angle that maximizes power output. The orientation can thus depend
upon factors such as the site location, time of day, etc.
[0155] FIGS. 3 and 3A show simplified perspective and
cross-sectional views, respectively, of an alternative embodiment
of an inflated concentrator. Concentrator 300 comprises a clear
film 302 joined to a reflective film 304 by a film seal 306. This
embodiment comprises two separate battens, a lower batten 320, and
an upper batten 322.
[0156] Lower batten 320 functions in a similar manner as batten 220
of FIG. 2 to define the shape of reflective film 304 and hence the
location of the elongated concentrated focus. Upper batten 322
functions to define the shape of clear film 302, for example to
determine whether a particular location (e.g. the position of the
receiver) lies inside or outside of the inflation space.
[0157] FIGS. 4 and 4A show simplified perspective and
cross-sectional views, respectively, of an alternative embodiment
of an inflated concentrator. Concentrator 400 comprises a clear
film 402 joined to a reflective film 404 by a hoop structure 406
having a thickness, with the material composition of the hoop
imparting stiffness to the concentrator and collector
structure.
[0158] Various techniques may be employed alone or in conjunction,
to enhance the effectiveness of harvesting of solar energy by a
solar collector comprising an inflatable concentrator. One such
technique is modification of the profile offered by the reflective
surface of the primary reflective optical element.
[0159] U.S. Non-provisional patent application Ser. No. 13/338,607
filed on Dec. 28, 2011 describes the use of embossing to control
the optical performance of films. Embodiments of solar collectors
employing inflatable concentrators may employ one or more
techniques described in that patent application.
[0160] One possible approach utilizing embossing, employs a linear
embossing pattern made by a linear (possibly roll-to-roll) process.
The result would be a film with a cross section centerline that
still has a cylindrical shape, but which has small deviations to
the active reflective surface. These deviations would ensure that
the effective slope of each point on the reflective surface is
determined explicitly, to achieve a particular optical result
(rather than just being the slope of a cylinder).
[0161] Optical results that can be obtained according to this
approach, include a spot exhibiting relatively uniform illumination
(a "flat" illumination profile), and/or exhibiting higher
concentrations than can be created with a cylindrical reflector. A
spot similar to that created by a parabolic reflecting profile
could be created if desired, although a parabola may not be an
optimal reflector shape for some concentrated photovoltaic (CPV)
applications. Another possible embossing approach corrects the
effective slope of the reflector, allowing off-axis placement. Such
an approach could allow unwanted shading from the receiver to be
reduced or eliminated.
Secondary Optic
[0162] The collection of solar energy from an inflated concentrator
structure may also benefit from the use of secondary optical
structures. Thus collectors according to various embodiments may
employ secondary optical structure(s) in addition to the inflated
reflective primary optic. Such a secondary optical structure can
perform one or more roles, including but not limited to, reducing
sensitivity to tracking error, enhancing uniformity of
illumination, steering light away from grout, and helping to define
optical boundaries.
[0163] U.S. patent application Ser. No. 12/720,429, filed on Mar.
9, 2010 describes certain types of secondary optics. This
application is hereby incorporated by reference herein for all
purposes.
[0164] Embodiments of collectors may include secondary optical
structures exhibiting one or more characteristics described in
these applications.
Receiver
[0165] Collectors according to various embodiments are not required
to be employed in conjunction with any specific type of receiver.
Thus receivers based upon thermal or photovoltaic principles may be
used. Other examples of receivers include but are not limited to a
chemical process receiver (i.e. use solar heat to drive a chemical
process), for example in fuel processing. A particular type of
thermal receiver may also create steam for oil extraction.
[0166] U.S. Pat. No. 7,866,035 describes various embodiments of
receivers. The above US patent is incorporated by reference herein
in its entirety for all purposes. Embodiments of collectors may
include receivers exhibiting one or more characteristics described
in the patent and provisional application.
[0167] FIG. 5 shows a simplified plan view of a section of one
particular embodiment of a receiver 500 which may be suited for use
with an inflated solar concentrator. Specifically, such a device is
particularly suited to receiving solar energy concentrated by a
factor of about 20.times., and up to about 40.times. or more, in an
elongated focus region.
[0168] A trough shaped reflective primary optic may create a region
of concentrated light by reflecting light rays inward toward each
other, so they are no longer parallel. This concentration created
by inward reflection or bending may occur about one axis. This is
somewhat different from concentration about two axes created by
reflective dishes, which typically have a central axis of
revolution, so that light concentrates to a point or a circular
spot rather than to a line. Certain embodiments described herein
concentrate light to a linear shaped region of increased intensity.
Receiver 500 includes receiver heat sink or substrate mount 502. In
certain embodiments, this heat sink or substrate mount may be made
out of aluminum, but this is not required. According to some
embodiments, the heat sink or substrate mount may itself comprise a
structural element of the receiver.
[0169] Arranged on the heat sink or substrate mount in row(s), and
aligned with the focused concentrated solar light from an
inflatable concentrator, are a plurality of silicon solar cells 504
and bypass diodes. These cells may be of any design, including
front-contact cells as described in the U.S. Provisional Patent
Application No. 61/475,483. This embodiment shows front contact
cells 504 in conjunction with conducting busbars 508 and fingers
506.
[0170] Fingers 506 will, in most cases, be electrically connected
to busbars 508. An alternate term for fingers 506 is "gridlines."
In an alternate embodiment, busbars similar to 508 may be on both
edges of each cell such that fingers 506 may connect to busbars at
both ends. This configuration can reduce current in the fingers,
especially near the busbars and thereby reduce energy lost in the
fingers and busbars. In another embodiment, fingers 506 could run
parallel to the long axis of the receiver and parallel to busbars
508. In such a configuration, some other electrical connection
between fingers 506 and busbars 508 may be used.
[0171] Electrical communication is established between cells
through a conductor 514. The conductor may comprise wire, foil,
mesh, or ribbon. The conductor may comprise, but is not limited to,
tinned copper. The conductor may be attached to the busbar and to
the diode through solder or an electrically conducting adhesive. As
used herein, the term conducting adhesive includes but is not
limited to a material selected from epoxy, acrylic, polyimide,
polyurethanes, cyanate esters, silicone, and combinations thereof,
allowing electrical communication.
[0172] As described in detail below in connection with FIG. 5A, a
transmissive optical element 516 overlies the active side of the
cells. A concentrated line focus 510 of light reflected by the
concentrator is shown on the cells.
[0173] Various embodiments of receivers may have a particular
designs and cell layouts. For example, cells may be arranged within
the receiver in any number of ways, including, e.g., as described
in the U.S. Provisional Patent Application No. 61/475,483. Solar
cell(s) may be arranged on an embodiment of a receiver to achieve
one or more of the following goals:
[0174] (a) the busbar is not normally illuminated;
[0175] (b) the cell gridlines are perpendicular to the light line
focus; and
[0176] (c) the concentrated light line focus normally illuminates
just half of the cell so as to provide more tolerance for tracking
errors.
[0177] FIG. 5A shows a simplified cross-sectional view of the
particular receiver embodiment of FIG. 5. This view shows
encapsulant 522 and the transmissive optical element 516.
Encapsulant 522 and transmissive optical element 516 serve to seal
and weatherize the receiver, as well as provide mechanical
protection for the cells. Sealing the cells and interconnects may
be important to reduce degradation in performance that can arise
from corrosion or electro migration of the solar cell
metallization.
[0178] The encapsulation material is chosen to match the index of
refraction of the transmissive element and minimize reflection.
Examples of materials that can be used as encapsulant include but
are not limited to, silicones or ethylene vinyl acetate (EVA).
Transmissive optical element 516 may be refractive and/or shaped
and/or include homogenizing properties. In certain embodiments
homogenizing properties can be achieved obtained through the use of
coatings or surface treatments, which minimize loss. Examples of
materials that can be used in transmissive optical element 516
include but are not limited to, low iron, tempered glass, or
TEFLON. Cells may be attached to the heat sink or substrate 502
directly using an insulating adhesive 520. Used herein, the term
insulating adhesive includes but is not limited to materials
selected from epoxy, acrylic, polyimide, polyurethanes, cyanate
esters, silicone, and combinations thereof that do not allow
electrical communication there through.
[0179] Adhesive 520 may also exhibit particular thermal properties.
In some embodiments the adhesive may be highly thermally conductive
to draw heat away from the cells to the heat sink. In some
embodiments, the thermal conductivity of the material used in the
adhesive may be between 0.005 W/m-k and 170 W/m-k. Minimizing the
number and thickness of layers between the cell and the heat sink
reduces the cell temperature and increase power output. Thermal
control over the receiver may be achieved by cooling, which can be
accomplished passively, actively, or by some combination of passive
and active approaches. In this particular embodiment three
conducting layers 514 are shown, separated by an insulating layer
518 which also lies between the cells 504 and the bypass diodes
512. The use of multiple conducting layers in a manner analogous to
the interconnect structures of integrated circuits, can allow for
internal power routing and reduce need for long external
cabling.
[0180] The particular embodiment of FIGS. 5-5A utilizes one bypass
diode arranged in parallel with each cell. However this is not
required. Alternative receiver embodiments may employ bypass diodes
in other configurations. A sealant 524 prevents the ingress of
moisture from reaching the cells and may also be used to attach the
transmissive optical element 516 to heat sink or substrate 502. As
used herein, the term sealant includes but is not limited to
materials such as epoxy, acrylic, polyimide, polyurethanes, cyanate
esters, silicone, and combinations thereof that serve to reduce the
transport of water there through.
[0181] FIG. 5B shows a simplified side view illustrating use of the
conductor 514 to connect cells 504 in series, according to one
embodiment. Typically, the top contact 544 is negative polarity and
the bottom contact 546 is positive polarity. The top of one cell is
in electrical communication with the bottom of the adjacent cell
via the conducting ribbon. The two end connections provide the
positive 542 terminal output and the negative terminal output
540.
[0182] The insulator 518 prevents shorting between the top cell
connections. The insulator also allows the negative terminal output
to be routed to the same area as the positive terminal output. In
the particular embodiment of FIG. 5B the cells are shown as being
connected in series, but this is not required. According to some
embodiments the cells may be connected in parallel, or in some
combination of series and parallel. FIG. 5C shows a receiver
configuration that uses a housing 530 in conjunction with the heat
sink or substrate 520 to hold the transmissive optical element 516.
FIG. 5D shows a non-planar receiver configuration. Here the bypass
diodes 512 are rotated 90 degrees from the cells 504 in order to
reduce shading losses. Thermal control over the receiver may be
achieved by cooling. Thus, the receiver plate/cell mount structure
may also serve as a heat sink. Cooling of the receiver may be
accomplished passively, actively, or by some combination of passive
and active approaches.
[0183] The location of a receiver relative to the concentrator, may
vary depending upon the particular collector embodiment. A range of
focal ratios can be workable from a minimum of about f/0.2 to a
workable maximum of about f/11 with no loss of light at 15.times.
concentration. Focal ratios above f/2 may be less desirable because
of increased sensitivity (losses) due to tracking errors and also
because the pressure differential required across the film may
become unfeasibly low unless large transverse forces are applied to
stretch the film tight. Focal ratios below f/0.2 require
concentration factors lower than 15.times. (i.e. larger receivers)
to avoid losing light off the receiver. One embodiment at a
15.times. concentration factor uses f/0.65 which creates a
reasonable balance between tracking error tolerance and tightness
of focus.
[0184] FIGS. 5E-5G illustrate a receiver system according to an
embodiment of the present invention. In this embodiment, a three
dimensional interconnect can be made to bypass diode 512 using
conductors 514. This may help to make the receiver assembly
smaller. In some embodiments, this design may help to reduce the
receiver width by up to 10% and may increase the power output of
the receiver by about 5%. In some embodiments, multiple bypass
diodes 512 may be stacked on top of busbar 508 along with
conducting layers 514. Such a three dimensional structure may allow
for a smaller footprint of the receiver system which may help
minimize shading losses.
[0185] Many benefits can be realized by coupling the various trough
concentration systems disclosed herein with a receiver arrangement
such as that in FIGS. 5-5G. In embodiments where solar cells are
arranged substantially end-to-end in individual rows where each row
corresponds to a concentrator, if a concentrator is partially
shaded along its length, the concentrated light that strikes each
cell in the receiver may be reduced by the amount of the shading
equally for each cell so that no cell receives substantially less
light than the others and "bottlenecks" or cells with much lower
current are avoided. Unlike most conventional solar panels, and
many other CPV systems, trough systems as described herein can be
installed so that they shade or partially shade one another at some
times of day or year without a disproportionate penalty to power
output.
[0186] According to certain embodiments, as shown in FIG. 6A, a
receiver 600 may be positioned completely within the inflation
space defined by the inflated concentrator. Certain such
embodiments may offer enhanced optical performance because the
sunlight does not have to pass through the front film twice before
striking the receiver. Inflated concentrators having receivers
positioned within an inflation space are described in U.S. patent
application Ser. No. 11/843,531 filed on Aug. 22, 2007, which is
incorporated by reference in its entirety herein for all purposes.
Embodiments may share one or more characteristics in common with
the apparatuses disclosed in that patent application.
[0187] Owing to its location within an enclosed space, such an
internally-positioned receiver may be cooled in an active manner,
for example by the flow of a liquid such as water. Incorporated by
reference herein for all purposes, is the U.S. patent application
Ser. No. 11/843,549 filed on Aug. 22, 2007 describing various forms
of interconnection structures, including interconnect structures
that are configured to carry liquids. Certain embodiments may
utilize interconnection structures sharing one or more
characteristics described in that published patent application.
Alternatively, a receiver 610 may be positioned outside of the
inflation space defined by the inflated concentrator, as shown in
FIG. 6B. Such a design may offer benefits of improved access for
cooling and also for maintenance/replacement. Here, the receiver is
shown to be passively cooled utilizing fin structures 612.
[0188] U.S. patent application Ser. No. 13/227,093, filed Sep. 7,
2011, discloses a solar collector having a receiver positioned
external to an inflation space or volume and is incorporated by
reference in its entirety herein for all purposes. Embodiments may
share one or more characteristics in common with the apparatuses
disclosed in that patent application.
[0189] Still further alternatively, hybrid versions are also
possible as shown in FIG. 6C. In this embodiment, the receiver 620
is present on both sides of the top clear film of the inflated
tubular concentrator. In this embodiment, the heat sink (here
including fins 622) is external to the inflation space, thereby
facilitating passive cooling. However the cell mount and the solar
cells themselves, lie within the inflation space, thereby reducing
optical losses. In this embodiment the clear film 699 may pass
between the receiver 620 and the heat sink 622 (it may be
sandwiched between the receiver and the heat sink). Alternatively,
the clear film 699 may have a gap or discontinuity to allow direct
contact and communication of heat between receiver 620 and heat
sink 622.
Support/Tracking
[0190] Embodiments of collectors may utilize pointing and tracking
apparatuses to maintain illumination over the path of the sun
across the sky. According to certain embodiments, the receiver
plate/cell mount may form a part of such a tracking structure.
[0191] U.S. patent application Ser. No. 11/844,877 filed on Aug.
24, 2007 describing rigging and pointing structures as well as
other concepts, is incorporated by reference in its entirety herein
for all purposes. Embodiments may share one or more characteristics
in common with the apparatuses disclosed in that published patent
application.
[0192] The U.S. patent application Ser. No. 13/015,339 filed on
Jan. 27, 2011 describes mounting and tracking structures and other
concepts. Embodiments may share one or more characteristics in
common with the apparatuses described in that patent application.
As mentioned above at least in connection with FIG. 2, a collector
may comprise an inflatable concentrator whose shape is defined by
one or more batten structures. Accordingly, FIG. 7 shows an
embodiment comprising an inflated tube 700 wherein the members
comprising the batten 702 may be oriented substantially parallel to
the ground.
[0193] As shown in FIG. 8, according to certain embodiments
multiple tubes 800 of inflated concentrators may be stacked in
order to reduce tracking costs. In certain embodiments, these
inflated tubes may be stacked according to tube diameter. According
to some embodiments, inflated tubes with smaller diameters may be
positioned higher than those having larger diameters, in order to
reduce possible shading effects.
[0194] FIG. 10A shows an overview of an embodiment of an inflatable
trough solar energy collection system 1002. System 1002 includes
one or more solar collector assemblies 1004 which are attached to
base posts 1006 via one or more pivot joints 1008.
[0195] Pivot joints 1008 define a tracking axis 1010 about which
collectors 1004 can rotate to track the sun in order to maximize
the capture of solar energy. Axis 1010 may be oriented North to
South, East to West, Northeast to Southwest or some other
orientation. A specific orientation of axis 1010 may be chosen so
as to maximize power output at a certain time of day and/or a
certain time of year. In this particular embodiment, tracking axis
1010 is shown horizontal and parallel to the length of collector
assemblies 1004. That is, axis 1010 goes through the long axis of
the collector assemblies 1004. However, this is not required and in
alternative embodiments a tracking axis may be inclined relative to
the ground or inclined relative to the collector assemblies, or
both. Other embodiments are possible in which collector assemblies
rotate about a vertical tracking axis. According to some
embodiments, each collector or collector assembly may have its own
axis about which it rotates to track the sun.
[0196] Certain embodiments may employ a linkage or other structure
which creates tracking motion that is not defined by an axis of
rotation. An example of this is a four-bar linkage. Other
embodiments are possible that cause the collector assemblies to
track the sun about 2 axes. This may allow the normal vector to the
plane of the collector assemblies to consistently point directly at
the sun, although some tracking error may be present.
[0197] Collectors 1004 are controllably actuated by drive system
1012 via a transmission element 1014. In this particular
embodiment, transmission element 1014 is shown as a curved gear
rack, but other linkages, components or forms of motion
transmission are possible. As drive system 1012 rotates, it causes
transmission element 1014 to move which in turn creates a rotation
of collectors 1004 about axis 1010. Base posts 1006 may be sunk
into the ground, or attached to ground screws, or attached to
ballast weights, or otherwise anchored to prevent unwanted motion
of system 1002.
[0198] In some embodiments, it may be possible to link multiple
rows of systems such as that shown in FIG. 10A together, so that
multiple rows may be actuated by a single actuator. Linked motion
may be accomplished via a linkage, belts, drive shafts, pushrods,
gears, cables, pulleys, hydraulics and many other types of
structures.
[0199] FIG. 10B shows a closer isometric view of one of the solar
collector assemblies 1004. Collector assembly 1004 includes an
inflated trough array 1016 which is supported by a frame 1018.
Receiver assemblies 1022 are supported by a receiver sub-frame 1020
which is in turn attached to frame 1018. Receiver assemblies 1022
may be fixed to sub-frame 1020 or they may have some degree of
lateral motion. Alignment between inflated trough array 1016 and
receivers 1022 is maintained by guides 1024. Guides 1024 may serve
to maintain only vertical alignment, or they may cause receivers
1022 to move laterally as necessary to stay centered above their
respective trough segments. This may be helpful if different
sections of trough array 1016 encounter different temperatures at
the same time and experience differential thermal expansion, or it
may serve to compensate for manufacturing and assembly tolerances
etc. In some embodiments, trough array 1016 may be made of polymer
films which can be replaced if they wear out or become damaged.
Guides 1024 may be easily removable to enable replacement of trough
array 1016.
[0200] In some embodiments, the length of the trough arrays may be
shorter or longer than the ones shown in FIG. 10B and can be varied
as needed. It may be desirable to have long trough arrays in order
to minimize end losses or other end effects. Still other
embodiments may employ trough arrays in a panel shape with
dimensions similar to standard 1-sun solar panels. This may allow
such panel-shaped trough arrays to be mounted on a variety of
commercially available frames and tracking equipment.
[0201] FIG. 10C shows a side view of the collector assembly 1004.
Frame 1018 may include pins 1026 for easy attachment of trough
array 1016. Other approaches to attaching trough array 1016 to
frame 1018 are possible. These include but are not limited to
clamps, bar clamps, tape, double sided tape, adhesives, pinch
rollers, hook and loop fasteners, threaded fasteners, magnets and
heat sealing, among others. Retaining caps, fasteners, barbs, or a
retaining strip (not shown) may aid trough array 1016 to remain
attached to pins 1026.
[0202] An inflation system (not shown) creates a pressure
differential between the chambers inside trough array 1016 and the
surrounding atmosphere. When inflated, trough array 1016 will
typically pull inward on frame 1018 if the segments of trough array
1016 are lenticular rather than circular.
[0203] FIG. 10D shows an enlarged section view of collector
assembly 1004 with frame 1018 and sub-frame 1020 removed for
clarity. Trough array 1016 can comprise a reflective bottom film
1035 and a transparent top film 1028. Films 1035 and 1028 may be
joined at their perimeter and along internal nodes to create
individual sections or chambers 1030. Trough array 1016 is retained
on guides 1024 via retaining clips 1032. Other possible approaches
to attachment include but are not limited to, clamps, bar clamps,
tape, double sided tape, adhesives, pinch rollers, hook and loop
fasteners, threaded fasteners, magnets and heat sealing, among
others.
[0204] FIG. 10E shows a partial cutaway isometric view of one
embodiment of trough array 1016. Edges of trough array 1016 may
have holes 1034 for mounting on pins 1026 (see FIGS. 10C and 10G).
Inflated chambers 1030 are separated by nodes 1036 which are areas
where the film is restrained against the inflation pressure
differential. Nodes 1036 may define mounting holes 1038 which align
trough array 1016 on guides 1024 (see FIG. 10D). In other
embodiments, the films that form trough array 1016 may be longer
than the effective length of the trough array, with extra deflated
trough stored on a roll system. If the portion of the films in use
becomes degraded or damaged, the roll system could advance the
films until a fresh section is ready for use.
[0205] FIG. 10F shows another view of frame 1018 and sub-frame
1020. Frame 1018 may include or be in the form of a truss
structure, which can serve to provide rigidity necessary to limit
deflections due to bending and torsion. An area A is circled which
is enlarged in the view in FIG. 10G.
[0206] FIG. 10G is an enlarged detail view of the area A denoted in
FIG. 10F. Pins 1026 are visible in this view. FIG. 10H shows a
schematic cross section view of the behavior of light for the
system in FIGS. 10A-G. Reflective film 1035 is shown being hit by
incident rays 1034 and creating reflected rays 1036. An axis of
symmetry 1038 of reflective film 1035 is shown. A line 1040
parallel to axis 1038 is shown. Incident rays 1034 are shown at a
slight angle to axis 1038 (in this case 1 degree) to illustrate the
behavior of light in the system with a small amount of mispointing.
Note that both incident rays 1034 and reflected rays 1036 may not
be symmetric about axis 1038, when mispointing exists. Therefore, a
region of focus 1042 will be also shifted relative to axis 1038 and
receiver 1022. FIG. 10I shows an enlarged view of the receiver
location and region of focused light circled in FIG. 10H. A plane
1044 is shown which is defined as being the location at which rays
1036 form the narrowest spot which includes all of rays 1036. It
may be desirable to locate the active surface of receiver 1022 at
or near plane 1044 to maximize the tolerance to tracking
errors.
[0207] FIG. 11 shows another embodiment of an inflated trough solar
collection system 1102. The principal difference between this
embodiment and the embodiment illustrated in FIGS. 10A-G, is that a
frame 1106 lies above a trough array 1104, facilitating replacement
of trough array 1104. In this embodiment, receivers 1110 are
aligned to trough array 1104 via guides 1108. Guides 1108 can stay
in position during replacement of trough array 1104 because there
is no frame or other members (except possibly retaining clips, not
shown) below trough array 1104. Trough array 1104 is held in place
by retaining bars 1112 which may employ pins (not shown), clamp(s),
or other approaches to retaining the film.
[0208] FIG. 12 shows another embodiment of an inflated trough solar
collection system 1202. A lower film 1204 is connected to an upper
transparent film 1206 via joining material 1208. Receivers 1210 may
be inside enclosed chambers 1212 formed by the surrounding films
and material or they may be outside.
[0209] FIG. 13 shows an embodiment of a solar collector trough
array that is shaped by a negative pressure differential (partial
vacuum). Trough array assembly 1302 includes reflective film 1304,
enclosing surface 1306, chamber(s) 1308, and receivers 1310. A
frame 1312 keeps film 1304 separated from enclosing surface 1306 to
create chambers 1308. Film 1304 has one or more concave shapes
which are created by a relatively lower air pressure in chambers
1308 as compared to the surrounding environment.
[0210] FIGS. 14A and 14B show another embodiment of solar
concentration trough system 1402 with a different frame. A
reflective film 1404 has a concave shape formed by a negative
pressure differential between a first chamber 1406 and the
surrounding environment. One or more inflated frame chambers 1408
may provide support for the edges of film 1404. Negative pressure
in chamber 1406 would tend to pull the edges of film 1404 together,
but this tendency is resisted by positive pressure inside frame
chambers 1408.
[0211] Optional frame members 1410 may be used to add additional
stiffness or to add height to chamber 1406. Chambers 1408 are
defined within a material 1412 which may be a film, fabric, polymer
sheet or other material and may be flexible, semi-flexible or
rigid. A solar receiver 1414 is positioned above film 1404 at a
region of light concentration and held in place by a receiver frame
1416. Frame chambers 1408 may rest on the ground 1420 and allow the
system to track the sun by rolling on the ground. Forces applied to
actuation arms 1418 cause system 1402 to rotate and roll on the
ground or some other support. Restraining straps (not shown) may be
employed to prevent translation of system 1402 while allowing
rotation for tracking FIG. 14B shows a cross section view of system
1402 with actuation arms 1418 removed for clarity. Optional clips
1422 may allow for easy film installation and replacement.
[0212] FIG. 16A shows an embodiment of a concentrator system 1602
with a modified film that can create performance enhancements. A
reflective film 1604 has modified surface shapes 1606. Incoming
light rays 1608 are reflected by surface shapes 1606 to create
reflected rays 1610. An optional front film 1612 is shown creating
an enclosed volume 1614 which may include a pressurized gas.
[0213] Surface shapes 1606 may be designed such that they create a
different pattern of reflected light than would be obtained by the
shape that would otherwise be formed by film 1604. While many
different patterns of reflected light are achievable, FIG. 16 shows
a pattern in which a higher concentration of light is created at a
receiver 1616 than would be possible with just inflated smooth
film. The reflected light by film 1604 converges to form a spot of
higher concentration. Different patterns could be achieved with
alternative surface shapes (not shown), to affect uniformity of
light distribution over receiver 1616 or a portion thereof.
[0214] FIG. 16B shows an enlarged view of a small portion of film
1604, with exaggerated surface shapes 1606 from FIG. 16A. These
surface shapes may be formed on film 1604 by processes including
but not limited to embossing, roll embossing, stamping, forming,
casting, laser machining, screen printing, spray deposition, inkjet
printing and photo etching and other photochemical processes, among
other methods. Surface shapes 1606 may cover some or all of the
surface of film 1604.
[0215] According to other embodiments, surface shapes 1606 may be
3-dimensional (rather than having a consistent cross section).
Three-dimensional shapes may be chosen so that light may be
directed away from inactive areas of a receiver. In some
embodiments, receiver 1616 may be placed outside the path of
incident sunlight 1608 (i.e. off-axis) so that it does not block
incident sunlight. Surface shapes similar to 1606 may have
different slopes chosen to reflect light to this off-axis receiver
location.
[0216] FIG. 17A shows a film-based solar concentration system 1702
that employs a transparent film 1704 with a modified shape.
Transparent film 1704 may be connected to an optional enclosing
material 1708 to form a closed chamber 1710, which may include an
inflation pressure. Film 1704 has a surface 1706 with a modified
shape which serves to refract light and create a desired light
pattern at a receiver 1712. Surface 1706 is shown in more detail in
FIG. 17B. Receiver 1712 may be inside chamber 1710 or it may be
outside or in another location. Incident rays 1714 come from the
sun and are nominally parallel with a small possible variation of
direction.
[0217] Surface 1706 is shaped such that rays arriving at different
locations are refracted differently, to create refracted rays 1716
at different angles as they exit film 1704 and travel to receiver
1712. In one embodiment, surface 1706 is designed so that refracted
rays 1716 form a region of higher concentration at receiver 1712,
than if film 1704 was smooth. A related embodiment (not shown) uses
a front film similar to 1704 which is stretched, so that it
maintains its shape and does not require inflation pressure or an
enclosed chamber.
[0218] FIG. 17B shows an exaggerated partial sectional enlarged
view of optical shapes for refracting light on a clear front film.
Surface 1706 can be made by a variety of manufacturing processes,
including but not limited to embossing, roll embossing, stamping,
forming, casting, laser machining, screen printing, spray
deposition, inkjet printing and photo etching and other
photochemical processes among other methods. Film 1704 can have a
modified surface shape such as 1706 on either a front surface as
shown in FIGS. 17A and 17B, or on a back surface (not shown) or on
both front and back surfaces.
[0219] In some embodiments, surface shapes 1706 may be
three-dimensional (rather than having a consistent cross section).
Three-dimensional shapes may be chosen so that light may be
directed away from inactive areas of a receiver.
[0220] FIG. 17C1 and 17C2 show a section view and a partial section
view, respectively, of another refractive front film 1718. Film
1718 is similar to film 1704 of FIG. 17B, except that prism shapes
1720 are formed on the concave side (inside) of the film. The prism
shapes in FIG. 17C2 are shown relatively large and relatively few
in number compared with the tube diameter and film thickness. In
practice it may be desirable to use a greater number of prisms with
smaller prism height relative to film thickness compared to what is
shown in this figure. In some embodiments, the prism height can
range between 0.2 .mu.m and 200 .mu.m.
[0221] Film 1718 may concentrate light in one direction (1D
concentration) if prisms 1720 are linear and have a continuous
cross section along their length (i.e. in a direction perpendicular
to the page). In some embodiments, film 1718 may concentrate light
in two directions (2D concentration) if prisms 1720 curve or if
their cross section changes along a direction perpendicular to the
page.
[0222] FIG. 17C1 shows schematically how light rays behave as they
are refracted through a 1D concentrating version of film 1718
according to an embodiment of the present invention. Incident rays
1722 are refracted a first time as they pass from the surrounding
air, gas, or vacuum into front film surface 1724 and are refracted
again as they pass through back surface 1726 of film 1718 and into
inside volume 1728 to form refracted rays 1730. Refracted rays 1730
converge to form a region of focus 1732. In an idealized case of 1D
concentration, region of focus 1732 is a line parallel to the axis
of the tubular film 1718.
[0223] FIG. 17C2 illustrates a partial close-up view of the
structure of film 1718 and prism shapes 1720 according to an
embodiment of the present invention. Incoming light rays 1722 are
refracted twice as they pass first through a front surface 1724
into film 1718 and then through a back surface 1726 into the
internal volume 1728.
[0224] FIG. 17D shows an inflated film based tubular refractive
concentrator system 1734 employing a 2D concentrating version of an
embodiment of a refractive film 1718. Refractive film 1718 is
attached to side films 1735 which may be of a different material
than film 1718. One reason to have side films 1735 to be of a
different material is to achieve cost savings or to take advantage
of different mechanical properties.
[0225] Films 1718 and 1735 are removably mounted to a receiver
assembly 1736. Films 1718 and 1735 may be formed from Polyethylene
Terephthalate (PET), poly(meth methacrylate) (PMMA) and co-, ter-,
tetra-, or other multimonomeric polymers of methacrylates or
acrylates including but not limited to monomers of ethyl, propyl
and butyl acrylate and methacrylates. Other examples of polymers
forming the upper transparent film include but are not limited to
polycarbonate (PC), polymethylpentane (TPX), cyclic olefin derived
polymers such as Cyclic olefin co-polymers (COC), cyclic olefin
polymer (COP), ionomer, fluorinated polymers such as polyvinilidene
fluoride and difluoride (PVF and PVDF), ethylene
tetrafluoroethylene (ETFE), ethylene chlorotrifluoroethylene
(ECTFE), fluorinated ethylene propylene (FEP), THV and derivatives
of fluorinated polymers, and co-extruded, coated, adhered, or
laminated species of the above. Examples of thicknesses of layers
of such materials may include from about 0.012 mm to 20 mm,
depending on the strength of the material and the size of the
collector. In some embodiments, films 1718 and 1735 may comprise
two or more layers. Each layer can be chosen from any of the
materials listed above. Receiver assembly 1736 has a heat sink 1738
to which cell assemblies 1740 are mounted. Incoming rays 1742 are
refracted through film 1718 to become refracted rays 1744 which
converge and are concentrated at points or tight regions of focus
1746 on cell assemblies 1740.
2D Concentration Via Refractive Cylindrical Optic
[0226] Refractive concentrating optics for CPV are typically in the
form of flat Fresnel lenses or aspheric solid lenses, but Fresnel
lenses can also be formed on a cylindrical surface. There are
existing techniques for creating a line focus (1D concentration) by
refraction of light through a cylinder. However, there are
fundamental limits to the concentration factor that can be achieved
with one-axis (or 1D) concentration. There may be economic benefits
and other advantages of operating at higher concentrations, which
are only achievable by concentrating in two axes (2D
concentration). 1D concentration with a cylindrical optic bends
light in a plane perpendicular to the axis of the cylinder,
creating a "line focus" parallel to the cylinder axis. Described
below are techniques that allow cylindrical optics to bend light
with precision in another axis as well to create 2D concentration.
This makes possible a series of point focuses, various regions of
focus, or a series of shortened lines of potentially higher
concentration than the line focus of 1D concentration.
[0227] FIG. 17E is a schematic that illustrates a tubular solar
concentration system 1750 according to an embodiment of the present
invention. Solar concentration system 1750 includes elongated 2D
concentrated light focus regions 1752 which can be created using a
cylindrical primary optic prism pattern 1748 which causes refracted
light to fill and define volumes 1754, which are shown to help
visualize the concentration action. Elongated focal regions 1752
are just one example of many possible focal region shapes that can
be created using the techniques and refractive elements described
below.
[0228] FIG. 17F shows another method of achieving higher net
concentration than is ordinarily possible with a 1-D refractive
film, according to an embodiment of the present invention. A 1-D
concentrating Fresnel lens 1756 in the form of a whole or partial
cylinder refracts incoming light to define and fill 1-D
concentrating volumes 1760, which are shown to help visualize the
concentration action. A secondary concentration device 1762 may be
used to concentrate light in an additional direction. Refracted
light after the secondary concentration device 1762 defines and
fills secondary concentration volumes 1764 and results in discrete
points or regions of focus 1766.
[0229] 1D or line focus on a linear Fresnel optic is accomplished
with prisms parallel to the axis of the optic which are typically
continuous. 1D concentrating Fresnel lenses may be flat, or they
may be curved in a circular arc or non-circular curve cross
section. If there is a curved cross section, the cross section is
kept relatively uniform along the length of the lens system in
order to maintain a consistent depth and width of focus on a linear
receiver. If the cross section is a circular arc, then the optic
forms a partial cylinder. The normals to the prism surfaces are all
normal to the cylinder axis (though they will not in general
intersect with the cylinder axis). In contrast, the canonical form
of 2D concentration uses a primary optic to create a point focus,
or a close approximation thereof. The most typical refractive
element used as a primary optic for 2-D concentration is a flat
Fresnel lens with concentric circular prisms or grooves.
[0230] Accomplishing 2D concentration with a tubular primary optic
is non-trivial if losses are to be kept to a minimum. Some level of
2D concentration may be achieved if the surface normals of some
locations on the prisms are not normal to the axis of the cylinder.
However, to achieve a high level of concentration in the additional
axis, e.g., the direction along the cylinder, it is beneficial to
have the surface normals of the prisms change in a prescribed way
so that light is refracted through different locations on the
cylinder to arrive at a similar point in space where concentration
is desired. In order to minimize the losses, additional geometry
constraints can be accommodated to avoid prism self-shading,
unwanted refractions and internal reflections. There are several
possible approaches to minimize the losses.
[0231] If a transparent cylinder has smooth, concentric inner and
outer surfaces (as in the case of a plain clear inflated film
tube), sunlight striking it will be refracted and its direction as
it exits the inside film surface into the inside air will in most
cases be changed slightly and shifted slightly, but it will not be
concentrated to a useful degree. Concentrating light using a
cylindrical optic involves modification of the surface slopes at
either the inner surface or the outer surface of the film or both.
In the particular case of thin film tubes to be deployed outdoors
for solar concentration, it may be advantageous to leave the outer
convex surface of the tube smooth and modify the inner surface of
the tube. The outer surface of tube then holds less soil and is
easier to clean. There are also a greater variety of material
choices and coatings and processes that can be employed with the
smooth surface. However, in some circumstances it may be desirable
to modify the shape of the outer surface or both surfaces.
[0232] In the instance where the outer surface is smooth and the
inner surface is modified, for any point on the cylinder inner
surface that refracts the incident ray to a specified target
location (a point in x,y,z space), there is a unique surface normal
for that point that will accomplish that goal. Accordingly, there
is a closed form solution for the case of surface normals which
will refract light to a common target for points on the upper half
of the interior surface of a cylinder. A derivation of this
solution is described below. When light is refracted through a
film, prism or other refractive optical structure, its direction is
changed. This change in the direction of the light as it passes
through a refractive optic is often referred to as the turning
angle. There are limits to the turning angle which depend upon
refractive indices of the material and medium contained inside the
tube. A similar solution can be found for modifying the surface at
points on the exterior of a cylinder.
[0233] FIGS. 17S1-17S9 schematically illustrate physical principles
relating to creating a curved body with optical elements (prisms or
grooves) with the goal of refracting and concentrating light and
creating controlled regions of focus according to an embodiment of
the present invention. These examples and figures discuss a partial
cylinder shape with prisms on an inner surface, but the methods
described here can also be generalized and adapted to the design of
other optical surface shapes and/or refractive elements (prisms) on
the outer surface or on both inner and outer surfaces.
[0234] FIG. 17S1 illustrates a cylindrical surface in a first set
of Cartesian coordinates x, y, z for purposes of mathematically
deriving the surface normals required for a prism pattern
superimposed on that cylindrical surface to refract incoming
collimated light striking the cylinder at a point {tilde over (g)}
to a target point {tilde over (f)}.
[0235] FIG. 17S2 illustrates the transformation of the cylindrical
surface to a flattened surface, for example when a cylindrical film
is straightened to become planar. A transformed set of Cartesian
coordinates i, j, k is shown that are used for referring to points
and vectors in the flattened transformation. A point {tilde over
(h)} represents the location where the point {tilde over (g)} ends
up after the transformation to flattened coordinates.
[0236] FIG. 17S3 shows the cylindrical surface of FIG. 17S1 with
the angle .theta..sub.1 from the central axis of the cylinder to
any point on the cylinder outer surface and a corresponding normal
vector {circumflex over (m)} at that point. Since points on the
surface lie on a cylinder whose central axis runs along the x-axis,
the angle .theta..sub.1 can be calculated from the y and z
coordinates:
.theta..sub.1=tan.sup.-1(y/z) (1)
[0237] FIG. 17S4 illustrates how an incoming ray w with an angle
.theta..sub.1 to the outer surface normal {circumflex over (m)} is
refracted to become initial refracted ray {circumflex over (v)}
within the material of the cylinder at a new angle .theta..sub.2
from {circumflex over (m)}. The refraction angle .theta..sub.2 can
be obtained by using the scalar form of Snell's Law.
.mu. sin .theta..sub.1=.eta. sin .theta..sub.2 (2)
.theta..sub.2=sin.sup.-1(.mu./.eta. sin .theta..sub.1) (3)
.mu. is the refractive index of ambient medium. .eta. is the
refractive index of the cylinder medium. .theta..sub.1 is the angle
between the incident ray and normal of exterior surface.
.theta..sub.2 is the angle between the refracted ray in cylinder
medium and normal of exterior surface.
[0238] The angle .alpha. between the ray {circumflex over (v)} and
the vertical z axis is used to calculate the components of the ray
{circumflex over (v)}.
.alpha. = .theta. 1 - .theta. 2 ( 4 ) v ^ = [ v x v y v z ] = [ 0 -
sin .alpha. - cos .alpha. ] ( 5 ) ##EQU00001##
[0239] FIG. 17S5 illustrates how initial refracted ray {circumflex
over (v)} is refracted again through the interior cylinder surface
(which may have a modified surface slope or shape--not shown) by an
angle .beta. to become final refracted ray u. Vector subtraction
can be used to find the vector between {tilde over (f)} and {tilde
over (g)}, which then may be scaled to a unit length.
[ .DELTA. x .DELTA. y .DELTA. z ] = f ~ - g ~ = [ f x f y f z ] - [
g x g y g z ] ( 6 ) u ^ = [ u x u y u z ] - [ .DELTA. x .DELTA. y
.DELTA. z ] 1 .DELTA. x 2 + .DELTA. y 2 + .DELTA. z 2 ( 7 )
##EQU00002##
The dot product can be used to find the angle between the two
vectors.
cos .beta.=u{circumflex over (v)} (8)
.beta.=cos.sup.-1(u{circumflex over (v)}) (9)
Expressing in terms of components:
.beta. = cos - 1 ( [ 0 v y v z ] [ u x u y u z ] ) ( 10 ) .beta. =
cos - 1 ( v y u y + v z u z ) ( 11 ) ##EQU00003##
[0240] FIG. 17S6 shows the relationship between {circumflex over
(v)}, u, .phi..sub.1, .beta., and .phi..sub.2. Solving for
.phi..sub.2
.mu. sin .phi..sub.2=.eta. sin .phi..sub.1 (12)
.phi..sub.1=.phi..sub.2.beta. (13)
The relationship between the refraction angles (13) can be used to
rewrite equation (12) in terms of .phi..sub.2 and .beta..
.mu. sin .phi..sub.2=.eta. sin(.phi..sub.2-.beta.) (14)
Using the angle sum trigonometry identity and substituting to
rewrite the right-hand side of equation (14).
sin(a-b)=sin(a)cos(b)-cos(a)sin(b) (15)
.mu. sin .phi..sub.2=.eta.(sin .phi..sub.2 cos .beta.-cos
.phi..sub.2 sin .beta.) (16)
Dividing by cos .phi..sub.2 and substituting tan .phi..sub.2:
.mu. sin .phi. 2 cos .phi. 2 = .eta. sin .phi. 2 cos phi 2 cos
.beta. - .eta. sin .beta. ( 17 ) .mu. tan .phi. 2 = .eta. tan .phi.
2 cos .beta. - .eta. sin .beta. ( 18 ) ##EQU00004##
Rearranging and solving for .phi..sub.2:
tan .phi..sub.2(.eta. cos .beta.-.mu.)=.eta. sin .beta. (19)
.phi..sub.2=tan.sup.-1(.eta. sin .beta./(.eta. cos .beta.-.mu.))
(20)
[0241] FIG. 17S7 shows the relationship between u, {circumflex over
(v)}, and a vector o representing a unit vector in the direction of
a rotation axis defined by u and {circumflex over (v)}. The axis of
the angle .beta. between the ray in the cylinder medium and the ray
to the focal point is defined by the cross product between the two
rays. The formula for the cross product is applied to the ray
vectors.
a ~ .times. b ~ = [ a 2 b 3 - a 3 b 2 a 3 b 1 - a 1 b 3 a 1 b 2 - a
2 b 2 ] ( 21 ) o ~ = v ~ .times. u ~ = [ 0 v y v z ] .times. [ u x
u y u z ] ( 22 ) o ~ = [ o 1 o 2 o 3 ] = [ v y u z - v z u y v z u
x - v y u x ] ( 23 ) ##EQU00005##
For creating a rotation matrix, the rotation axis can be expressed
as a unit vector and the magnitude can be found as:
o ^ = [ o x o y o z ] = [ o 1 o 2 o 3 ] 1 o 1 2 + o 2 2 + o 3 2 (
24 ) ##EQU00006##
The vector is rotated by angle .phi..sub.2 about the axis defined
by unit vector u. The corresponding rotation matrix R is
created.
R=I cos .phi..sub.2+[o].times.sin .phi..sub.2+oo(1-cos .phi..sub.2)
(25)
I is the identity matrix and is given by
I = [ 1 0 0 0 1 0 0 0 1 ] ( 26 ) ##EQU00007##
[o].sub.x is the cross product matrix of o.
[ o ^ ] x = [ 0 - o z o y o z 0 - o x - o y o x 0 ] ( 27 )
##EQU00008##
[0242] oo is the tensor product of o and itself
o ^ o ^ = [ o x 2 o x o y o x o z o x o y o y 2 o y o y o x o z o y
o z o z 2 ] ( 28 ) ##EQU00009##
[0243] The components of the rotation matrix R are listed
below:
R = [ R 1 , 1 R 1 , 2 R 1 , 3 R 2 , 1 R 2 , 2 R 2 , 3 R 3 , 1 R 3 ,
2 R 3 , 3 ] ( 29 ) R 1 , 1 = cos .phi. 2 + o x 2 ( 1 - cos .phi. 2
) R 2 , 2 = cos .phi. 2 + o y 2 ( 1 - cos .phi. 2 ) R 3 , 3 = cos
.phi. 2 + o z 2 ( 1 - cos .phi. 2 ) R 1 , 2 = o x o y ( 1 - cos
.phi. 2 ) - o z sin .phi. 2 R 2 , 1 = o x o y ( 1 - cos .phi. 2 ) +
o z sin .phi. 2 R 1 , 3 = o x o z ( 1 - cos .phi. 2 ) + o y sin
.phi. 2 R 3 , 1 = o x o z ( 1 - cos .phi. 2 ) - o y sin .phi. 2 R 2
, 3 = o y o z ( 1 - cos .phi. 2 ) - o x sin .phi. 2 R 3 , 2 = o y o
z ( 1 - cos .phi. 2 ) + o x sin .phi. 2 ( 30 ) ##EQU00010##
[0244] FIG. 17S8 shows the relationship between u, the desired
interior surface normal vector {circumflex over (n)}, o and
.phi..sub.2. The rotation matrix R is multiplied by vector u to
obtain the unit normal vector {circumflex over (n)}.
n ^ = - R u ^ = - [ R 1 , 1 R 1 , 2 R 1 , 3 R 2 , 1 R 2 , 2 R 2 , 3
R 3 , 1 R 3 , 2 R 3 , 3 ] [ u x u y u z ] ( 31 ) n ^ = [ n x n y n
z ] = - [ R 1 , 1 u x + R 1 , 2 u y + R 1 , 3 u z R 2 , 1 u x + R 2
, 2 u y + R 2 , 3 u z R 3 , 1 u x + R 3 , 2 u y + R 3 , 3 u z ] (
32 ) ##EQU00011##
[0245] FIG. 17S9 shows the relationship of the desired interior
surface normal vector {circumflex over (n)} in the coordinates of
the cylindrical surface to that of the corresponding vector
{circumflex over (l)} in the coordinates of the flattened surface.
{circumflex over (l)} is {circumflex over (n)} rotated about the x
axis (which is equivalent to the i-axis in the flattened
coordinates) by an angle of .theta..sub.1 using a rotation matrix
Q. To convert the vector to the flattened film coordinates, it is
rotated about the x-axis by rotation matrix Q.
l ^ = Q n ^ = [ 1 0 0 0 cos .theta. - sin .theta. 0 sin .theta. cos
.theta. ] [ n x n y n z ] ( 33 ) l ^ = [ l i l j l k ] = [ n x n y
cos .theta. - n z sin .theta. n y sin .theta. + n z cos .theta. ] (
34 ) ##EQU00012##
The location {tilde over (g)} on the cylindrical surface is
converted to the location {tilde over (h)} on the flattened
surface.
h ~ = [ h i h j h k ] = [ g x r .theta. 1 g y 2 + g z 2 - r ] ( 35
) ##EQU00013##
The location {tilde over (h)} on the flattened surface is converted
to the location {tilde over (g)} on the cylindrical surface
using:
g ~ = [ g x g y g z ] = [ h i ( r + h k ) sin ( h j / r ) ( r + h k
) cos ( h j / r ) ] ( 36 ) ##EQU00014##
The condensed form of the solution is written as:
.theta. 1 = tan - 1 ( g y g z ) ( 37 ) .alpha. = .theta. 1 - sin -
1 ( ( .mu. .eta. ) sin .theta. 1 ) ( 38 ) v ^ = [ v x v y v z ] = [
0 - sin .alpha. - cos .alpha. ] ( 39 ) u ^ = f ~ - g ~ f ~ - g ~ (
40 ) .beta. = cos - 1 ( u ^ v ^ ) ( 41 ) .phi. 2 = tan - 1 ( .eta.
sin .beta. .eta. cos .beta. - .mu. ) ( 42 ) o ^ = v ^ .times. u ^ v
^ .times. u ^ ( 43 ) R = I cos .phi. 2 + [ o ^ ] x sin .phi. 2 + o
^ o ^ ( 1 - cos .phi. 2 ) ( 44 ) n ^ = [ n x n y n z ] = - R u ^ (
45 ) l ^ = [ l i l j l k ] = [ 1 0 0 0 cos .theta. 1 - sin .theta.
1 0 sin .theta. 1 cos .theta. 1 ] n ^ ( 46 ) ##EQU00015##
[0246] A section of cylinder where every point on the surface has a
constant target location will create a point focus (for collimated
incoming light). Other focal shapes can also be created by making
the target location dependent on the location on the cylinder.
[0247] In practice, knowing the desired surface normals at points
on the cylinder surface is not enough to completely specify the
whole surface pattern, particularly if thickness is important. This
is because a continuous inner surface with the calculated surface
normals would no longer be a cylinder and therefore given that the
outer surface still is a cylinder, the material thickness across
the surface of the tube would deviate greatly, which is often
impractical. If the average material thickness across the surface
is to be kept within reasonable bounds (or nearly constant as in
the case of embossing of films), the surface being changed to
redirect the light (in this case the inner surface of the film)
must be in some way divided into active sub-surfaces with in-active
joining surfaces in between them. As one moves across an active
sub-surface, the material thickness reaches a limit (e.g., a
limiting minimum thickness in the case of a valley or a limiting
maximum thickness in the case of a peak) and a joining surface may
be needed to traverse across the thickness to the other extreme,
where another active surface can begin. This is the basis of
traditional flat Fresnel lens design but also applies to other
approaches, some of which differ substantially from traditional
flat Fresnel lenses with axis-symmetric grooves.
[0248] Although a closed form solution of a particular prism active
face pattern may be possible from the map of surface normals, it is
generally more feasible to use discrete approximations.
[0249] FIGS. 17G1-17G6 show one approach to discretization of a 2D
concentrating refractive film which uses tetragonal faces 1776 on a
rectilinear grid, each with a unique surface normal (not shown)
that refracts light passing through the face towards the target
(not shown), according to an embodiment of the present invention.
FIG. 17G1 shows the overall film system with actively refracting
region 1778.
[0250] FIG. 17G2 shows a top view of part of refractive region
1778. Four regions of prisms 1768, 1770, 1772 and 1774 are labeled
and shown in detail in the figures below. FIG. 17G3 shows a detail
view of prisms 1776 in region 1768. For embossing processes, the
surfaces between the optical faces must have a draft angle. In
FIGS. 17G3-17G6 this draft angle can be designed into and
manufactured into draft faces 1775 and 1777. For draft faces 1775
which are largely parallel to the longitudinal cylinder axis, the
refraction of light through the outer surface of the cylinder makes
it possible to use draft faces that do not block the light.
However, for faces 1777 that touch the other sides of active faces
and which are closer to perpendicular to the cylinder axis, the
draft angle will cause some vignetting losses where light is either
blocked by or misdirected by a non-optically active face such as
face 1777.
[0251] FIG. 17G4 shows a detail view of prisms 1776 in region 1770.
FIG. 17G5 shows a detail view of prisms 1776 in region 1774, which
is the most central region and has the least losses due to the
draft faces. FIG. 17G6 shows a detail view of prisms 1776 in region
1772.
[0252] FIG. 17H shows another approach which uses axisymmetric
curved prisms in groups or "tiles" combined together in an array to
create the overall prism pattern according to another embodiment of
the present invention. FIG. 17H illustrates a top view of a tubular
optic 1780 formed of a film 1782 which has an overall prism pattern
1790 which is repeated along a longitudinal axis 1784. The overall
prism pattern 1790 is made up of a variety of individual tiles
1786. Each tile 1786 may have many individual prism grooves 1788.
Prism grooves 1788 may be axisymmetric (when the film is flat,
before it is turned into a cylinder). Although the surface normals
across the entire primary optic region do not match an axisymmetric
description, the surface normals across small subsection tiles 1786
can be feasibly approximated by an axisymmetric pattern. This has
distinct advantages for manufacturing because it allows for the
creation of tools using diamond turning or other processes that
require axis-symmetry. In FIG. 17H, the grooves do not necessarily
line up with each other from tile to tile. The further a tile is
from the center of the pattern, the more the slope of the active
faces of prism grooves 1788 deviates from the active groove faces
used on a flat Fresnel lens. This means that for a given acceptable
error of surface normals within a tile, smaller tiles must be used
further from the center or target (the target may or may not be
located directly below the cylinder center). Note that in FIG. 17H,
tiles near axis 1784 are larger and tiles further from axis 1784
are smaller.
[0253] In a different optical design, the Fresnel groove paths may
be made as continuous circular arcs wrapped onto the cylindrical
surface. If the prism angles are kept constant on these circular
arc grooves, then the refracted light would not form a tight region
of focus and the results would generally not be useful for HCPV. To
correct this problem, the prism angle would need to change along
the groove path. However, even with a changing prism angle along
the path, for any point on the primary there is a finite band of
angles through which the refracted light could be directed. If we
assume that the cylindrical optic is correctly pointed at the sun,
the direction of any ray refracted through the smooth front surface
of the optic lies in a plane perpendicular to the cylinder axis
(+/- a portion of sun's angular subtense). Then after refraction
through a prism on the inside of the optic surface whose normal
must nearly be perpendicular to the groove path, the resulting
ray's direction will have been changed in two angular directions
relative to the orientation of the principal axes, but the range of
resulting ray angles will be limited to a narrow band governed by
the plane of the ray directions after the front surface refraction
and the plane of possible prism surface normals. This narrow band
of possible resultant ray directions will be different for every
point on the primary and importantly, each band for each point on
the primary will not generally intersect at a consistent location
in space. This would still present a significant limitation to the
degree of concentration, shape of the region of concentration and
distribution of light that could be achieved within the region of
concentration. In some embodiments, another degree of freedom in
the design may be needed if the optical design is to achieve a
common spatial point of intersection of all the rays (or even a
narrow region within which a majority of the resultant rays
pass).
[0254] Prism depth and width may be chosen based on several factors
and constraints. The width and depth of the prisms may often be
linked because when the depth and angle are specified, the width is
determined. The optical performance is determined in largest part
by the prism angle (prism width and spacing if too small will
generally have a detrimental effect on focus due to diffraction
phenomena). Prism depths may be constrained by manufacturing and
cost concerns. For example, in the case of embossed film refractive
optics, economics are optimized by using small prism depths because
less material is required, but some embossing processes may require
a minimum feature size. Also losses due to tool radii and
diffractive effects may impose limits on how small the prism
spacing and widths can be and therefore also on the depths.
[0255] If desirable, it may be possible to vary the prism depth
from prism to prism. Alternatively, for manufacturing reasons or
other reasons, it may be desirable to design the pattern of prisms
so that the groove valleys are all at the same depth.
Alternatively, prism peaks may be designed to be all at the same
height. In other embodiments, the mid-plane of each prism may be
designed to be at a constant depth. The latter principle can enable
optimal embossing performance by providing equal volumes of
material displaced from the troughs on one side of the mid plane
and filled into the peaks on the other side of the mid plane. In
this way resin displaced from valleys exactly equals that which
flows to form the peaks.
[0256] In one particular embodiment, the design for 2-D
concentrating light to a point or tight focus with prisms on a
cylindrical surface uses a continuous grooved pattern of curving
prisms. Because of the unique nature of the desirable surface
normals on a cylindrical Fresnel lens, the angle of both the
groove's optical and non-optical faces varies along its curve, and
the curve is not accurately described by either circular or
polynomial descriptions. A collection of grooves can be held to a
uniform depth (if desired for manufacturing for example) by
determining the spacing between grooves such that the maximum depth
in each groove is consistent. In an embodiment, a desired focal
point defines a map of ray directions and therefore the desired
surface normals over the cylinder surface that refract the light
appropriately as described above. The groove pattern is generated
based on these surface normals. The focal region can be a point, or
it may be a compressed line, or other arbitrary mapping of
locations on the cylinder surface to x,y,z locations in space.
[0257] It is a challenging problem to design continuous facet
grooves on a cylindrical surface that redirect incident light into
a point focus. The design methodology is briefly summarized as
follows. For a point on the cylinder, calculate the direction of
the ray refracted through the exterior surface. In the case of an
inflated tube, the surface is a cylinder. Thus, once the cylinder
diameter is chosen, the behavior of the light refracting through
the outer surface is known. Thereafter calculate the interior
surface normal that is needed to refract that ray to its desired
target. This gives every point on the cylinder a unique ideal
surface normal (as described above in FIG. 17S). This design
process creates a pattern of grooves that match these ideal surface
normals. In one embodiment where the target lies on a vertical
center plane of the cylinder, there are two planes of symmetry. One
plane is orthogonal to the cylinder axis and intersects the point
focus. The second plane splits the cylinder into two equal halves.
These two planes split the cylinder surface into quadrants; only
one of them needs to be determined in order to obtain the others
due to the symmetry conditions. Note that there are also other,
more complex cases that don't have this same symmetry, some of
which will be described later.
[0258] Unlike the surfaces of a flat Fresnel lens, for a 2-D
concentrating cylindrical Fresnel lens, an analytical description
of the groove pattern (groove paths and surfaces) is challenging.
Therefore, in some embodiments, an irregular 3D mesh of discrete
triangular elements can be used to describe the paths of the
grooves as well as the local slopes of the active and inactive
prism faces. At sufficiently high mesh resolution, this can yield
excellent optical results. Once the mesh is solved, 3D curves can
be fitted along the nodes to create a set of continuous curves that
describe the Fresnel groove pattern including groove paths, optical
surfaces and draft surfaces. The draft surfaces created by this
methodology are surfaces which are not optically active, and which
are created primarily as connectors or segway surfaces between the
optically active faces. In general, draft faces and their angles
are designed such that they have a minimal effect on the light
passing through the optically active surfaces. Minimizing their
effect or losses due to draft faces is challenging and is one of
the advantages of the design methodology described herein.
[0259] Specific steps of this general method are described below.
Four triangular elements are grouped into a segment; a chain of
segments describes a groove, and the set of grooves describes the
Fresnel. Each segment has three edges along the groove direction;
two "valley" edges and a "peak" edge. The other edges lay along the
optical and draft faces, two on each.
[0260] FIG. 17I1 shows an isometric view of a subset of points 1792
that are generated and used by the groove calculation described
herein. A smaller subset of points 1794 lie in and define a slice
plane 1796. FIG. 17I2 shows how points 1792 are connected to form
curves 1798 that define valleys between prisms and curves 17100
that define peaks of prisms. FIG. 17I3 shows how points 1794 may be
used as vertices of triangular mesh elements 17102. FIG. 17I4 shows
partial prism segments 17104 made using points 1792, and curves
1798 and 17100. FIG. 17I5 shows optical faces 17106 and draft faces
17108 on idealized curved prisms. FIG. 17I8 shows a small segment
17114 which approximates a small length of an idealized curved
prism. Optical faces 17106 and draft faces 17108 can be
approximated by triangular elements. FIG. 17I7 shows how a single
curved prism 17112 can be made up of many approximated segments
17114 which are connected along common edges and vertices. FIG.
17I6 shows how a prism array 17110 can be made up of many curved
prisms 17112.
[0261] To create a mesh representation of grooves and groove paths
that match the ideal mapping of surface normals, each groove starts
with points or vertices one "slice", whose properties are used to
find the location of the next "slice" in the groove. This process
is iterated to populate the full length of the groove. If needed,
every "slice" in the groove is then scaled by the same amount to
ensure the tallest "slice" in the groove stays within depth
restrictions for the Fresnel pattern. Then the first "slice" in the
groove is used to predict the first "slice" in the next groove, and
the process is repeated until the entire primary optic surface is
covered.
[0262] A more detailed description of one implementation of this
general algorithm according to an embodiment of the present
invention is provided below. The very first parameter of a groove
is the starting location of the first slice's inner valley vertex.
This lies in the plane of transverse symmetry. For the first
groove, this is calculated by finding the location on the film that
is directly over the target location, then stepping an offset
distance away from it. For all following grooves, the first slice's
inner valley vertex is determined by the previous groove's outer
valley vertex with the same slice number. From this starting point,
the step size to the center of the optical face is guessed by using
the corresponding distance of the prior groove's first slice. The
location at the center of the optical face is used to calculate
vectors for the surface normal, ray through refracting medium, and
refracted ray. The surface normal vector defines the plane of the
slice. The surface normal determines the optical face angle of the
slice. The draft face angle is chosen based on the angle of the
refracted ray and the angle of the ray in the refracting medium
projected onto the slice plane, as well as the minimum draft angle
of the manufacturing process. In most locations on the primary
surface, it is possible to pick a draft angle that lies between the
angle of the projected refracted ray and the angle of the projected
internal ray, such that the peak vertex of the slice lies in an
optically inactive zone.
[0263] The next slice in the groove is found as follows. The vector
that defines the slice plane is tangent to the ideal continuous
curve of the groove. Although a discrete step could be taken along
this vector, it is possible to increase the accuracy of the mesh
without increasing the number of points by stepping along a
circular path that better approximates the ideal shape. The
circular path is found by taking a very small linear step along the
plane's vector, then finding a new surface normal at that location.
Both surface normals are projected into the surface plane. Their
intersection determines the center of the arc. The circular step
size is determined by whichever is smaller, a minimum angular step
or a minimum path length step. The location of the outer valley
vertex of the new slice can then be found by the circular path
parameters and the step size. This step is repeated, generating the
outer vertex of each slice from the properties of the previous one
until the groove leaves the active area of the primary.
[0264] Upon completing the propagation of the groove, each slice
within the groove can be scaled. A spline function is fit to the
outer valley points of the previous groove, using cylindrical
coordinates. A simple circle of constant radius is used if there is
no previous groove. The angle of each slice vertex in the
cylindrical coordinate system is used to find the radius of the
inner valley points that lay along the same radii as the outer
valley points. The angular difference between the axisymmetric
coordinate and the actual curve along the outer valley radii can be
used in conjunction with the radial gap in order to find the base
dimension of each slice. From this base dimension, the slice
optical and draft angles can be used to determine the height of the
slice peak. The entire groove can be then scaled by some amount to
fit within the maximum depth & width constraints. Scaling is
done by using the inner points of the groove and scaling the base
width along the slice plane.
[0265] The above steps are repeated, filling the primary area with
grooves until it is entirely covered. This involves extending the
groove start and end locations past the active primary area, such
that the corners can be completely filled.
[0266] There are many possible prism patterns and associated
locations of focal regions. Some possibilities are described here.
All of these may be designed using the tubular prism design
algorithm described above, with some slight variations to account
for changes to symmetry conditions and target locations.
[0267] FIG. 17J1 illustrates a top view of a composite prism
pattern 17116 made up of tiled refractive prism groups 17118 that
may be wrapped to form a cylindrical or tubular concentrating
optic. Prism groups 17118 are made up of prisms 17120 which are
designed to efficiently concentrate incoming light in two
directions. Note that the prisms in this figure and many of the
following figures are drawn schematically and that only selected
contours are drawn, since in many actual designs the prisms are so
tightly spaced and so numerous that it is impractical to depict
them all in an overview figure. It is to be noted that the prisms
need not be symmetrical despite the symmetry shown in some of the
figures. FIGS. 17J1 and 17J2 schematically represent an idealized
prism pattern that can create optimal 2-axis light concentration
with prisms that are continuous over relatively large areas of the
cylindrical optic--i.e. the prisms are not constrained to be
axisymmetric arcs and the optic surface does not have to be
subdivided into many small tiles to achieve high concentration in a
relatively small target region.
[0268] FIG. 17J2 illustrates an isometric view of a composite
pattern 17116 wrapped to form a tubular optic structure 17122.
Prism pattern 17116 may be located on the inner (concave) surface
of tubular optic 17122, or it may be located on the outer (convex)
surface of tubular optic 17122, or both. Prisms pattern 17116 may
be made of prisms with desired optimal shapes. Alternatively the
prisms may be circular arc approximations of desired shapes or they
may be some other curve that approximates an idealized
concentrating prism shape.
[0269] FIG. 17K1 illustrates a top view of another composite
refractive prism pattern 17124 according to an embodiment of the
present invention. The prism pattern shown in FIGS. 17K1 and 17K2
is divided into many small tiles which may be useful to achieve
near ideal optical concentration with prisms that are circular arc
approximations of the ideal non-circular prism shapes. Tile
refractive pattern 17124 includes repeating sections 17126 of tiled
refractive prism groups 17130 each of which is in turn made up of
prisms 17128. Prism pattern 17124 is designed to be wrapped to form
a cylindrical or tubular concentrating optic.
[0270] Note that while prism groups shown here are in square
sections, other shapes may also be used as the boundary of prism
groups. Possible shapes include hexagons, triangles, rectangles,
other tessellating shapes, or other shapes. FIG. 17K2 illustrates
an isometric view of prism pattern 17124 of FIG. 17K1, as wrapped
onto a tubular optic 17122, according to an embodiment of the
present invention.
[0271] FIG. 17L1 illustrates a top view of a mapping pattern 17132
for a tubular refractive optic which is divided into sections 17134
and respective regions of focus 17140, according to an embodiment
of the present invention. Optional cells 17136 are shown to
illustrate one possible location of cells with respect to the
regions of focus 17140. FIG. 17L2 illustrates an isometric view of
the mapping pattern 17132 of FIG. 17L1 according to an embodiment
of the present invention. Spatial volumes 17138 enclose light rays
refracted through respective sections 17134, and are shown to
illustrate how light arriving at the different sections 17134
travels through different spatial volumes 17138 and is therefore
mapped to different respective regions of focus 17140. Locations
17142 represent the areas of highest light intensity within regions
of focus 17140 and may represent a cell location.
[0272] FIG. 17M1 Illustrates a top view of another mapping pattern
17144 for a tubular refractive optic according to another
embodiment of the present invention. Mapping pattern 17144 is
divided into sections 17146 and respective regions of focus 17152.
Optional cells 17148 are shown to illustrate one possible location
of cells with respect to the regions of focus 17152. This figure
shows how a mapping pattern may be divided into pairs of sections
and corresponding regions of focus.
[0273] FIG. 17M2 illustrates an isometric view of the mapping
pattern 17144 of FIG. 17M1 according to an embodiment of the
present invention. Spatial volumes 17150 enclose light rays
refracted through respective sections 17146 and are shown to
illustrate how light arriving at the different sections 17146
travels through different spatial volumes 17150 and is therefore
mapped to different respective regions of focus 17152. Locations
17154 represent the areas of highest light intensity within regions
of focus 17152 and may represent a cell location.
[0274] FIG. 17N1 Illustrates a top view of another mapping pattern
17156 for a tubular refractive optic which is divided into sections
17158 and respective regions of focus 17164, according to an
embodiment of the present invention. Optional cells 17160 are shown
to illustrate one possible location of cells with respect to the
regions of focus 17164. Pattern 17156 differs from the pattern
17144 of FIGS. 17M1 and 17M2 in that sections are not in aligned
pairs, rather they are offset in the longitudinal direction of the
tube.
[0275] FIG. 17N21 illustrates an isometric view of the mapping
pattern 17156 of FIG. 17N1 according to an embodiment of the
present invention. Spatial volumes 17162 enclose light rays
refracted through respective sections 17158 and are shown to
illustrate how light arriving at the different sections 17158
travels through different spatial volumes 17162 and is therefore
mapped to different respective regions of focus 17164. Locations
17166 represent the areas of highest light intensity within regions
of focus 17164 and may represent a cell location.
[0276] FIG. 17O1 Illustrates a top view of another mapping pattern
17168 for a tubular refractive optic which is divided into sections
17170 and respective regions of focus 17176, according to an
embodiment of the present invention. Optional cells 17172 are shown
to illustrate one possible location of cells with respect to the
regions of focus 17176. Pattern 17168 differs from the pattern
17156 of FIGS. 17N1 and 17N2 in that ray directions and therefore
regions of focus are shifted in a direction transverse to the
longitudinal axis of the tube so that cells may lie in a line
rather than being offset from the centerline as in FIGS. 17N1 and
17N2.
[0277] FIG. 17O2 illustrates an isometric view of the mapping
pattern 17168 of FIG. 17O1 according to an embodiment of the
present invention. Spatial volumes 17174 enclose light rays
refracted through respective sections 17170 and are shown to
illustrate how light arriving at the different sections 17170
travels through different spatial volumes 17174 and is therefore
mapped to different respective regions of focus 17176. Locations
17178 represent the areas of highest light intensity within regions
of focus 17176 and may represent a cell location.
[0278] FIG. 17P1 Illustrates a top view of another mapping pattern
17180 for a tubular refractive optic which is divided into sections
17182 and respective regions of focus 17188 according to an
embodiment of the present invention. Cells 17184 are shown to
illustrate one possible location of cells with respect to the
regions of focus 17188. Pattern 17180 differs from the pattern
17168 of FIGS. 17O1 and 17O2 in that ray directions and therefore
regions of focus are shifted alternately in a direction parallel to
the axis of the tube, which allows sections of the optic pattern
17182 to be aligned pairs such that there may be an integer number
of sections on a tube and no sections are cut in half while at the
same time cells 17184 may still be arranged in a line or other
widely spaced pattern to maximize heat transfer effectiveness. This
arrangement may offer benefits in heat transfer and cell cooling,
and may create freedom in cell placement and spacing useful in
receiver design and optimization of an integrated system.
[0279] FIG. 17P2 illustrates an isometric view of the mapping
pattern 17180 of FIG. 17P1 according to an embodiment of the
present invention. Spatial volumes 17186 enclose light rays
refracted through respective sections 17182, and are shown to
illustrate how light arriving at the different sections 17182
travels through different spatial volumes 17186 and is therefore
mapped to different respective regions of focus 17188. Locations
17190 represent the areas of highest light intensity within regions
of focus 17188 and may represent a cell location.
[0280] FIG. 17Q1 Illustrates a top view of another mapping pattern
17192 for a tubular refractive optic which is divided into sections
17196 and respective regions of focus 17200 according to an
embodiment of the present invention. Optional cells 17194 are shown
to illustrate one possible location of cells with respect to the
regions of focus 17200. Regions of focus 17200 are spaced more
widely in this version compared with FIGS. 17P1 and 17P2, which may
allow for better heat transfer or larger heat sink elements.
[0281] FIG. 17Q2 illustrates an isometric view of the mapping
pattern 17192 of FIG. 17Q1 according to an embodiment of the
present invention. Spatial volumes 17198 enclose light rays
refracted through respective sections 17196, and are shown to
illustrate how light arriving at the different sections 17196
travels through different spatial volumes 17198 and is therefore
mapped to different respective regions of focus 17200. Locations
17202 represent the areas of highest light intensity within regions
of focus 17200 and may represent a cell location.
[0282] FIGS. 17R1-17R8C illustrate a refractive 2D concentrating
primary optic film for use in a tubular shape with tiles of
axis-symmetric grooves similar to that of FIGS. 17K1-17K2. In FIGS.
17R1-17R8C, selected tiles are shown in greater detail and light
ray trace results and resulting focal spot shapes and irradiance
maps are shown in order to observe and discuss the quality of focus
that can be attained with axisymmetric approximations to the ideal
continuous grooves. The shapes and locations of tiles shown are for
illustration of effects of tile shape and size on spot quality and
not necessarily the ideal tile configuration.
[0283] FIG. 17R1 shows a top view of an overall refractive film
prism pattern 17216 in a flattened configuration. Overall pattern
17216 is divided into symmetrical quadrants 17204 in this
embodiment. In other embodiments, pattern 17216 may not have
symmetric quadrants. The symmetrical embodiment is discussed below
for ease of explanation. However it is to be noted that the
description of the symmetrical pattern embodiment below is equally
applicable to non-symmetric patterns. In FIG. 17R1 five specific
tiles are shown for comparison. Tile 17206 is a large tile near the
center of the repeating pattern 17216 and may generally have the
widest prisms. Prism grooves within each tile are drawn selectively
(not all are shown) for clarity. Tile 17208 is a large tile further
from the axis of the tube (not shown, but lies vertically at the
center of overall pattern 17216). Prism widths may be smaller in
this tile if average height is to be held constant. This is because
when in a tubular shape, the slope of the general tube surface
would be higher in the region where tile 17208 is located, so the
"stair steps" would be narrower if the step height is held fixed.
Tile 17210 is a large tile at the extreme corner of pattern 17216.
Prism spacing may generally be the narrowest here. Tile 17212 is a
medium sized tile located at the center of the extreme lateral edge
of pattern 17216. Tile 17214 is a small tile near the extreme
lateral edge of pattern 17216 and also next to a lateral axis of
symmetry of pattern 17216. In this embodiment the large tiles are
shown as approximately 0.95 inches square.
[0284] FIG. 17R2 illustrates a refractive film prism pattern 17218
that represents the prism pattern 17216 of FIG. 17R1 when it is
wrapped, formed or inflated to become a part of a cylinder or tube.
Tiles 17220, 17222, 17224, 17226 and 17228 are correspondingly the
cylindrically wrapped versions of tiles 17206, 17208, 17210, 17212
and 17214 respectively. Refracted light rays 17230 are shown
passing through the center of the tiles to illustrate the change in
direction that is created when sunlight passes through the tiles
from above. Refracted rays 17230 converge at a common focal point
17232.
[0285] FIG. 17R3A-C illustrate the illumination profile that would
be created by, e.g., tile 17220 of FIG. 17R2, on a horizontal plane
at the general location of focal point 17232. FIG. 17R3A shows
isolines of the illumination profile (the spot of light) from tile
17220. Note that while tile 17220 is 0.95 inches square, the spot
of light is only roughly 0.2 inches by 0.05 inches in size. This
happens because the prisms within tile 17220, when formed in the
correct geometry, concentrate the incoming (nearly parallel) light
in two dimensions to create a smaller spot near focal point
17232.
[0286] FIG. 17R3B shows a cross section graph of illumination (in
"suns" where 1 "sun" represents 1000 w/m.sup.2 of irradiation)
along the y axis of the spot of FIG. 17R3A. FIG. 17R3C shows a
cross section graph of illumination along the x axis of the spot of
FIG. 17R3A.
[0287] FIGS. 17R4A-C illustrate the illumination profile that would
be created, e.g., by tile 17222 of FIG. 17R2, on a horizontal plane
at the general location of focal point 17232. FIG. 17R4A shows
isolines of the illumination profile (the spot of light) from tile
17222. Note that the spot from tile 17222 is larger than that from
tile 17220 even though the tiles themselves are the same size.
There are several reasons for this. One reason is that light
arriving at focal point 17232 from tile 17222 on a horizontal plane
arrives at a larger incident angle than light from tile 17220, so
it gets spread out. A second reason is that tiles with axisymmetric
prisms farther away from the center of pattern 17218 would need to
be smaller to achieve the same approximation accuracy of the
refracted light (compared to the ideal continuous prisms), so tiles
of the same size that are farther from the center will create less
accurate (larger) regions of concentration in general.
[0288] FIG. 17R4B shows a cross section graph of illumination in
"suns" along the y axis of the spot of FIG. 17R4A. FIG. 17R4C shows
a cross section graph of illumination along the x axis of the spot
of FIG. 17R4A. FIGS. 17R5A-C illustrate the illumination profile
that would be created, e.g., by tile 17224 of FIG. 17R2, on a
horizontal plane at the general location of focal point 17232. FIG.
17R5A shows isolines of the illumination profile (the spot of
light) from tile 17224. Note that the illumination profile from
tile 17224 is larger still than either that from tile 17220 or tile
17222, for the same reasons discussed above. FIG. 17R5B shows a
cross section graph of illumination in "suns" along the y axis of
the spot of FIG. 17R5A. FIG. 17R5C shows a cross section graph of
illumination along the x axis of the spot of FIG. 17R5A.
[0289] FIGS. 17R6A-C illustrate the illumination profile that would
be created, e.g., by tile 17226 of FIG. 17R2, on a horizontal plane
at the general location of focal point 17232. FIG. 17R6A shows
isolines of the illumination profile (the spot of light) from tile
17226. Note that tile 17226 is smaller than tile 17224, and has a
smaller spot than tile 17224 even though it is in a similar
location. This shows how smaller tiles can be used to reduce the
"focus errors" out toward the extreme edges of the overall pattern.
FIG. 17R6B shows a cross section graph of illumination (in "suns"
where 1 "sun" represents 1000 w/m.sup.2 of irradiation) along the y
axis of the spot of FIG. 17R6A. FIG. 17R6C shows a cross section
graph of illumination along the x axis of the spot of FIG.
17R6A.
[0290] FIGS. 17R7A-C illustrate the illumination profile that would
be created, e.g., by tile 17228 of FIG. 17R2, on a horizontal plane
at the general location of focal point 17232. FIG. 17R7A shows
isolines of the illumination profile (the spot of light) from tile
17226. Note that tile 17228 is even smaller than tile 17226, and
has a resulting spot that is somewhat more "well behaved", but is
still relatively spread out because of the location of the tile at
the extreme edge of the pattern. The spread effects related to the
error of the axisymmetric prism shape approximation (compared to
the ideal amorphous continuous groove shapes) can be made
arbitrarily small by using arbitrarily small tiles, but the
spreading of the light due to the extreme incident angle of the
light hitting a horizontal target from the locations near the
extreme edge of the optic pattern do not diminish with decreasing
tile size. These spread errors would be present even in similar
regions in an optic with the ideal continuous grooves.
[0291] FIG. 17R7B shows a cross section graph of illumination in
"suns" along the y axis of the spot of FIG. 17R7A. FIG. 17R7C shows
a cross section graph of illumination along the x axis of the spot
of FIG. 17R7A.
[0292] FIGS. 17R8A-C show the net illumination profile ("spot")
created when illumination profiles due to tiles 17220, 17222,
17224, 17226 and 17228 are combined as they would be in a generally
useful system for high concentration. As noted above, only a
limited number of tiles are illustrated for ease of explanation. In
practice, the entirety of the area of pattern 17218 may be filled
with contiguous tiles.
[0293] FIG. 17R8A shows isolines of the net illumination profile
(the spot of light) from tiles 17220, 17222, 17224, 17226, and
17228. FIG. 17R8B shows a cross section graph of illumination in
"suns" along the y axis of the spot of FIG. 17R8A. FIG. 17R8C shows
a cross section graph of illumination along the x axis of the spot
of FIG. 17R8A.
[0294] FIGS. 17T1A-17T1C show an illumination profile ("spot")
created by a tubular refractive optic similar to that of FIG. 17R
at a similar target location, but with idealized continuous grooves
for comparison. The illumination values have been normalized to be
comparable to the same primary optic area as in FIG. 17R.
[0295] FIG. 17T1A shows isolines of the net illumination profile
from a refractive tubular optic with idealized grooves (i.e.
grooves that are not constrained to be circular arcs and are not
divided into patches) with illumination normalized to the optic of
FIG. 17R for ease of comparison. Notice that the scale on the graph
is smaller than that of FIG. 17R8A and that the overall spot size
is smaller and the peak irradiance value is higher in the center
(see also FIGS. 17T1A,B). This is because the idealized grooves
create less angular error or spread than the axisymmetric
approximation grooves of the large tiles of FIG. 17R. However, if
the tiles of axisymmetric grooves similar to those of FIG. 17R are
made small enough, the spot of FIGS. 17R8A-C could be made to be
substantially similar to the spot of FIG. 17T1A-C.
[0296] FIG. 17T1B shows a cross section graph of illumination in
"suns" along the y axis of the spot of FIG. 17T1A. FIG. 17T1C shows
a cross section graph of illumination along the x axis of the spot
of FIG. 17T1A.
[0297] FIGS. 17T2A-C show the net illumination profile created when
all four quadrants of a completely populated tubular refractive
primary optic prism pattern are combined as they might be on a
commercial CPV system described herein. These illumination profiles
are generated using a prism pattern that is the same total size as
in FIGS. 17R and 17T, but with a completely populated surface--i.e.
all the available aperture area is covered by the continuous curved
ideal prisms. These figures illustrate the very high concentration
possible with such a design. In a commercial system, secondary
optics may be used to further homogenize or otherwise reshape the
illumination profile and eliminate hot spots and/or create
improvements to spectral distribution.
[0298] FIG. 17T2A shows isolines of an illumination profile created
when all four quadrants of a complete tubular refractive primary
optic prism pattern are combined. FIG. 17T2B shows a cross section
graph of illumination in "suns" along the y axis of the spot of
FIG. 17T2A. FIG. 17T2C shows a cross section graph of illumination
along the x axis of the spot of FIG. 17T2A.
[0299] FIG. 18 shows a film based solar concentration trough system
with stationary trough array and moveable receivers according to an
embodiment of the present invention. FIG. 18A shows a side view of
the embodiment of the system of FIG. 18.
[0300] In this embodiment, a reflective film 1802 and enclosing
material 1804 define a chamber 1806. A negative pressure
differential is maintained between chamber 1806 and the surrounding
environment. A frame 1808 keeps reflective film 1802 separated from
enclosing material 1804. Optional base frame members or legs 1810
may be used to anchor, locate or support the system. Receivers 1812
are supported above reflective film 1802 by support arms 1814.
Support arms 1814 rotate to allow receivers 1812 to move. One or
more actuation members 1816 are linked to receivers 1812 and cause
receivers 1812 and support arms 1814 to move when a force is
applied. Other embodiments are possible that use different support
for the receivers.
[0301] In this particular embodiment, receivers 1812 follow a
rotational path but have a fixed orientation. In other words, their
active face stays parallel to actuation arms 1816. In alternative
embodiments, receivers 1812 may move in a different way. Their
orientation may change; they may move in a linear fashion, and/or
they may move with according to some other predetermined motion.
Film 1802 may be made to change shape by varying the pressure or
vacuum in chamber 1806. Aside from this (slight) shape change, the
troughs formed by film 1802 may otherwise remain mainly
stationary.
[0302] As the sun moves over the course of time, rays of sunlight
are reflected off of film 1802 at different angles. With a
correctly chosen pressure or vacuum in chamber 1806 and the correct
movement of receivers 1812, a region of concentrated reflected
sunlight from each trough made be made to coincide with its
respective receiver for most times of day or year. A system
employing moveable receivers may be simpler and less expensive than
a system where the troughs move to follow the sun. Generally, this
is because the receivers are more compact, offer less wind
resistance, and are geometrically easier to be coupled together for
actuation.
[0303] The embodiment of FIG. 18A employs a negative pressure
differential. However, other embodiments having a moveable receiver
may employ a positive pressure differential and/or a clear front
film. Still other embodiments may not require a pressure
differential or enclosed chambers. A reflective film similar to
1802 may be stretched over a frame similar to 1808 to create
deterministic reflective shapes.
[0304] FIG. 19 shows an embodiment of a solar concentration trough
system 1902 that uses inflation air and a membrane to eliminate the
need for a rigid frame according to an embodiment of the present
invention. A clear front film 1904 is connected to a reflective
film 1906 to form a first enclosed chamber 1908. A rear enclosing
material 1910 is connected to film 1904 or film 1906 to form a
second enclosed chamber 1912. Material 1910 may be a ruggedized
fabric or film or it may be another material.
[0305] Reflective film 1906 takes on an arc shaped cross section
under the influence of a pressure differential between chamber 1908
and chamber 1912. Film 1904, film 1906 and enclosing material 1910,
may all be used in tension without the need for additional frame
members if both chamber 1908 and chamber 1912 include a fluid with
a greater pressure than the pressure of the surrounding
environment. This condition can be achieved, with the desired arc
shape of film 1906 formed, with different positive pressures in
chambers 1908 and 1912.
[0306] A receiver 1914 can be positioned inside chamber 1908 or it
may be positioned outside of chamber 1908. Receiver 1914 may be
attached to front film 1904 if desired. Film 1904 may have
cutout(s) or may be in multiple parts to allow an inner surface of
receiver 1914 to face reflective film 1906 while an outer surface
of receiver 1914 is in direct communication with the surrounding
environment (not shown). Solar power costs may be reduced by
creating such a system that utilized primarily inexpensive films
and fabrics rather than expensive rigid frames. System 1902 may
articulate to track the sun by rolling directly on the ground, or
it may be articulated in some other way.
[0307] FIG. 19A shows an isometric view of the system of FIG. 19
with additional components. FIG. 19A shows multiple systems 1902
configured to track the sun together. Systems 1902 are supported by
support frames 1916. Systems 1902 may rotate about one or more
pivot joints 1918, and may be moved to track the sun by an
actuating apparatus such as a motor, hydraulic system, cables,
linkage or other actuator (not shown). Pivot joints 1918 may define
a tracking axis 1920 about which systems may be rotated to track
the sun.
[0308] Some embodiments of the present invention may employ shorter
or longer trough segments. Long trough segments may be desirable to
minimize end effects. Other embodiments may employ multiple troughs
per post. Troughs may be disposed next to one another, above/below
one another, or in a diagonal arrangement. Troughs may be linked to
rotate together about their respective tracking axes.
[0309] In still other embodiments, the films and membrane forming
the trough segment may be much longer than the effective length of
the trough segment, with extra deflated trough stored on a roll
system. If the portion of the films in use becomes degraded or
damaged, the roll system could advance the films until a fresh
section is ready for use.
[0310] FIGS. 20A and 20B show a solar concentration system 2002
that uses a film without inflation pressure according to an
embodiment of the present invention. FIG. 20A shows an isometric
view and FIG. 20B shows a cross section view from the front.
Receivers are not shown for clarity. Film 2004 is stretched over
one or more guide bar(s) 2006. Guide bars 2006 are formed with a
shape chosen to create a desired pattern of reflected light. FIG.
20B shows an embodiment in which film 2004 passes alternately over
and under bars 2006. This creates transverse force between film
2004 and bars 2006 sufficient to keep film 2004 pressed against
bars 2006 so that it takes the shape of bars 2004.
[0311] FIG. 20C shows a solar collection system similar to the
system 2002, but with the addition of a roll-to-roll film
replacement system 2008. Roll fill replacement system 2008 turns in
order to advance or replace the film, for example when the
currently exposed film loses some of its reflectivity or other
functional properties. Roll film replacement system 2008 may also
provide the desired tension on the film. Roll system 2008 may
operate manually or automatically or it may be indexed by or
otherwise linked to a sun tracking system (not shown).
[0312] In another embodiment, film 2004 on roll to roll system 2008
may be stretched over a rigid trough shape (not shown) rather than
guide bars. The film may take the shape of the rigid trough because
of film tension, and/or a vacuum may be applied through holes in
the rigid trough shape to temporarily secure the film to the rigid
trough. The vacuum may be released to allow the film to be advanced
or replaced.
[0313] FIG. 21 shows an alternative embodiment of a solar
concentration trough system 2102 that is similar to the system of
FIGS. 19 and 19A. System 2102 uses inflation air and a membrane to
eliminate the need for a rigid frame. A clear front film 2104 is
connected to a reflective film 2106 to form a first enclosed
chamber 2108. A rear enclosing material 2110 is connected to film
2104 or film 2106 to form a second enclosed chamber 2112. Material
2110 may be a ruggedized fabric or film or it may be another
material.
[0314] Reflective film 2106 assumes an arc shaped cross-section
under the influence of a pressure differential arising between
chamber 2108 and chamber 2112. Film 2104, film 2106, and enclosing
material 2110, may all be used in tension without the need for
additional frame members if both chamber 2108 and chamber 2112
include a fluid with a greater pressure than a pressure of the
surrounding environment. This condition can be achieved (and the
desired arc shape of film 2106 formed), utilizing different
positive pressures in chambers 2108 and 2112.
[0315] A receiver 2114 can be positioned completely inside chamber
2108. Alternatively, the receiver may be positioned completely or
partially outside of chamber 2108.
[0316] Receiver 2114 may be attached to front film 2104 if desired.
Film 2104 may have cutout(s) or may be in multiple parts to allow
an inner surface of receiver 2114 to face reflective film 2106,
while an outer surface of receiver 2114 is in direct communication
with the surrounding environment (not shown). Frame members 2116
support system 2102 via ring 2118 and rollers 2120. The films and
chambers can be rotated to track the sun when ring 2118 rolls on
rollers 2120. Rollers 2120 may also be driven by an actuator to
create tracking motion for the system. Chambers 2108 and 2112 may
be sealed at the ends to allow pressure differentials to be
maintained. Seals or additional enclosing members are omitted here
for clarity. Additional films or material covering the ends could
be used for this purpose. Also, multiple systems such as that shown
in FIG. 21 may be joined end-to-end and may be actuated together.
In this instance, enclosing material between each of the
intermediate systems may not be needed.
[0317] FIG. 9A shows an isometric view of another embodiment of a
film-based solar collector trough structure 902. FIG. 9B shows a
top view of the solar collector trough of FIG. 9A. FIG. 9C shows a
side view of the solar collector trough of FIG. 9A. FIG. 9D shows a
close-up detail view of a region circled in FIG. 9B. FIG. 9E shows
a close-up detail view of a region circled in FIG. 9C.
[0318] Trough array 902 has multiple trough segments 904.
Communication of inflation fluid between each segment 904 may be
accomplished via transverse chambers 906 on each end, which connect
to each trough segment 904. This geometry may reduce the length of
the deformed or unusable portion of inflated trough segments. Seams
908 divide the trough segments. Seams may be made by joining the
front and rear films together. This may be accomplished via a
number of possible methods, including but not limited to heat
sealing, RF welding, sonic welding, adhesives, mechanical
attachment, and/or other methods.
[0319] FIG. 15A shows a simplified isometric view of another
embodiment 1502 of a film-based solar collector trough structure.
FIG. 15B shows a simplified end view of the structure of FIG.
15A.
[0320] This system has a reflective film 1504 which faces outward.
Chambers 1506 behind reflective film 1504 may contain a negative
pressure differential (for example a partial vacuum) to the
surrounding environment. A containing material 1508 prevents fluid
leakage along the back and sides of chambers 1506. Reflective film
1504 is held apart from containing material 1508 by a second set of
chambers 1510, which contain a positive pressure differential to
the surrounding environment. The pressure in chambers 1510 creates
outward force resisting inward forces created by the negative
pressure differential in chambers 1506.
[0321] The embodiment of a system shown in FIGS. 15A-B can
potentially avoid the need for, or reduce the number and/or mass
of, rigid frame members. This could in turn reduce the mass and/or
cost of the system as a whole.
[0322] FIGS. 22A-22D illustrate different secondary optic
possibilities for use with a receiver similar to that of FIG. 5
according to an embodiment of the present invention.
[0323] FIG. 22A illustrates a receiver system with straight
reflective secondary optic components according to an embodiment of
the present invention. A heat sink 2202 is attached to a solar cell
2204. Reflective elements 2206 are positioned on either side of
cell 2204. An incoming ray of sunlight 2208 is shown striking cell
2204 directly. Another incoming ray 2210 is shown striking
reflective element 2206 and creating reflected ray 2212. Reflected
ray 2212 hits cell 2204. Thus, reflective element 2206 helps
capture ray 2210 and direct it towards cell 2204. Without
reflective element 2206, ray 2210 would have missed cell 2204.
Reflective elements 2206 may be silvered glass mirrors, coated
aluminum mirrors, other polished metal, metalized film, multi-layer
reflective polymers or other reflective material.
[0324] FIG. 22B illustrates a receiver system with curved
reflective secondary optic components according to another
embodiment of the present invention. A heat sink 2202 is attached
to a solar cell 2204. Curved reflective elements 2216 are
positioned on either side of cell 2204. An incoming ray of sunlight
2208 is shown striking cell 2204 directly. Other incoming rays 2210
are shown striking reflective element 2216 and creating reflected
rays 2214. Reflected rays 2214 hit cell 2204. Without reflective
element 2216 rays 2210 would have missed cell 2204. Reflective
elements 2216 help capture these rays 2210 to increase the amount
of usable sunlight thus increasing the efficiency of the receiver
system. In some embodiments, reflective elements 2216 may have an
arc shape or other shape. In some embodiments, reflective elements
2216 may be shaped so that reflected rays 2214 generated by typical
incoming rays 2210 will be distributed over the width of cell 2204.
In other embodiments, reflective elements 2216 may be designed to
create another predetermined distribution of light on cell 2204.
Reflective elements 2216 may include silvered glass mirrors, coated
aluminum mirrors, other polished metal, metalized film, multi-layer
reflective polymers or other reflective material.
[0325] FIG. 22C illustrates a receiver system with multi-segmented
shaped reflective secondary optic components according to yet
another embodiment of the present invention. A heat sink 2202 is
attached to a solar cell 2204. Multi-segmented shaped reflective
elements 2218 are positioned on either side of cell 2204. Each
reflective element 2218 can have multiple shape segments 2220. An
incoming ray of sunlight 2208 is shown striking cell 2204 directly.
Other incoming rays 2210 are shown striking different positions on
one segment 2220 of an reflective element 2218 and creating
reflected rays 2222. Reflected rays 2222 hit cell 2204. Without
reflective element 2218, rays 2210 would have missed cell 2204.
Reflective elements 2218 may be shaped so that reflected rays 2222
reaching each segment 2220 will be distributed over the width of
cell 2204, so that hotspots may be avoided. In some embodiments,
segments 2220 may be designed to create another predetermined
distribution of light on cell 2204. Reflective elements 2218 may be
silvered glass mirrors, coated aluminum mirrors, other polished
metal, metalized film, multi-layer reflective polymers or other
reflective material.
[0326] FIG. 22D illustrates a receiver system with a transparent
refractive secondary optic component according to still another
embodiment of the present invention. A heat sink 2202 is attached
to a solar cell 2204. A refractive secondary optic 2224 is
positioned so that it is in optical communication with cell 2204.
Incoming rays 2226 are refracted as they pass into optic 2224 to
create refracted rays 2228 which may have a different direction
than rays 2226. Optic 2224 may be designed to redirect rays that
would have normally missed cell 2204 so that they strike cell 2204.
FIG. 22D illustrates one embodiment of a secondary optic with a
particular shape. One skilled in the art will realize that many
other optic shapes are possible to create a wide variety of light
distributions on the cell or to direct light away from in-active
areas of the cell.
[0327] Certain embodiments of the present invention provide tubular
or cylindrical solar concentrators. These tubular/cylindrical solar
concentrators may be either 1-dimensional or 2-dimensional.
[0328] There are existing techniques for creating a line focus by
refraction of light through a cylindrical surface. However, there
are fundamental limits to the concentration factor that can be
achieved with one-axis concentration while maintaining high net
optical efficiency. There are economic benefits and other
advantages of operating at higher concentrations, which are only
possible by concentrating in two axes. A line focus is created by
concentrating the light in the transverse axis of the cylindrical
primary; it is possible to also concentrate in the longitudinal
axis of the tube. By adding this second axis, a string or array of
point-like-focuses can be generated that can achieve very high
concentrations.
[0329] 2D concentration for Concentrating Photovoltaic (CPV)
systems with refractive elements may be typically accomplished
using flat Fresnel lenses or aspherical solid lenses. However with
the embodiments described herein it is also possible to concentrate
light in 2 axes using an optic with an overall cylindrical shape,
such as a tube. Some embodiments of the present invention provide
inflated thin film tubes, but the methods and geometries described
herein are equally applicable with non-inflated and/or thicker
cylindrical optics.
[0330] 1D or line focus on a cylindrical Fresnel optic is
accomplished with prisms parallel to the axis of the cylinder which
are usually continuous. The normals to the prism surfaces are all
normal to the cylinder axis (though they will not in general
intersect with the Cylinder axis). 2D concentration usually
involves the primary optic creating a point focus. When the primary
optic for 2D concentration is a refractive element, it may be a
flat Fresnel lens with concentric circular prisms or grooves.
[0331] Accomplishing 2D concentration with a tubular primary optic
is non-trivial if losses are to be kept to a minimum. One of the
requirements to achieve 2D concentration is that the surface
normals of some locations on the prisms be not normal to the axis
of the cylinder. To achieve a high level of concentration in the
additional axis, e.g., the direction along the cylinder, requires
that the surface normals of the prisms change in a prescribed way
across the cylinder's active optical surface(s) so that light is
refracted through different locations on the cylinder to arrive at
a similar point in space where concentration is desired. There are
several possible approaches to do this.
[0332] One approach is to use discrete prisms with planar surfaces
as shown in FIG. 17G. In this approach, each prism has normal
vectors chosen such that the light passing through each prism
refracts in a different, specific direction so that it reaches a
common target or region. A challenge with this approach is that
discrete prisms have end faces that are almost certain to either
block or misdirect light that passes through them, unlike an optic
with prism faces formed as continuous grooves which by definition
have no end faces.
[0333] One approach uses non circular prisms designed such that the
angle of the active face of the prism (and potentially draft angle)
and the prism width and depth varies along the prism. This approach
is shown in FIGS. 17J1 and 17J2. In some embodiments, the angle of
the draft (non-active) face may also vary along the prism. In some
embodiments, prism angles vary from one groove or prism to the
next, but depth may be constant. The overall trend of the groove
paths may resemble a thumbprint when the cylinder is flattened out.
The specific curved paths that the grooves must follow depend on
the location and nature of the region of concentration and whether
it is to be a point focus, a region of focus or a specific shaped
region of concentrated light. If a point focus is desired, the
slopes of the prism faces that lie on the cylinder are
deterministic and can be represented by a set of equations or
closed-form solution. Other distributions and shapes of
concentrated light may be represented as variations of this
solution or specific deviations from this solution. In some
embodiments, the complex optimization problem of creating perfectly
matched primary optic prism design plus secondary optic location
and design to minimize optical losses for given tracking errors and
other design tolerances may be accomplished through automated
iterative optimization routines. In this embodiment, most of the
prism draft faces generally lie in optical "shadow" regions,
meaning that light refracting through the smooth front surface of
the film almost always reaches the active prism faces on the back
(inside surface) of the film and almost never passes through the
draft faces of the prisms. This approach minimizes losses from
misdirection of light. The degree to which this is true depends
upon the specific draft angles required in the embossing process.
No light passes through any draft faces if the draft angle is zero
(i.e. draft faces are perpendicular to the nominal film surface
when the film is flat) if the refractive index of the cylinder
medium is greater than that of the medium that the incident rays
were travelling in (e.g. air) This is because rays travelling in
air that are refracted by a material with higher refractive index
will always have a refraction angle that is less than the angle
between the refracting surface and the plane normal to the initial
ray direction. The draft angles for typical flat Fresnel lenses
must generally be non-zero due to manufacturing concerns and result
in real optical losses for almost all cases of practical flat
Fresnel designs. In contrast, cylindrical Fresnel optics made from
films may have lower losses. This is because draft angles that are
positive when the film is flat can become zero or even negative
when wrapped to form a cylinder, thereby avoiding some or all of
the losses that would have been incurred in the flat case. In this
way, cylindrical Fresnel optics can generally be made to have lower
losses than flat Fresnel optics. For the case of 2D concentration,
prism paths and optic and draft faces must be designed according to
certain constraints to maintain minimal losses.
[0334] In some embodiments, the Fresnel groove paths may be made as
continuous circular arcs wrapped onto the cylindrical surface. If
the prism angles were kept constant on these circular arc grooves,
then the refracted light would not form a tight region of focus and
the results would generally not be useful for HCPV. To correct this
problem, the prism angle would need to change along the groove
path. Even with a changing prism angle along the path, for any
point on the primary there is a finite band of angles through which
the refracted light could be directed. If we assume that the
cylindrical optic is correctly pointed at the sun, the direction
any ray refracted through the smooth front surface of the optic
lies in a plane perpendicular to the cylinder axis (+/- a portion
of sun's angular subtense). Then after refraction through a prism
on the inside of the optic surface whose normal must nearly be
perpendicular to the groove path, the resulting ray's direction
will have been changed in two angular directions relative to the
orientation of the principal axes, but the range of resulting
angles will be limited to a narrow band governed by the plane of
the ray directions after the front surface refraction and the plane
of possible prism surface normals. This narrow band of possible
resultant ray directions will be different for every point on the
primary and importantly, each band for each point on the primary
will not generally intersect at a consistent location in space.
Continuous circular arc prism paths with constant prism angles
would therefore present a significant limitation to the degree of
concentration, shape of the region of concentration and
distribution of light that could be achieved within the region of
concentration. Put another way, another degree of freedom in the
design may be needed if the optical design is to achieve a common
spatial point of intersection of all the rays (or even a narrow
region within which a majority of the resultant rays pass). One way
to attain the needed additional degree of freedom is to use paths
that are not circular arcs. Another way is to use non-continuous
prisms, for example prism paths could be arcs that are kept very
short so as to approximate the ideal, non-circular arc path to
within a chosen error limit. In order to employ the latter approach
effectively, it is helpful to first understand both a method for
finding the ideal non-circular arc prism paths and also the
additional design variables.
[0335] Prism depth and width may be chosen based on several factors
and constraints. The width and depth of the prisms will often be
linked because when the depth and angle are specified, the width is
determined. The optical performance is determined in part by the
prism angle (prism width and spacing if too small will generally
have a detrimental effect on focus due to diffraction phenomena).
Prism depths may be constrained by manufacturing and cost concerns.
For example, in the case of embossed film refractive optics,
economics are optimized by using small prism depths because less
material is required, but some embossing processes may require a
minimum feature size. Also losses due to tool radii and diffractive
effects may impose limits on how small the prism spacing and widths
can be and therefore also on the depths.
[0336] In some embodiments, it may be possible to vary the prism
depth from prism to prism. Alternatively, for manufacturing reasons
or other reasons, it may be desirable to design the pattern of
prisms so that the groove valleys are all at the same depth.
Alternatively, prism peaks may be designed to be all at the same
height. Alternatively, the mid-plane of each prism may be designed
to be at a constant depth. The latter can enable optimal embossing
performance by providing equal volumes of material displaced from
the troughs on one side of the mid plane and filled into the peaks
on the other side of the mid plane. In this way resin displaced
from valleys exactly equals that which flows to form the peaks.
[0337] To design continuous facet grooves on a cylindrical surface
that redirect incident light into a point focus, the following
methodology may be used. For a point on the cylinder, it is
straightforward to calculate the direction of the ray refracted
through the exterior surface. From there it is possible to
calculate the interior surface normal that will refract that ray to
its desired target. This gives every point on the cylinder a unique
surface normal. The goal of the Fresnel design is to create a
pattern of grooves that match these ideal surface normals. There
are two planes of symmetry. One is orthogonal to the cylinder axis
and intersects the point focus. The second splits the cylinder into
two equal halves. These two planes split the cylinder surface into
quadrants; only one of them needs to be solved in order to obtain
the others from the symmetry conditions.
[0338] At sufficiently high resolution, an irregular 3D mesh of
triangular elements can be used to accurately describe the Fresnel
pattern. Four triangular elements are grouped into a segment; a
chain of segments describes a groove, and the set of grooves
describes the Fresnel. Each segment may include up to four
triangular elements, with two vertices along the "peak." Each
trapezoidal element may be on a plane and has two vertices along
the "peak" and two along the "valley" of its groove. A groove is
described by a series of trapezoidal elements joined
end-to-end.
[0339] Some optical systems described herein include a large
primary optic with specific discrete zones that concentrate light
to a particular point or small area. Light from each zone of the
primary is further concentrated by secondary optics that in turn
illuminates the PV cells. Variations for the optical systems may
include: (1) A single primary zone, a single secondary optic, and a
single cell, (2) Multiple primary zones, multiple secondary optics,
each coupled to individual cells in a cell array (3) Multiple
primary zones, and a compound secondary optic that overlays the
power from the individual primary zones onto the same cell. In some
embodiments, secondary optics can be: internally-reflecting
reflective optics, truncated cone or pyramid, or with curved
surfaces. In other embodiments, the secondary optics can be
pyramidal, conical, or compound parabolic concentrator (CPC) and
made of glass or plastic. Secondary optics may have planar entrance
faces, or have spherical or aspherical curved entrance faces. In
some embodiments, they may be Kohler-type concentrators for optimal
irradiance uniformity from multiple primary zones illuminating a
single cell.
[0340] The above-mentioned design maximizes system efficiency and
therefore power output by minimizing losses in and between
components. The design also avoids excessive non-uniformity of cell
irradiance.
[0341] In some embodiments, a cylindrical primary concentrator of a
given diameter with a specified spacing between the primary and the
cell plane can be created. Optionally, the lateral location of the
cells of a given size, and a specified length/area for the primary
zone(s) such that the desired concentration factor and hence total
power on each cell can be achieved. Then iteratively, the
parameters that define the different components and their geometry
can be adjusted to achieve maximum power output. The rays that are
traced fully sample the entrance aperture of the relevant
concentrator primary zone or zones, and include the extremes in
incident wavelength that the cell can convert into usable power, as
well as the extremes in incident angle expected given the finite
angular subtense of the sun and the expected range in angular
errors due to combined tracking errors and fabrication and assembly
alignment tolerances.
[0342] Once the optical design is completed, an embossing pattern
that is a close approximation to the design surface and can be
fabricated onto the primary concentrator using available
techniques. Material usage considerations and, in some instances,
embossing techniques employed might tend toward smaller embossing
depths, perhaps as small as several microns. In certain
embodiments, depths may be as small as 0.1 to 10 microns. However,
smaller depths correlate with tighter groove spacing, which
increases the angular extent of diffraction from the periodic
structure of the embossed film, to the extent that the
as-fabricated embossed film disperses light into the different
diffraction orders. Consequently, depending on the fabrication
quality, the embossing depth may need to be held in a range such
that the minimum groove spacing is at least some tens of
microns.
[0343] Many advantages may be realized using the embodiments
described herein. One of the great benefits of cylindrical
refractive optics is that they can be made in long lengths, which
minimizes various significant costs related to end effects. This
also takes advantage of lowest-cost continuous manufacturing
processes (which tend to produce linear features) in several areas
of the design to create additional cost savings. Examples of
manufacturing processes include continuous embossing, extrusion,
roll forming, web based operations, Heat sealing, impulse welding,
sonic welding, RF welding, adhesive application and other film
converting applications.
[0344] If 2D concentration is accomplished with cylindrical
refractive optics of long length, it may be desirable to have the
primary optic divided into regions, each of which directs light to
a separate focus region, secondary optic and/or solar cell. For
example, if the desired optical concentration at the cell is
1000.times. nominal sunlight irradiance, and each cell is 1
cm.sup.2, and the whole cylindrical primary optic has an area of 5
m.sup.2 (=50,000 cm.sup.2), and the net optical efficiency
including primary, secondary, and any other optical losses is 88%,
then the primary would need to be divided into 44 regions, each of
which would direct light to a different cell. In some embodiments,
it would also be possible to arrange prisms in a given region to
direct light to multiple cells or multiple focus regions. In some
embodiments, the mapping may include dividing the primary into
separate areas with division lines or planes perpendicular to the
cylinder axis at equally spaced intervals along the length of the
cylinder. In this scheme, cells may be spaced at even intervals
along the length of the concentrator assembly. Primary divisions
and/or cells may of course also be at un-even intervals if
desired.
[0345] In another embodiment, the primary to cell mapping and
corresponding primary division can be accomplished as follows. In
this embodiment, cells are arranged in pairs at even intervals
along the length of the concentrator assembly. This may be
desirable, for example, if the acceptance angle of cells or
secondary optics is too limited to accommodate the range of ray
angles from the full film breadth (direction transverse to tube
axis) without incurring losses. In this case, the primary is
divided into corresponding pairs of regions along its length and
the light entering each individual primary region is directed
toward the corresponding cell.
[0346] In yet another embodiment, the cells are offset from the
centerline of the receiver and staggered from one side of the
centerline to the other along the length of the concentrator.
Corresponding areas of primary optic are also staggered along the
length of optic. This arrangement may be advantageous because it
preserves the narrower angular range of rays of the paired
arrangement of one of the previously described embodiments, but
puts the cells farther apart which can enable better heat spreading
into heat sink and therefore lower cell temperatures.
[0347] In a particular embodiment, cells are spaced in a similar
staggered fashion as described above, and the lower cell
temperature and small angular range of rays are preserved. In this
embodiment, a primary is divided into pairs of regions where each
pair has a common transverse boundary. Each primary region must be
designed to refract light in a manner such that a region of focus
is created which is offset away from the center of the patch. Each
cell and/or secondary is arranged to be optimally positioned with
respect to the region of focus. The benefit created by this
arrangement is that there is no in-active area of the primary optic
at the ends of each assembly. This is important because a
significant fraction of the cost of any CPV system is driven by the
area of the primary, so best performance is attained with the
highest possible fraction of that area actively able to illuminate
a receiver.
[0348] There are many other possible ways to divide cylindrical
refractive primary optics and to map sections of the primary to
cells or secondary optics. For example, as shown in the figures, a
primary may be divided into sets of 3 regions evenly spaced along
the length of the primary. The primary may also be divided into
groups of regions of 4, 5 or any other number spaced along the
primary. Individual regions of prisms on the primary may be
aligned, or they may be staggered. Regions on the primary may also
be designed to direct light to more than one cell or to distribute
light which may mitigate potential detrimental effects of partial
shading of the primary.
[0349] A prism structure for 2D concentration by prisms on a
cylindrical surface may be approximated by a different pattern in
some cases for ease of manufacturing. FIGS. 17K1 and 17K2 show one
such approximation pattern. The "thumbprint" paths of the grooves
in the ideal case may be continuous and have continuous first and
second derivatives. As such, a given curved path may be
approximated with predictable errors by a circular arc or a
straight line or any other path which is easier to manufacture as
shown in FIGS. 17K1 and 17K2. For example, grooves may be cut into
tooling plates with a lathe, which is most capable of cutting
circular arcs with best surface finish. The ideal case "thumbprint"
non-axisymmetric pattern may be arbitrarily divided into regions of
any size or shape and circular arcs substituted within each region
in the place of the more difficult to manufacture "thumbprint"
curved grooves. The curvature of each arc segment may be chosen to
match the curvature of the "thumbprint" groove it is replacing at
any desired point on the curve being replaced--often the midpoint.
Or the average curvature of the "thumbprint" grooves may be taken
in the patch region and the curvature of the substituted arcs may
vary based on distance from the middle point of the region. The
center point of the substituted arcs may be chosen to minimize the
differences in curvature of the arcs from the original groove paths
over the patch area. Other schemes for choosing arc parameters are
also possible, for example averages of curvature might be weighted,
etc.
[0350] In a similar way, active prism angles for the grooves that
follow the new circular arc path approximations may be chosen such
that they match the prism angle of the ideal case at a specified
point. At other locations along the groove path, the prism angles
and the path itself may not match the ideal design, but will be
close enough to small errors in the refracted light. Angles of each
approximated prism may be chosen to match the angle of the specific
ideal prism they are approximating, or one prism in a patch may be
designed with the others varying from that in a prescribed manner.
Using this approximation technique, it is possible to design
tooling that can be manufactured using typical machine tools in the
optics industry. This tooling may be used to emboss a set of
grooves into the surface of a film which when used as prisms to
refract light with the film in the form of a cylinder (inflated for
example), will concentrate light in two directions or dimensions so
that concentrations of 50 to over 1000 times are possible. Some
embodiments may concentrate light in two directions or dimension to
concentrations of greater than 1000 times. According to some
embodiments, the tools may be used to form the grooves by
techniques other than embossing.
[0351] Under some circumstances it may be desirable to use straight
line groove paths as approximations rather than arcs. In this case,
the curvature is zero, and while the prism angles may still be
matched similarly closely, either the resulting errors in refracted
light direction will be larger for a patch of a given size, or
patches may be made arbitrarily smaller to achieve a given desired
maximum error in the direction of the refracted light.
[0352] In some embodiments, light arriving at the entry aperture of
the secondary may be specifically guided by the primary to arrive
at different locations on the secondary depending on where it is
coming from. This would create a region of concentrated light
rather than a point focus. The potential benefit is that greater
optical efficiency may be possible if the secondary is able to
direct light arriving at different angles from different distances
in more specific ways.
[0353] The accompanying figures illustrate some methods for
manufacturing the solar concentrator. In one embodiment, a film
attachment system shows elements that attach a straight edge of a
film to a straight edge of a receiver. A film is attached to a
retaining element via a bond. The bond may be a heat seal, RF weld,
adhesive, tape or other joining element. The Film may also wrap
around the retaining element to increase either the area of bond or
friction between the film and the retaining element or both. The
retaining bracket has a channel into which the film and the
retaining element fit. The Channel has a narrow area through which
the film can pass. When a force is applied to a free are of the
film, forces are in turn applied by the film to the retaining
element which ensure that the element maintains an orientation in
which it will stay seated in the channel without becoming
disengaged. In this way, the film can be removably attached to the
bracket.
[0354] An optic may be made of custom plastic film and inflated to
shape. Using air and thin plastic film may reduce the cost of both
the primary optic and balance of system (BOS) tracking structures.
Embodiments of a Concentrated Photovoltaic (CPV) system concentrate
sunlight using inflation air as a structural element, reducing
expensive materials and improving system efficiency. Embodiments
focus on the integration of low-cost inflated CPV optics and a
tracking structure coupled with high efficiency multi junction (MJ)
cells.
[0355] Embodiments thus leverage a baseline low-concentration
design, and address hardware upgrades to achieve an overall module
efficiency of 30% for greater energy yield, while maintaining a low
system cost. An objective is to integrate the high efficiency
photovoltaics into a baseline tubular concentrator and system
architecture, while maintaining low costs similar to the low
concentration (1D) baseline system. After taking baseline tracking
and system efficiency measurements, a prototype two-dimensional
(2D) concentrating sub-scale refractive optic may be created. A
full-scale, 2D primary optic film may be designed adaptable to high
volume, low cost roll-to-roll volume manufacturing.
[0356] A tracking system may mate to the tubular inflated
concentrator balloons incorporating the new 2D concentrating optic
film. Receiver components may be built and qualified through
baseline IEC testing. When the primary film and secondary optics
assembly are finalized, laboratory testing may commence to qualify
the primary and secondary optic components. Finally, the receivers,
secondaries, and 2D inflated concentrators can be assembled onto
the structure. The final, integrated system may achieve the overall
project objective of 30% optical system efficiency, and accelerate
the development of high efficiency PV technology while maintaining
a low cost structure, and meet a LCOE target when manufactured in
high volumes.
[0357] Certain specific system areas may either lead to cost and
performance, or may be targeted for upgrading. The frame and
tracking system may create cost advantages and will have
incremental design refinements as part of the integration work. The
receiver may be upgraded to a multi junction (MJ) cell and
secondary optic assembly from LCPV Si. The MJ receiver technology
may be similar to some conventional HCPV systems. The inflated
primary optic design may offer certain benefits. For example, this
approach may maintaining low system cost while moving from 1D to
2D.
[0358] The tubular inflated refractive optic may be commercially
manufactured and produces approximately 89% specular transmission.
This particular embodiment of an optic is made of relatively thick
(0.012-in.) acrylic, and it concentrates only in one dimension.
This may affect its economic feasibility for use with expensive, 3J
cells. Accordingly, some embodiments may use inexpensive PET as the
basis of its optical materials, and may replace the film several
times over the system lifetime. There are at least four reasons for
using PET materials. First, PET film has the highest modulus to
price ratio of all commodity polymers. Second, its microcrystalline
nature makes it tough: it is able to withstand IEC hail testing
with as little as 0.002-in. thick films, where acrylics need at
least 15.times. that thickness. Third, PET films are amongst the
most dimensionally stable polymers known. The modulus and
crystalline nature of PET make it one of the most heat and creep
resistant commodity polymers. The creep is within tolerable limits
for a 2D concentrating optic. The coefficient of thermal expansion
is low at approximately 17 ppm or close to that of Aluminum or at
least a factor of 4 lower than any other commodity polymer film
material making its overall dimensional stability very good. The
water absorption and swell are unusually low, about half of
acrylics. Fourth, PET has good heat resistance within typical
atmospheric conditions, even in the hottest regions of the world.
Hence, PET offers an inexpensive weatherizable and dimensionally
stable material. Fifth, the energy to break for PET is well over an
order of magnitude higher than any other commodity polymer (with
the possible exception of ionomer), thus resulting in hail
resistance that is orders of magnitude better than acrylics.
[0359] PET may offer challenges in that its index of refraction may
makes for higher reflection losses, and it is subject to some level
of hydrolysis and photolysis. However, the hydrolysis issue is
largely historical, due to older manufacturing methods and
formulations. Modern materials made for PV backsheets in one-sun
applications are less susceptible to this phenomenon. If films are
replaced at regular intervals and deployed in dry climates, this
should not be a problem. Photolysis is a more stringent issue, but
may be subverted to the point that systems can operate effectively.
It is also known that the result of PET photolysis is a drop in
elongation to break and yellowing of the material with exposure to
UV.
[0360] The designs according to certain embodiments may also be
migrating from a 1D concentrating embossing pattern, to a 2D
embossed concentration pattern on its primary optic films, as is
discussed below. Such high precision, 2D refractive optics can not
only be mapped to a tube, but may seamlessly integrate industry
standard film processing practices to allow the manufacture of this
optic film at scale.
[0361] Embodiments of the present invention may offer a baseline,
reliable receiver for integration into the system. A conservative
approach to MJ receiver fabrication utilizes commercial triple
junction (3J) cell packages. Such cell packages already qualified
for use in HCPV, are available from several manufacturers.
Embodiments may utilize a typical adhesive to the heat sink, and a
custom-shaped glass secondary optically coupled to the cell via
silicone. One success factor is that the cell package pass the
standard IEC testing while configured for the system.
[0362] An embodiment of an integrated system may be produced that
achieves greater than 30% overall conversion efficiency,
pre-inverter. An embodiment of the system may be based on
refractive, inflated tube technology mounted on a ground contact
tracker with an in-house Supervisory Control and Data Acquisition
(SCADA) system.
[0363] In some embodiments, certain patterns may be embossed into
film surfaces in order to better concentrate and focus the incident
sunlight onto defined focus areas for greater efficiency. The basic
functionality of the primary optic can be provided by forming a
pattern of facets of millimeter or smaller spacing, such that the
facets refract sunlight and direct it to the receivers or PV cells.
In one embodiment, the refractive grooves might be the only pattern
present on the film surface. The refractive primary optic or
primary film functionality can also be enhanced by applying
additional patterns to the other surface.
[0364] One pattern that may be applied may include sub-wavelength
features that modify the effective index of refraction with depth
of the surface of the film. For example, applying a "moth's-eye"
pattern to the outer surface of the primary optic/film to reduce
Fresnel reflection losses can increase the system efficiency by a
significant amount. It may also be possible to use a smaller scale
pattern superimposed on the facet structure on the same side or
surface of the primary film.
[0365] In another embodiment, the polymer materials in which the
embossed refractive facets are made may have indices of refraction
that vary with wavelength. The resulting chromatic aberration
increases the lateral spread of sunlight at the receiver, thereby
effectively reducing the amount of angular error due to
mis-alignment or tracking offsets that can be tolerated without
losing some power output. With refraction, blue light is deflected
more strongly than red light. However, diffraction disperses red
light through greater angles than blue. Hence, applying the
appropriate diffractive groove pattern on the outer face of a
refractive film to make a hybrid optic can correct the intrinsic
chromatic aberration of the purely refractive film.
[0366] In yet another embodiment, an optic can be used that focuses
light by diffraction. The diffraction can be achieved with a
regularly spaced surface relief profile, with a gradient-index
material whose index of refraction varies in a repeating pattern,
or by etching sub-wavelength features of a variety of shapes that
similarly vary in shape or spacing to produce a laterally-changing
effective index of refraction. However, a standard diffractive
optic will provide high efficiency (i.e., close to unity) only for
one particular wavelength in one particular diffraction order.
Solar power generation, however, requires achieving nearly unity
efficiency over the very broad wavelength of about 400 nm to 1600
nm. Achieving high efficiency over a wide wavelength range, in
addition to the regular surface height or effective index of
refraction lateral variation, requires inclusion of a variation of
effective index of refraction with depth. The vertical variation
can be achieved by: (a) making surface relief profile structures
with different dispersion characteristics that are co-aligned and
separated by an air gap, (b) laminating two gradient-index
materials, with coincident boundaries for the zones of varying
gradient, but with their gradients running in opposite directions
such that a high index part of the lower layer lies directly below
a low index part of the upper layer, and conversely, or (c) using a
single layer consisting of strips of alternating materials in
sub-wavelength structures with the appropriate effective index
variation and spacing to provide high efficiency over the specified
wavelength range.
[0367] Embodiments may feature one or more elements or
characteristics as are now described. FIG. 23A shows an isometric
view of one version of an integrated CPV system 2302 according to
an embodiment of the present invention. Four inflated film tubular
solar concentrators 2304 are configured to track the sun in
elevation and azimuth. Concentrators 2304 are mounted to an upper
structure 2306 which pivots on rollers 2320 about a virtual
elevation axis 2308 with respect to a lower structure 2310. Lower
structure 2310 is rotatably connected to the ground via ground
anchor 2312. Ground anchor 2312 defines an azimuth axis 2314.
System 2302 rotates with respect to the ground about azimuth axis
2314 and is driven by a drive wheel 2316. A follower wheel 2318
provides additional ground support for system 2302. Together,
ground anchor 2312 and the two wheels 2316 and 2318 create 3 points
of ground contact at or near the maximum spatial extents of the
system for greatest system stiffness and stability. Azimuth
actuation through wheel 2318 happens at or near the largest
distance from azimuth axis 2314 which reduces the actuation forces
required, increases stiffness and reduces the cost and complexity
of actuator transmission components by allowing less gear reduction
for a given amount of torque to be applied to system 2302.
Similarly, an elevation actuator 2322 acts to apply actuation
torque to upper structure 2306 at or near the largest possible
distance from elevation axis 2308 in order to reduce forces and
elevation actuator transmission cost and complexity. Wheels 2316
and 2318 are configured to operate directly on unprepared ground or
soil which reduces system costs and installation costs. System 2302
is able to track the sun's position despite ground irregularities,
bumps, holes or obstacles. As wheels 2316 and 2318 travel to create
azimuth motion and pass over an obstacle. Elevation actuator 2322
can adjust the position of upper structure 2306 so that a desired
elevation orientation is maintained despite the ground
irregularities.
[0368] One advantage of the virtual pivot axis frame geometry of
this system is that the elevation axis is able to run through the
center of gravity of upper structure 2306 and concentrator
assembly, even if the center of gravity is inside the volume of the
concentrators or even if it conflicts with the frame members.
Another advantage is that the concentrators can be made very long,
but still can be rotationally supported mid-span. Without the
virtual pivot axis, the concentrator and upper frame assembly would
have to be rotationally supported at the ends which could result in
large deformations in the middle of the concentrators, or
alternately the need for more frame material to prevent large
deformations. With the virtual pivot axis, the upper structure 2306
can be supported at any point or points along its length without
interfering with the rotational operation.
[0369] FIG. 23B shows a top view of the system of FIG. 23A. FIG.
23C shows a side view of the system of FIG. 23A.
[0370] FIGS. 24A and 24B show views of another system 2402
configured with inflated tubular solar concentrators 2404 according
to an embodiment of the present invention. FIG. 24A shows an
isometric view. The tracking system 2410 is similar to that of FIG.
23A except that inflated concentrators 2404 each rotate about their
own respective elevation axes 2406 so that the rest of tracking
system 2410 can move as a rigid unit. FIG. 24B shows a side view of
the system 2402 of FIG. 24A.
[0371] FIG. 25 shows a method of attaching and removing a film from
a film holder which may be employed to allow changing of film-based
inflated optics or other applications where that benefit from an
easily changeable film with or without air seal, according to an
embodiment of the present invention. Film 2502 wraps around one or
more corners of an anchoring strip 2506. An optional attachment
zone 2508 prevents film 2502 from slipping off anchor 2506.
Attachment zone 2508 may be a heat seal, dielectric weld, RF weld,
ultrasonic weld, adhesive bond, tape joint or other form of joining
or friction enhancement. Anchor 2506 has a shape designed to fit
into a film holder 2504 that has a slot or other retaining feature.
Film 2502 and anchor 2506 are designed to slide into film holder
2504. When forces are applied perpendicular to the direction of the
insertion sliding motion (for example forces related to film
inflation), anchor 2506 holds film 2502 securely in film holder
2504. If forces on film 2502 are going to be very high, additional
wraps around anchor 2506 may be made so that capstan friction can
be increased to enhance film retention.
[0372] FIGS. 26A-26D show another method of attaching and removing
a film from a film holder which may be employed to allow changing
of film-based inflated optics, according to an embodiment of the
present invention. A film 2604 is joined to an anchor strip 2606
which can snap into a film holder 2602. The film installation in
FIGS. 26A-D is accomplished by snapping the film and anchor in from
the side rather than sliding in from the end as in the film
retention system of FIG. 25. FIGS. 26A-D show end views of
sequential installation steps. Film removal can be accomplished
either by reversing the actions of installation or by sliding the
film and anchor out in a direction parallel to the length of the
film holder or by using a specially designed tool (not shown).
[0373] FIGS. 27A-27C show variations of film anchor features and
sealing materials which can be used in conjunction with the film
retaining and sealing methods of FIGS. 25 and 26, according to an
embodiment of the present invention.
[0374] FIG. 27A shows an end view of a film 2704 attached to an
anchor 2702 which is retained in a film holder 2706. Sealing, e.g.,
to prevent pressurized gas or liquid from passing from one side of
film 2704 to the other side of the film by going between film 2704
and anchor 2706, can be accomplished by taking advantage of the 3
tight junctions that are created when film 2704 is tensioned or
force is applied. The first sealing junction is formed between film
2704 and film holder 2706. The second is between film 2704 and a
different edge of film holder 2706. The third is between anchor
2702 and film holder 2706. The sealing function of all three
junctions is improved as tension in the film increases. Sealing
function can be compromised by imperfections of shape or surface
finish in the surfaces of the components forming the junctions, or
by particles that prevent perfect surface contact. When sealing
function is compromised, the leak rate increases. One way to
mitigate leaks is to design compliance into one or both of the
sealing surfaces at a junction. For example film holder 2706 may be
made in part or in whole out of a compliant elastomeric material.
Another mitigation technique is to use a sealing agent (not shown)
such as a thick liquid, gel or grease in between film 2704 and
holder 2706 and/or in between anchor 2702 and holder 2706.
[0375] FIG. 27B shows another way to improve the sealing function
of a film holder. Film 2710, anchor 2708, and film holder 2712 are
configured similarly to the analogous components in FIG. 27A. An
additional component, sealing agent 2714 is attached to film holder
2712. Sealing agent 2714 may be an elastomer, rubber, polymer, wax
or other compliant material. Tension in film 2710 will cause anchor
2708 to press into and deform sealing agent 2714. If sealing agent
2714 is compliant enough it can conform to any irregularities in
the surface of anchor 2708.
[0376] FIG. 27C shows another way to improve the sealing function
of a film holder. Film 2718 anchor 2716, and film holder 2720 are
configured similarly to the analogous components of FIG. 27A. To
improve the sealing function a sealing agent 2722 is attached to
anchor 2716. This is very similar to FIG. 27B except that sealing
agent 2722 can be removed with the film. This can be useful because
whenever the film is replaced, the sealing agent can be
renewed.
[0377] FIGS. 28A-28D show variations of film anchor construction.
FIG. 28A shows an anchor that is formed out of several or many
folds of a film 2802 itself. If film 2802 is folded over enough
times it may form a body that is rigid enough to perform the anchor
function. It may be helpful to use an adhesive or tape (not shown)
in between the folds that form anchor 2804. FIG. 28B shows another
anchor construction variation. Film 2806 wraps around anchor 2810
with an optional adhesive, heat seal or tape between them. An outer
layer of tape, film or other thin material 2808 is wrapped around
anchor 2810 outside of film 2806. If 2808 is an adhesive tape, then
it can be adhered to both sides of film 2806 and it may be
sufficient to keep the anchor intact. FIG. 28C shows another anchor
construction. Film 2812 is adhered, glued, bonded, heat sealed,
mechanically attached or welded to anchor 2814 without needing to
wrap around it. FIG. 28D shows another anchor construction. Film
2818 wraps around an anchor 2822 and is adhered, glued, bonded,
welded, heat sealed or mechanically attached to itself at bond area
2820.
[0378] FIGS. 29A-29C show additional variations of film anchor and
seal materials. FIG. 29A shows an anchor 2902 adhered directly to a
film 2904. Anchor 2902 may be molded onto film 2904 or it may be
made by a co-extrusion process with the film or it may be adhered
onto the film via an adhesive, heat seal, or weld. FIG. 29B shows
an anchor construction with a film 2908 wrapping around a bead 2906
and then adhered, taped, heat sealed, glued or welded to itself at
bond area 2910. FIG. 29C shows an anchor constructed from a film
2914 which is joined to a round bead 2912 by a joining material
2916. Joining material may itself be an adhesive tape, or it may be
joined to film 2914 by an adhesive, glue, weld or heat seal.
[0379] FIGS. 30A and 30B show variations of film anchors and
related structures to which film is secured. FIG. 30A shows a film
anchor 3002 that slides into a holder 3004. The teardrop or wedge
shape of anchor 3002 is useful in creating a seal with low leak
rate because the gradual slope of the sides creates mechanical
advantage and a wedging action which increases the sealing forces
between anchor 3002 and holder 3004. FIG. 30B shows an anchor 3006
which gets trapped in a holder 3008 by a retainer 3010.
[0380] FIG. 31A shows another method of attaching a film to a solar
receiver or heat sink. Film 3102 can be sandwiched between a first
retainer 3104 and a second retainer 3106. Retainer 3104 may serve
as a structural base of a solar receiver and also may be made of a
material with high thermal conductivity in order to facilitate heat
transfer through film 3102 to retainer 3106. Retainer 3106 may be
attached to or may be an integral part of a heat sink 3108.
[0381] FIG. 31B shows another method of attaching a film to a solar
receiver or heat sink. Film 3110 can be sandwiched between a first
retainer 3112 and a second retainer 3114.
[0382] FIGS. 32A and 32B show a method of sealing the end of a
tubular inflated solar concentrator according to an embodiment of
the present invention. FIG. 32A shows the concentrator prior to
inflation in a flat form. Film 3202 forms a closed tube with an end
3206. A seal 3204 closes the end so that gas cannot escape. Seal
3204 may be in the form of a heat seal, a mechanical clamp, glue,
adhesive tape, an ultrasonic weld or other bond. FIG. 32B shows an
inflated configuration of the sealed film of FIG. 32A. Film 3208,
end 3212 and seal 3210 correspond to the film, end and seal of FIG.
32A.
[0383] FIGS. 33A and 33B show another method of sealing the end of
a tubular inflated solar concentrator according to an embodiment of
the present invention. FIG. 33A shows a flat configuration of a
film 3302 which forms a continuous tube but is folded flat with
pleats on opposite sides. An end 3306 is closed with a seal 3304
which joins adjoining pieces of film 3302 across each pleat. Seal
3304 may be heat seal, a mechanical clamp, glue, adhesive tape, an
ultrasonic weld or other bond. FIG. 33B shows an inflated version
of the film of FIG. 33A. Film 3308, end 3312 and seal 3310
correspond to the analogous items in FIG. 33A.
[0384] FIGS. 34A and 34B show another method of sealing the end of
a tubular inflated solar concentrator according to an embodiment of
the present invention. Film 3402, end 3406 and seal 3404 are
similar to the analogous items in FIGS. 33A and 33B except that
seal 3404 is curved which promotes a more round shape when inflated
as shown in FIG. 34B. Film 3408, end 3412, and seal 3410 in FIG.
34B are shown in the inflated shape of the analogous items in FIG.
34A.
[0385] FIG. 35 shows another method of sealing the end of a tubular
inflated solar concentrator according to an embodiment of the
present invention. A film 3502 has a bunched end 3506. As a result
of bunching the end, pleats 3504 are formed. Bunched end 3506 may
be held in place by a mechanical clip, glue, zip-tie, string, wire,
fiber, heat seal, weld or other joint.
[0386] FIGS. 36A-B show isometric cutaway and partial section views
respectively of another method of sealing the end of a tubular
inflated solar concentrator according to an embodiment of the
present invention. FIG. 36A shows an inflated tubular film 3602 and
a removable endcap 3604. A stop ring 3614 is attached to film 3602
as shown in FIG. 36B. A seal ring 3606 fits between the edge of
endcap 3604 and stop ring 3608 and is made of a compliant material
which is squeezed to form a seal and prevents air from escaping.
When inflation pressure is removed, endcap 3604 is easily removed
so that the film can be replaced. When inflation pressure is added,
it creates forces on endcap 3604 which keep it in place. FIG. 36B
shows a partial section view of components analogous to those of
FIG. 36A including tubular inflated film 3610, endcap 3616, stop
ring 3614, and seal ring 3612.
[0387] FIGS. 37A-37B show isometric cutaway and partial section
views respectively of another method of sealing the end of a
tubular inflated solar concentrator according to an embodiment of
the present invention. FIG. 37A shows an inflated tubular film 3702
which is connected to a removable endcap 3704 by a stop ring 3706
which is attached to the outside surface of film 3702. A seal ring
3708 is made of a compliant material and creates a seal between
film 3702 and endcap 3704 so that air does not leak out. The
geometry of these components differs from that of FIG. 36A in that
stop ring 3706 and seal ring 3708 are on the outside of film 3702
so that a flange on endcap 3704 can fit around the outside of the
end of film 3702. FIG. 37B shows a partial section view of
components analogous to those of FIG. 37A including film 3710,
endcap 3716, stop ring 3714 and seal ring 3712.
[0388] FIGS. 38A and 38B show isometric cutaway and partial section
views respectively of another method of sealing the end of a
tubular inflated solar concentrator according to an embodiment of
the present invention. FIG. 38A shows a tubular inflated film 3802
and a removable endcap 3804. A compliant seal and stop ring 3806 is
attached to film 3802 and prevents endcap 3802 from separating from
the film and resists forces created by inflation pressure. FIG. 38B
shows a partial section view of components analogous to those of
FIG. 38A including film 3808, endcap 3812 and stop and seal ring
3810. Stop and seal ring 3810 has a flange which fits into a groove
on endcap 3812.
[0389] FIGS. 39A and 39B show isometric cutaway and partial section
views respectively of another method of sealing the end of a
tubular inflated solar concentrator. FIG. 39A shows an inflated
tubular film 3902 and an endcap 3904 which are connected and sealed
by a seal ring 3906 which is attached to endcap 3904. FIG. 39B
shows a partial section view of components analogous to those in
FIG. 39A including film 3908, endcap 3904 and seal ring 3906.
[0390] FIGS. 40A-40E show another method of attaching an inflatable
tubular concentrator to a receiver heat sink and sealing its ends.
FIG. 40A shows a cross section view of a tubular inflated solar
concentrator system 4002 which includes a receiver assembly 4004
and inflated film 4006. FIG. 40B shows a partial detail view of an
area indicated in FIG. 40A. Film 4006 has a cutout which enables it
to fit onto receiver assembly 4004. Film 4006 is clamped between
flange 4012 and gasket 4010. These components are held onto a heat
sink 4008 and clamping pressure is created by screws 4014 which are
tightened into heat sink 4008 to create clamping forces required to
hold and seal to film 4006. Gasket 4010 may be made of thermally
insulating materials if desired to minimize the temperature that
film 4006 is subjected to even if heat sink 4008 reaches a high
temperature. FIG. 40C shows an exploded end view of the solar
concentrator system 4002 of FIGS. 40A and 40B. To assemble system
4002, gasket 4010 fits around the edge of heat sink 4008. Then film
4006 slips over heat sink 4008 until it meets gasket 4010. Then
flange 4012 fits on to hold film 4006 securely up against gasket
4010. Then bolts 4014 are screwed in to secure the components
together. This view also shows the end seal 4016 which joins and
closes the end of film 4006. Film 4006 may have pleats that fold
inward in order to reduce the width of end seal 4016 (similar to a
typical candy bar wrapper). Seal 4016 may be a mechanical clamp, or
it may be a heat seal or an adhesive tape, glue, weld or other form
of joining which prevents air from escaping out the end of film
4002. FIG. 40D shows an isometric exploded view of system 4002. The
pleats in film 4006 are more clearly visible here as is seal 4016.
FIG. 40E shows a bottom isometric exploded view of concentrator
system 4002. Film 4006 has a cutout to allow it to fit over heat
sink 4008 which is more clearly visible in this view.
[0391] The film attachment and sealing features shown in FIGS.
40A-40E provide a workable film replacement, attachment and sealing
system for an inflatable solar concentrator. Another variation of
system 4002 provides features which allow the film to slide onto
the receiver assembly and snap in place without the need for
additional mechanical fasteners. Another variation facilitates
creating a seal between the film and the receiver assembly by
having the film slide over a taper on the receiver assembly. Other
variations are possible.
[0392] FIGS. 41A and 41B show another method of sealing the ends of
an inflatable tube such as an inflatable tubular solar
concentrator. FIG. 41A shows an inflatable film tube 4102 with an
end seal across the tube width. One side of the seal is labeled
4104 and the other side of the seal is labeled 4106. Because this
seal flattens film tube 4102, it becomes wider than the diameter of
film tube 4102 which may or may not be acceptable. FIG. 41B shows
an alternate version of an end seal configuration for an inflatable
film tube 4108. In this case, the sides of the end seal are labeled
4110 and 4112. Seal sides 4110 and 4112 are wrapped inward toward
each other and joined together which significantly reduces the
effective width of the seal and the end of the tubular
concentrator.
[0393] FIGS. 42A-42D show partial cross section views of variations
of another end seal for an inflated tubular solar concentrator
according to an embodiment of the present invention. FIGS. 42A-42D
all represent geometries that are substantially axisymmetric about
horizontal axes (not shown). FIG. 42A shows a tubular film 4204
which is joined to an end film at bond 4206. bond 4206 may be a
heat seal, RF weld, sonic weld, glue, adhesive tape, chemical bond,
mechanical bond, clamped joint, or other form of joining Film 4202
is shown overlapping film 4204 on the outside, but it could also
overlap on the inside of film 4204. The overlap order depends upon
the particular film materials being used and the type of bond being
created. For example it is possible to use laminated films that
have a PET layer for the benefit of its strength and modulus plus a
polyolefin layer which can be more easily heat-sealed. One
arrangement with these films would be to use an overlap order as
shown in this figure with end film 4202 having the polyolefin layer
on the inside and tubular film 4204 having the polyolefin layer on
the outside so that the two may be heat sealed together, polyolefin
to polyolefin to form bond 4206. Many other types of films, film
layers, materials and bond geometries are also possible. FIG. 42B
shows a tubular film 4210 which is joined to an end film 4208 via a
joint film 4212 and bonds 4214. Bonds 4214 may be any of the types
described in FIG. 42A. Using a separate joint film 4212 would allow
films 4208 and 4210 to have the same side facing out. Joint film
4212 may or may not be the same material as films 4208 and 4210. It
may be advantageous to make joint film a different material
specifically tailored for joints since it need not cover larger
areas.
[0394] FIG. 42C shows a tubular film 4218 which is joined to an end
film 4216 via a joint element 4220 and bonds 4222. Joint element
4220 is shown having an angled cross section which may be
facilitate creation of bonds 4222. However, joint element 4220 may
be made in a variety of other shapes and configurations. Joint
element 4220 may have a cross section that is rectangular,
circular, tubular, flexible, rigid, open, or closed. Bonds 4222 may
be any of the bond types described in FIG. 42A. FIG. 42D shows a
tubular film 4226 joined to an end film 4224 via joint element 4228
and bonds 4230. FIG. 42D is similar to FIG. 42C except that joint
element 4228 is shown on the outside of the films rather than on
the inside of the films. The particular configuration used will
depend upon the specific film materials and orientations, the
environment and processing parameter considerations.
[0395] FIGS. 43A-J show aspects of another method of attaching an
inflatable tubular concentrator to a receiver heat sink and sealing
its ends according to an embodiment of the present invention.
[0396] FIG. 43A shows a bottom isometric view of a tubular
inflatable film solar concentrator system 4302. An upper film 4304
is joined to side films 4306 (one film 4306 on each side). Film
4304 may be a refractive concentrating film with embossed prisms
(not shown). Side films 4306 may have different characteristics
than film 4304. Films 4306 are joined to a receiver assembly 4308
which includes a heat sink 4310. Film holders 4316 are joined to
and run the length of receiver assembly 4308. An end transition
flange 4312 is attached to an end of receiver assembly 4308 and
also attached to film holders 4316. End transition flange 4312 has
a curved and tapered shape so that film holders 4316 may come
together at one end while still being separated along the length of
receiver assembly 4308. End transition flange 4312 may be sealed to
receiver assembly 4308 and to film holders 4316 so that no air
escapes between these components. Films 4306 are held on and sealed
to film holders 4316 (see FIG. 43B). The end of the tube formed by
films 4304 and 4306 is shown sealed in a different way than
described in some of the previous figures. In some cases,
refractive films such as 4304 may be damaged by folding or bending
beyond a certain limit, but side films such as 4306 may be more
flexible. FIG. 43A shows a T-seal 4314 that seals the end of system
4302 in a space equal to or narrower than the diameter of tube
system 4302 without requiring refractive film 4304 to be tightly
folded. T-seal 4314 may be a mechanical clamp system or it may be
an adhesive bond, tape, weld, or other type of joint which prevents
air from escaping out the end of system 4302. The circled area
labeled "A" is shown enlarged in FIG. 43B.
[0397] FIG. 43B shows a detail view of the circled area labeled "A"
in FIG. 43A. In this view anchors 4318 can be seen. Anchors 4318
are joined to films 4306 and fit into slots on film holders 4316.
The film installation procedure for system 4312 involves 1) sliding
films 4306 and 4304 with anchors 4318 into film holders 4316 from
one end and then 2) applying seals 4314 to each end of system
4302.
[0398] FIG. 43C shows an end view of system 4302. A detail circle
labeled "B" is shown enlarged in FIG. 43D. FIG. 43D shows an
enlarged view of the region labeled "B" in FIG. 43C. This view more
clearly shows how films 4306 wrap around anchors 4318 and how
anchors 4318 fit into channels in film holders 4316. FIG. 43E shows
another view of the end of system 4302. This view shows optional
seal clamping flanges 4320 and also an optional gasket member 4322
that may be useful in adding compliance and making a better seal.
Seal 4314 is made in between flanges 4320 and around gasket 4322
(if present). FIG. 43F shows a partial cutaway 3D view of just
receiver assembly 4308 without the films installed. This view more
clearly shows channels in film holders 4316. This view also shows
an insulating material 4326 between heat sink 4310 and film holders
4316 which limits heat transfer from heat sink 4310 to film holders
4316.
[0399] FIG. 43G shows construction of a film in flat form related
to the method of FIGS. 43A-F. Films 4304 and 4306 are joined at 2
joint areas 4328 which may be heat seals, dielectric welds,
ultrasonic welds, tape joints, chemical bonds, mechanical bonds or
some other form of joints. FIG. 43H shows a close-up view of the
construction of an anchor feature for the film and attachment
method of FIGS. 43A-G. Side film 4306 is connected to anchor 4318
by wrapping around it, with an optional bond 4330 between film 4306
and anchor 4318. Bond 4330 may be a heat seal, adhesive bond, tape,
RF weld, ultrasonic weld, glue or other form of bond. FIG. 43I
shows the film of FIG. 43H when curved into a tubular configuration
just before installation. FIG. 43J shows the shape that the film of
FIGS. 43H-I takes when it is installed. Film holders 4316 and
receiver assembly 4308 are not shown for clarity.
[0400] FIGS. 44A-44E show embodiments of solar concentrators that
employ flexible cells mounted directly on the inflated concentrator
film and/or receivers with minimal structure.
[0401] FIG. 44A shows a solar concentrator with an optically active
film 4404, side film 4402, ends 4410, and a receiver 4408 made up
of cells 4406. In this version, cells 4406 are mounted directly to
film 4402. Cells 4406 may be mounted to film 4402 via a lamination
process, using an adhesive, by heat sealing, sonic welding, RF
welding or via a different bonding process or agent. Cells 4406 may
or may not be coated with an encapsulant material such as ethyl
vinyl alcohol (EVA), silicone or other encapsulant. Cells 4406 may
be flexible so that they conform to the circular cross section that
film 4402 naturally tends to assume, or they may be rigid and flat
and film 4402 may conform to the shape of the cells.
[0402] FIG. 44B shows another version of an inflated solar
concentrator with a front optical film 4414, a side film 4412, ends
4420 and a receiver 4418 made up of cells 4416 and a carrier sheet
4422. In this case carrier sheet 4422 is attached to side film 4412
along a bond area 4424. Bond area 4424 may be an adhesive seal,
heat seal, RF weld, tape joint, mechanical joint, clamped joint or
other form of bond. Receiver 4418 may be removable from side film
4412 so that the concentrator films may be removed and replaced and
the receiver 4418 may be reused. Carrier sheet 4422 may be a
polymer film such as PET or PET laminate or another polymer, or it
may be another material. Carrier sheet 4422 may also function as a
heat sink. Carrier sheet 4422 may for example be made of aluminum
and have fins (not shown) that protrude from the bottom in order to
better transfer heat from the receiver to air outside the solar
concentrator. It may also serve as a structural member in which
case it could be made of steel or composite material. It may have
ridges, corrugations or other features which enhance its strength
or stiffness. Carrier sheet 4422 may have channels (not shown) in
which fluid for active cooling could flow.
[0403] FIG. 44C shows a detail cutaway view of a version of a
receiver and film for a solar concentrator after the film has been
replaced several times. Cells 4426 and carrier sheet 4428 are
attached to layers of old film 4430 that may remain after a film
change. Old film layers 4430 are in turn attached to current side
film 4432. During the next film change, side film 4432 would be cut
away and would turn into another layer 4430 and another side film
similar to 4432 would be attached. Carrier sheet 4428 may be a
polymer or it may be a more traditional heat sink material such as
a metal like aluminum or copper. It may have fins (not shown) to
better transfer heat from the receiver to the air outside the
concentrator. It may also serve as a structural member in which
case it may be made of steel. It may have ridges, corrugations or
other features which enhance its strength or stiffness. Carrier
sheet 4428 may have channels (not shown) in which fluid for active
cooling could flow.
[0404] FIG. 44D shows a detail cutaway view of another version of a
receiver and film arrangement for a solar concentrator. Cells 4434
are mounted to a carrier sheet 4436 which is in turn mounted to a
side film or member 4442 via interlock connectors 4438 and 4440.
Carrier sheet 4436 is bonded to interlock connectors 4438 and side
film 4442 is bonded to interlock connectors 4440. Interlock
connectors 4438 and 4440 have features which allow them to lock
together to create a releasable mechanical connection and also
optionally and air-tight seal (if the concentrator is to be
inflated for example). With this arrangement, the side film 4442
(and the entire film construction including optical film which is
not shown here) may be replaced as needed and the receiver and
particularly cells 4434 may continue to be used for the extent of
their lifetime (which may be greater than that of the films). Note
that there are many possible types of interlock connectors 4438 and
4440. FIG. 44D shows schematic interlocking ridges that may be used
to form both a mechanical connection and an air seal, but there are
many other geometries or components that could be used. These
include, but are not limited to Zip-Loc.RTM. style features,
plastic ridges and grooves, metal interlocking fingers, snaps, hook
and loop fasteners, mechanical fasteners, screws, removable
adhesives, strips of nano-scale interlocking features, zippers,
interlocking hooks, and other bonds or interlocks. Carrier sheet
4436 may be a simple polymer sheet such as PET or a laminate of
different polymers, or it may be a high-temperature polymer such as
Kapton.RTM., or it may be another material. In some embodiments,
carrier sheet 4436 may also serve as a heat sink and may be made of
a material with a high thermal conductivity such as aluminum alloy,
magnesium alloy or copper. Carrier sheet 4436 may have longitudinal
or transverse fins, pins or other features designed to enhance heat
transfer from cells 4434 to the air outside the concentrator.
Carrier 4436 could also be made of steel or stainless steel, and
could be made very thin so as to be flexible, or it could be made
thicker or with ribs, corrugations or other features so that it
enhances the stiffness of the concentrator structure.
[0405] FIG. 44E shows a detail cutaway view of another version of a
receiver and film arrangement for a solar concentrator. Cells 4444
are mounted to a carrier sheet 4446 which is in turn mounted to a
side film or member 4452 via interlock connectors 4448 and 4450.
Carrier sheet 4446 is bonded to interlock connectors 4448 and side
film 4452 is bonded to interlock connectors 4450. Interlock
connectors 4448 and 4450 have features which allow them to lock
together to create a releasable mechanical connection and also
optionally and air-tight seal (if the concentrator is to be
inflated for example). Connectors 4448 have a circular cross
section bead shape which slides into or snaps into a mating female
"C" channel cross section shape of connector 4450. With this
arrangement, the side film 4452 (and the entire film construction
including optical film which is not shown here) may be replaced as
needed and the receiver and particularly cells 4444 may continue to
be used for the extent of their lifetime (which may be greater than
that of the films). Carrier sheet 4446 may be a simple polymer
sheet such as PET or a laminate of different polymers, or it may be
a high-temperature polymer such as Kapton.RTM., or it may be
another material. In some embodiments, carrier sheet 4446 may also
serve as a heat sink and may be made of a material with a high
thermal conductivity such as aluminum alloy, magnesium alloy or
copper. Carrier sheet 4446 may have longitudinal or transverse
fins, pins or other features designed to enhance heat transfer from
cells 4444 to the air outside the concentrator. Carrier 4446 could
also be made of steel or stainless steel, and could be made very
thin so as to be flexible, or it could be made thicker or with
ribs, corrugations or other features so that it enhances the
stiffness of the concentrator structure.
[0406] FIG. 45 shows another variation of an inflated tubular solar
concentrator construction. Endcaps 4502 may be removable from film
4504, so that film 4504 may be replaced. A partial cutaway of film
4504 is shown so that internal cables or rods 4506 are visible.
Cables 4506 connect to endcaps 4502 and resist forces created by
gas pressure inside the concentrator system which would tend to
push endcaps 4502 apart and out of the concentrator.
[0407] FIGS. 46A and 46B shows a solar receiver with secondary
optics and heat sinks according to an embodiment of the present
invention.
[0408] FIG. 46A shows receiver assembly 4602 truncated for clarity.
Receiver assembly 4602 includes a heat sink 4606 which may also
serve as a primary structural element for receiver assembly 4602. A
number of cell assemblies 4604 are attached to heat sink 4606. The
parts of the cell assemblies visible in this figure are secondary
optics holders 4612 and secondary optics 4610. Secondary optics
4610 are made from optical glass, quartz, silicone or other
transparent optical material capable of handling highly
concentrated sunlight. Secondary optics 4610 may typically have an
entrance aperture at which reflection is generally minimized and
also a total internal reflection (TIR) section which serves to
create additional concentration as well as to homogenize the light
on the cell (not shown) and change its direction if necessary so
that it will hit the cell. Holder 4612 serves to align secondary
optics 4610 to the cells and also to the incoming patches of
concentrated sunlight created by the primary (not shown). Film
holders 4608 are attached to receiver assembly 4602, with an
optional insulating material 4614 between them and heat sink 4606
to limit heat transfer to film holder 4608.
[0409] FIG. 46B is a partial cutaway view of a cell assembly 4604.
Cell assembly 4604 includes secondary optics 4610, secondary optic
holder 4612, substrate 4622 and cell 4616. Substrate 4622 has
mechanical and electrical connections for cell 4616. Secondary
optic holder 4612 may have a lower opening 4618 which aligns the
base of a secondary optic 4610 with its respective cell 4616.
Secondary optic holder 4612 may also have spring tabs 4620 which
serve to align the entrance aperture of a secondary optic 4610 and
also to mechanically restrain it while compensating for variations
in part tolerances. Cell assembly 4604 is shown here with two
secondary optics 4610 and two cells 4616, however in other in other
variations it may only have one or it may have more than two of
both secondaries and cells.
[0410] FIGS. 47A-C show details of another receiver assembly that
uses individual heat sink and cell assemblies that join to a
carrier chassis according to an embodiment of the present
invention.
[0411] FIG. 47A shows an exploded view of a receiver assembly 4700.
A top chassis 4702 is joined to a bottom chassis 4704 to create a
stiff, effectively tubular structure. Individual cell assemblies
4706 are each joined to lower chassis 4704 and each protrude
through a respective hole in bottom chassis 4704. Cell assemblies
4706 may be joined to lower chassis 4704 in a number of ways
including adhesive, welding, or mechanical fastening. Light reaches
the individual cell assemblies 4706 through apertures in the upper
chassis 4702. Cell assemblies 4706 may or may not be joined to
upper chassis 4702 at the perimeter of the apertures. If the cell
assemblies 4706 are joined to the upper chassis 4702, then receiver
4700 may become a sealed or semi-sealed unit which has the benefits
that liquid water may be kept away from the cell and electrical
conductors and also exposure of people to high voltage components
can be avoided. It may be desirable to include a breathable
moisture barrier (not shown) somewhere in the perimeter of receiver
assembly 4700 to allow the pressure inside receiver assembly 4700
to equalize with the pressure outside, but without allowing liquid
water to pass inside. Upper chassis 4702 and lower chassis 4704 may
be formed from steel or other material with a moderate coefficient
of thermal expansion (CTE), even though individual heat sinks may
be made from aluminum or other material with a higher CTE. Such a
scheme may avoid potential problems posed by long lengths of
receiver with continuous aluminum (or other high CTE) as the
primary structural member. Note that many different materials could
be used for the upper and lower chassis. It may be of particular
interest for inflated film-based concentrators to match the net
expansion at operating temperature of the chassis components with
the expected expansion of the primary optic films at their
operating temperature.
[0412] FIG. 47B shows an exploded view of one embodiment of a cell
assembly. A heat sink 4716 serves as a structural base for the
assembly. A cell 4712 is electrically and mechanically attached to
a substrate 4714 which is in turn attached (at least thermally
coupled) to heat sink 4716. A secondary optic 4710 has an entry
aperture (top face) and angled sides which allow total internal
reflection (TIR) to concentrate, homogenize and guide incoming
light. Optic 4710 is placed on top of cell 4712, with an optional
optical coupling agent such as a silicone or other compound
formulated to withstand highly concentrated sunlight. A holder 4708
captures optic 4710, aligns it to cell 4712 and substrate 4714 and
keeps it in place on heat sink 4716. Holder 4708 may or may not
include compliant elements ("fingers") to ensure contact and
accommodate component tolerance variations.
[0413] FIG. 47C shows another variation of a cell assembly that
does require a substrate. Instead, a cell 4722 may be bonded
(thermally or mechanically or both) directly to a heat sink 4724.
Secondary optic 4720 and holder 4718 are similar in structure and
function to their analogous components in FIG. 47B. An advantage of
bonding cell 4722 directly to heat sink 4724 is that cell 4722 may
be able to operate at a lower temperature for a given incident
power because heat transfer to heat sink 4724 may be enhanced. A
thermal grease, or thermally conductive adhesive or both may be
used between cell 4722 and heat sink 4724. In some embodiments, a
material having a thermal conductivity between 0.005 W/m-k and 180
W/m-k may be used in the grease/adhesive. It may be advantageous to
use grease or a low-modulus adhesive so that stresses in cell 4722
due to CTE mismatch may be minimized. Two other schemes that may be
considered: 1) a "dot" of adhesive in the center of cell 4722
surrounded by a ring of thermal grease out to the edges of cell
4722 may be employed to both retain the cell while allowing its
corners to float or expand and contract as needed. 2) The perimeter
of the cell may be bonded with adhesive while the center may be
covered by thermal grease. This would restrain the perimeter of the
cell and prevent grease from escaping, while allowing the center of
the cell to move to mitigate stresses.
[0414] FIGS. 48A-48D show a parts of a partial receiver assembly
including thermal insulation according to an embodiment of the
present invention. FIG. 48A shows an exploded view of a partial
receiver assembly similar to that of FIG. 47. The cell and
secondary are omitted for clarity. Heat sink 4810 mounts from below
to lower receiver chassis 4802. An insulating material 4806 such as
a foamed isocyanurate, or filled or foamed epoxy or silicone filled
with glass spheres ("microballoons") or other insulating material
is mounted between heat sink 4810 and lower chassis 4802.
Insulating material 4806 may be mounted or sealed with adhesive or
sealant layers 4804 and 4808. FIG. 48B shows the receiver assembly
of FIG. 48A assembled. FIG. 48C shows a receiver assembly
variation. This assembly is similar to that of FIG. 48A except that
heat sink 4810 mounts to lower chassis 4802 from the top instead of
from the bottom. FIG. 48D shows the receiver assembly of FIG. 48C
assembled.
[0415] FIGS. 49A-49F show another tubular solar concentrator design
and associated frame and tracking system according to an embodiment
of the present invention.
[0416] FIG. 49A shows an isometric overview of a solar concentrator
and tracking system 4900. Three individual tubular concentrators
4902 are shown mounted to a simple base frame and tracking system
4914 to form a row of repeating units. In practice, any number of
concentrators 4902 and tracker units 4914 may be connected together
to form a system 4900 of arbitrary length. Each tubular
concentrator 4902 has a refractive front film 4904, one or more
side film(s) or membrane(s) 4906, end cap(s) 4912, and pivot joints
4908 which define a pivot axis 4910. System 4900 may be installed
so that pivot axes 4910 runs north-south, or east-west, or at a
different orientation. Frame and tracker 4914 may be anchored to
the ground via ground screws or footings, or it may be simply held
in place by sufficient ballast weight. Frame and tracker 4914 may
include adjustment mechanisms (not shown) for accommodating un-even
ground while maintaining pivot axes level and/or collinear.
Concentrators 4902 may be made to rotate about pivot axes 4910 by a
simple actuator (not shown) that applies a torque to them with
respect to frame 4914. Axes 4910 may be located at approximately
the center of gravity of concentrators 4902 so that actuation loads
may be minimized. System 4900 has features which enable it to be
used with concentrating refractive optics. While 1-D concentrating
reflective trough type systems are able to operate with a 1-axis
tracker without excessive energy losses, 1-D refractive optics
would conventionally require a 2-axis tracking system because their
region of focus would move substantially away from the plane of the
receiver if they were used on a conventional 1-D tracking system.
This would cause the receiver to be positioned away from the region
of tight focus and would result in light missing the receiver.
However, 1-axis trackers can be mechanically simpler and can enable
a higher system power rating per area of land used than 2-axis
trackers, and it would be desirable to use a refractive system on a
1-axis tracker if possible. The system described in FIGS. 49A-F
makes it possible to use refractive optics in a system that has
only a single pivot axis and has the benefits of simplicity and
packing density of traditional 1-axis trackers, while adding only
minor additional components.
[0417] FIG. 49B shows a view of one concentrator 4902 with endcaps
removed for clarity. Concentrator system 4902 keeps a receiver
assembly 4916 in the plane of focus for most or all incident light
angles by moving the plane of receiver assembly 4916 up and down as
needed. Motion of receiver 4916 is accomplished via actuator screws
4924 which act on threaded crossbars 4922 to change the distance
between receiver 4916 and the plane of film attachment defined by
crossbars 4922 and film holders 4920. There are many mechanisms and
types of actuators that can be used to move the receiver. This
particular version depicts a screw arrangement which could be
turned by a motor (not shown), but linkages, sliders, inflatable
actuators, hydraulics, linear motors, rack and pinion sets and many
other actuator types are also possible. Side films 4906 are
attached to film holders 4920. Film holders 4920 are also attached
to and sealed to expanding webs 4918 which are in turn attached and
sealed to receiver 4916. Expanding webs 4918 may have a bellows or
accordion-like shape, or they may be made of an elastic material
that stretches as needed, or they may be constructed of multiple
sealed but sliding leaves to allow the required expansion as
receiver 4916 moves. Crossbars 4922 maintain the appropriate
distance between film holders 4920 on each side so that receiver
4916 can be moved while the overall shape of concentrator 4902 and
in particular the shape of refractive film 4904 remains
constant.
[0418] FIGS. 49C and 49D show cross section views of the inflated
concentrator of system 4902 in different states. A volume 4926
containing all the refracted light paths from refractive film 4904
is shown, along with the area of tightest focus 4928. FIG. 49C
shows receiver 4916 at its most distant position from refractive
film 4904. FIG. 49D shows receiver 4916 at its closest position to
refractive film 4904.
[0419] FIGS. 49E and 49F show isometric views of the same receiver
positions shown in FIGS. 49C and 49D respectively. FIG. 49E
represents the direction of refracted rays and the correspondingly
defined volume 4926 that occur when incident light strikes
concentrator system 4902 in a direction normal to the cylinder
axis. Receiver 4916 is in a corresponding position coincident with
the tightest region of focus 4928 in volume 4926. FIG. 49F shows
what happens to the shape of volume 4926 and the location of region
of focus 4928 when incident light is not normal to the axis of the
cylinder, or is arriving at a skew angle. This would happen to some
degree most of the time because with only one axis of rotation 4910
(see FIG. 49A) which in this particular version of the system would
allow the system to track the sun's elevation angle in the sky, the
sun's varying azimuth position would generally cause light to
arrive at the refractive film with some non-normal angle with
respect to the tubular concentrator axis. The case of light
arriving exactly normal to the cylinder axis would happen
instantaneously only once per day. Note that the skewed shape of
volume 4926 in FIG. 49F would tend to suggest that the active
length of the primary optic (film 4904) be longer than the active
portion of receiver 4916. While not needed, such an arrangement
would prevent ends of the active portion of receiver 4916 from
"going dark", i.e. having some cells illuminated while others are
not.
[0420] FIGS. 50A-50E show another solar concentrator construction
according to an embodiment of the present invention. FIG. 50A shows
an exploded or pre-assembly view of some of the components of a
solar concentrator assembly 5000. An optical front film 5002 is
joined to one or more side film(s) 5004. An anchor bead 5006 lines
the perimeter of a cutout in side film 5004. During assembly,
anchor bead 5006 serves to retain film 5004 on a receiver assembly
5008. Receiver 5008 has a retaining groove 5010 into which anchor
bead 5006 fits. Anchor bead 5006 is held in place by retainer band
5012. Retainer band 5012 wedges into retaining groove 5012. FIG.
50B shows an assembled configuration of concentrator 5000. Films
5002 and 5004 are secured to receiver 5008. FIG. 50C shows a
partial cutaway view of one end of receiver assembly 5008. A heat
sink 5016 supports film retainers 5018 and round ends 5014. Both
film retainers 5018 and ends 5014 contain retaining groove 5010.
Ends 5014 have a rounded shape so that groove 5010 can be
continuous all the way around receiver 5008. FIG. 50D shows a
partial cross section view of receiver 5008. Film 5004 is retained
in film retainer 5018. Heat sink 5016 provides structural support
to the assembly. FIG. 50E shows a close-up partial cross section
view of one side of receiver 5008. Film 5004 is held in film
retainer 5018 via bead 5006. Bead 5006 is held in retaining groove
5010 by deformed retaining band 5020 which gets deformed during
installation. Retaining band 5012 shows the shape of the retaining
band before installation. Note that the shape of retaining band
5012 may be circular or square or some other shape. Retaining band
5012 may be made of rubber or other elastomer, or some other
material that can be deformed during installation. An inner groove
5024 provides extra space where bead 5006 can move while the other
side of the bead and films is being pulled over the other side of
the receiver during installation. Heat sink 5016 may be separated
from film retainer 5018 by an insulating material 5022 in order to
limit heat transfer from heat sink 5016 to film retainer 5018.
[0421] FIGS. 51A and 51B show another method for creating a heat
sink with transverse fins according to an embodiment of the present
invention. FIG. 51A shows how an extrusion of heat sink material
5102 may be cut perpendicular to its length into sections 5104.
Sections 5104 may be re-joined together perpendicular to their
original orientation at dovetail joints 5106 to form a heat sink of
arbitrary length 5108 with fins that are perpendicular to the
length of heat sink 5108. Having fins perpendicular to the length
can increase heat transfer effectiveness in many circumstances.
FIG. 51B shows a partial cross section view of a similar heat sink
dovetail joint 5112. The cross section of fins 5110 is shown more
clearly in this view.
[0422] Minimizing the levelized cost of energy depends heavily on
the choice of film material. One form of polymer which may be
suitable for use as the refractive film is polyester, examples of
which include but are not limited to polyethylene terephthalate
(PET) and similar or derivative polyesters such as polyethylene
napthalate (PEN), or polyesters made from isophthalic acid, or
other diols such as but not limited to butyl, 2,2,4,4
tetramethylcyclobutyl or cyclohexane. Polyethylene terephthalate
(PET) is good material for solar optic applications contrary to
conventional belief.
[0423] PET is the least expensive film known for the elastic
modulus. The energy to break per unit thickness for PET is well
over an order of magnitude higher than any other commodity polymer
(with the possible exception of ionomer), thus resulting in hail
resistance that is orders of magnitude better than acrylics. Third,
PET is one of the most dimensionally stable polymers with respect
to creep, temperature, coefficient of thermal expansion and water
absorption. Typically, PET has a coefficient to thermal expansion
as low as 18-20 ppm/C, which is very close to common structural
metals such as aluminum (.about.20-25 ppm/C) and less expensive
steels (.about.10-20 ppm/C), and a factor of 2-3.times. less than
typical polymers. This is an important consideration for long
structures. In the present embodiment, for example, the tubes are
23 ft. long. Even when the outside temperature is about 45.degree.
C., which is about 23.degree. C. above design reference
temperature, for PET, with a difference of less than 5 ppm/C
compared to an Al heat sink, this amounts a differential expansion
of only 0.16 inches from neutral axis (length 11.5 feet). A typical
polymer other than PET will have a differential expansion of about
30 ppm from Al, and hence a differential expansion of nearly 1 inch
under a similar scenario. Equilibrium water absorption is on the
order of 1-2%, whereas acrylics are more on the order of 2-4%.
While polymers typically have a range of properties such that an
"acrylic" can match PET in one particular property, it is clear
that PET is a superior material for the price when considering
weatherizable optical elements. Lastly PET is inexpensive enough
such that replacement of the film with time is a practical
solution. PET-based optics allow one to spend less up front, and
create an advantage due to the lower net-present-value (NPV) of
future film replacement costs.
[0424] Conventionally, PET has been regarded as generally a poor
choice for refractive optics application because PET has a high
index of refraction, high wavelength dispersion, and high
hydrolysis and photolysis conditions. However, using the techniques
described herein it is possible to use PET for refractive optic
applications. This has never been accomplished previously and all
conventional literature regarding PET teaches away from using PET
in refractive optics.
[0425] FIG. 52A shows a cross-section of an inflatable refractive
optics film 5202 for CPV according to an embodiment of the present
invention. In this figure, the film is monolithic and thus the
photolysis resistance must be within the PET material itself since
the PET faces the sun. In addition, the embossed pattern must be
embossed within the PET material. A typical thickness of bulk PET
material in this case would be 0.05 mm, while thickness between
0.012 mm and 20 mm would be possible.
[0426] FIG. 52B shows a different configuration where the embossed
pattern is made in a layer of acrylic type material 5206 which is
part of a laminate with a layer of PET 5204, according to another
embodiment of the present invention. In this embodiment, the
reflection loss exiting the prism drops from about 5.5% to about 4%
due to the lower index of the acrylic layer 5206, and the
wavelength dispersion problem is minimized as the photons are
largely refracted between air and acrylic vs. air and PET. In this
configuration, the thickness of the acrylic material 5206 would be
on the order of the thickness of the PET as long as the modulus of
the acrylic was substantially less than that of the PET. If the
acrylic modulus were high (1/2 to 1.times. the modulus of PET),
then it may be desirable to make the acrylic thinner to prevent the
dimensional instabilities of the acrylic from altering the shape
and dimensional stability of the PET.
[0427] Choosing thicknesses of the acrylic and PET layers is
non-trivial. In general, the acrylic thickness can be slightly
larger than the embossing depth. Acrylic thicknesses of 1.5 to 2
times the embossing depth are typical. For a refractive optic
design, diffraction effects will start to occur that potentially
create losses by misdirecting light when the embossing depth and/or
acrylic layer is thinner than about 0.03 mm, so for a purely
refractive design, one would tend to choose an acrylic layer
thickness that is as thin as possible (to minimize cost for
example) but thick enough so that unwanted diffraction does not
occur. However, it is also possible to create sophisticated
diffractive optic designs that use diffraction to concentrate light
and therefore are able to use shallower embossing patterns and a
correspondingly thinner layer of acrylic. Using the thinnest
practical layer of acrylic will generally create the least
expensive and most dimensionally stable films. The acrylic may also
include additives to remove unwanted portions of the solar
spectrum; those that are not converted to electricity by the cells.
For example, for a silicon receiver, the acrylic may include
absorbers that absorb radiation above 1100 nm, thereby keep the
heat off of the receiver where it can reduce the efficiency of the
cells. The acrylic can be embossed directly on the PET, or can be
embossed separately and laminated to the PET in an additional
step.
[0428] FIG. 52C shows a further optimized configuration. In this
figure, a fluorinated, silicone or otherwise radiation resistant
material coating 5208 is coated on top of a PET layer 5210, which
in turn is on top of the embossed structures in an acrylic layer
5212. There are two major advantages to this structure. First, the
UV absorbers may be placed within the radiation resistant coating
5208, which can eliminate the photolysis of the PET altogether.
Absorbed energy in the thin coating 5208 can be more easily
directed toward heat or toward less damaging mechanisms if the
polymer itself is not susceptible to radiolyis. Second
fluoropolymers in particular have low indices of refraction, on the
order of 1.3-1.4, lowering the overall reflection loss of the new
front surface of coating 5208. In addition, embossing or other
shaping of the front film may be used for the purposes of
refracting light, or may be used for the purposes of lowering the
reflection even more such as the known "moth's eye" structure
anti-reflection coating. Hence, a coating that is thick enough to
contain enough UV absorber to not allow passage of light below 390
nm, but also capable of handling the absorber without being hazy
would be preferred. Other metrics of the coating may include
abrasion resistance, oxidation resistance from either water,
oxygen, ozone or the like. A typical coating may be as thin as
0.001 mm to as thick as 1 mm, but most preferably between 0.003 mm
and 0.015 mm. The optimal coating thickness is dependent on the
lifetime that that the coating imparts vs. the cost of the coating
and is a complex trade-off with the time value of money and labor
costs, etc.
[0429] The strength, dimensional stability, and cost of PET-based
materials makes them attractive materials for inflated and
replaceable optics for CPV applications. A thin, acrylic-type
material for the actively shaped or embossed refracting optic layer
may be beneficial to improve performance. The use of an optional
weather resistant coating can keep the UV photons from reaching and
damaging the PET material. The materials mentioned above can be
used in conjunction with or in addition to the materials mentioned
herein.
[0430] FIGS. 53A-53C show another solar concentrator and receiver
system. FIG. 53A shows an exploded view of a partial length of an
inflated solar concentrator 5300 and receiver assembly 5306. The
ends of the inflated concentrator 5300 and receiver 5306 have been
removed for clarity. Inflated concentrator 5300 includes an optical
film 5302 and a back film 5304. Back film 5304 has one or more
holes 5308. An inner plate 5310 has retaining holes 5312. O-rings
5314 form a seal between film 5304 and receiver 5306 to limit the
leak rate of inflation gas out of concentrator 5300. Receiver 5306
includes securing features 5316 which secure inner plate 5310 to
receiver 5306 with film 5304 in between them. Securing features
5316 may be snap features, barbs, bayonet features, twist-lock
features, threaded retainers, taper-lock members, retaining
ring/groove members or other such feature or member that may serve
to provide positive retention between inner plate 5310, film 5304,
and receiver 5306. It may also be desirable to include spacers or
thermal insulators (not shown) between film 5304 and inner plate
5310. Inner plate 5310 serves both to provide a rigid member for
securing features 5316 to attach to or pull on and also it may
serve to protect film 5304 from concentrated sunlight during
planned or unplanned mis-pointing (i.e. times when concentrator
5300 is not perfectly pointing at the sun). Securing features 5316
pass through film holes 5308 and retaining holes 5312 in order to
securely retain inner plate 5310. Holes 5308 and 5312 allow
concentrated sunlight to reach individual cell assemblies 5318 on
receiver 5306 without being impeded by film 5304 or inner plate
5310. Inflated concentrator 5300 and receiver 5306 may be shorter
or longer than shown here. For example, a solar concentrator may
use a version longer than shown here and may include more cell
assemblies 5318 than shown here. In an embodiment, the solar
concentrator may include just a single cell assembly. Securing
features 5316 are latched onto inner plate 5310 which traps film
5304 and retains concentrator 5300 to receiver 5306. While the ends
of concentrator 5300 and receiver 5306 are removed for clarity,
these would be enclosed or sealed to limit the leak rate of gas out
of concentrator 5300. In other related embodiments, cell assemblies
similar to 5318 may attach to a film similar to 5304 independently
and without other plates or receiver structures.
[0431] FIG. 53B shows the system of FIG. 53A now assembled with
concentrator 5300 attached and sealed to receiver 5306. The ends of
concentrator 5300 and receiver 5306 are removed for clarity, but
these would be enclosed or sealed to limit the leak rate of gas out
of concentrator 5300. FIG. 53C shows a partial cross section view
of the system of FIG. 53A. This view more clearly shows how O-ring
5314 forms a seal between film 5304 and receiver assembly 5306.
Securing features 5316 are shown latched onto inner plate 5310.
Light rays 5324 are shown passing through plate 5310 and film 5304
to reach a secondary optic element 5320 which is part of cell
assembly 5318. A solar cell 5326 may be located at the lower end of
optic element 5320. Film 5304 is shown truncated for clarity. A
heat sink 5322 is in thermal communication with cell 5326 to
facilitate heat transfer from cell 5326 to the ambient air. This
view also shows more clearly how inner plate 5310 can shield film
5304 from concentrated light due to mispointing or other
phenomena.
[0432] There are several advantages of curved refractive optics
described herein. The first is that the curved lens lowers the
losses due to fabrication errors. Specifically, the shapes created
by the radius of curvature of the diamond tool can be hidden in
optically inactive areas created by the curve. This is not possible
with flat Fresnel lenses. The second advantage is that it keeps the
light away from the non-optical draft faces. The third advantage is
that it improves the chromatic aberration problem. The fourth
advantage is improved tolerance to displacement of the lens.
[0433] Having thus described exemplary embodiments of the present
invention, it should be noted by those skilled in the art that the
within disclosures are exemplary only and that various other
alternatives, adaptations, and modifications may be made within the
scope of the present invention. Accordingly, the present invention
is not limited to the specific embodiments as illustrated herein,
but is only limited by the following claims.
* * * * *