U.S. patent application number 15/055431 was filed with the patent office on 2016-06-23 for system and method for solar energy capture and related method of manufacturing.
The applicant listed for this patent is The Regents of the University of California. Invention is credited to Joseph E. Ford, Justin Matthew Hallas, Jason Harris Karp, Eric Tremblay.
Application Number | 20160178879 15/055431 |
Document ID | / |
Family ID | 42040170 |
Filed Date | 2016-06-23 |
United States Patent
Application |
20160178879 |
Kind Code |
A1 |
Ford; Joseph E. ; et
al. |
June 23, 2016 |
SYSTEM AND METHOD FOR SOLAR ENERGY CAPTURE AND RELATED METHOD OF
MANUFACTURING
Abstract
A system and method of capturing solar energy, and related
method of manufacturing, are disclosed. In at least one embodiment,
the system includes a first lens array having a plurality of
lenses, and a first waveguide component adjacent to the lens array,
where the waveguide component receives light, and where the
waveguide component includes an array of prism/mirrored facets
arranged along at least one surface of the waveguide component. The
system further includes at least one photovoltaic cell positioned
so as to receive at least a portion of the light that is directed
out of the waveguide. A least some of the light passing into the
waveguide component is restricted from leaving the waveguide
component upon being reflected by at least one of the
prism/mirrored facets, hereby the at least some light restricted
from leaving the waveguide component is directed by the waveguide
toward the at least one photovoltaic cell.
Inventors: |
Ford; Joseph E.; (Solana
Beach, CA) ; Karp; Jason Harris; (La Jolla, CA)
; Tremblay; Eric; (Fruitvale, CA) ; Hallas; Justin
Matthew; (San Diego, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Regents of the University of California |
Oakland |
CA |
US |
|
|
Family ID: |
42040170 |
Appl. No.: |
15/055431 |
Filed: |
February 26, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13119955 |
Jun 3, 2011 |
9274266 |
|
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PCT/US09/57567 |
Sep 18, 2009 |
|
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15055431 |
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61098279 |
Sep 19, 2008 |
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Current U.S.
Class: |
250/203.4 ;
29/527.1; 29/527.2; 359/622 |
Current CPC
Class: |
G02B 6/26 20130101; Y02B
10/20 20130101; G02B 3/0037 20130101; G02B 6/0078 20130101; H01L
31/0543 20141201; B29D 11/00865 20130101; Y02E 10/40 20130101; B29D
11/00009 20130101; F24S 23/12 20180501; Y02B 10/10 20130101; Y02E
10/52 20130101; G02B 3/0056 20130101; G02B 6/0053 20130101; G02B
19/0028 20130101; Y02E 10/47 20130101; G02B 19/0042 20130101; H01L
31/0547 20141201; F24S 23/31 20180501; G02B 7/005 20130101; G02B
6/0038 20130101; G01S 3/7861 20130101 |
International
Class: |
G02B 19/00 20060101
G02B019/00; B29D 11/00 20060101 B29D011/00; G02B 7/00 20060101
G02B007/00; G01S 3/786 20060101 G01S003/786; G02B 3/00 20060101
G02B003/00; G02B 6/26 20060101 G02B006/26 |
Claims
1. A system for capturing solar energy, the system comprising: a
first lens array having a plurality of lenses; a second lens array,
wherein the second lens array has a mirrored surface and wherein
each of the lenses of the second lens array is aligned with a
respective one of the lenses of the first lens array such that
light received by the system for capturing solar energy is
partially focused upon passing through the first lens array and
further focused upon reflection from the second lens array to form
a focal spot on a plane that lies between the first and the second
lens arrays.
2. The system of claim 1, further comprising an optical waveguide
layer located between the first lens array and the second lens
array, wherein the optical waveguide layer includes an array of
injection features wherein each of the injection features is
positioned to receive reflected light from at least one of the
lenses of the second lens array and to direct the reflected light
to the optical waveguide layer.
3. The system of claim 2, wherein each injection feature is
triangular in cross section, with at least one reflective surface
that directs the reflected light in the optical waveguide
layer.
4. The system of claim 2, wherein the optical waveguide layer is
separated from the first lens array by a first cladding layer and
is separated from the second lens array by a second cladding
layer.
5. The system of claim 2, wherein each of the injection features is
located at or near a focal spot of a lens of the second lens array,
and wherein each injection feature is oriented so that at least
some of the reflected light is coupled into the optical waveguide
layer.
6. The system of claim 2, wherein at least some of the light
directed to the optical waveguide layer is guided within the
optical waveguide layer by total internal reflection.
7. The system of claim 2, wherein the optical waveguide layer is
laterally shiftable relative to the first or the second lens arrays
so that incident light arriving at the system for capturing solar
energy is received by the injection features upon reflection from
the second lens array even though an angle of incidence of the
incident light arriving at the system for capturing solar energy
varies with time.
8. The system of claim 7, further comprising one or more actuators
and a controller that adjusts a position of the optical waveguide
layer relative to the first lens array and the second lens array to
enable alignment of the injection features with the foci of the
light reflected from the second lens array.
9. The system of claim 7, further comprising one or more sensors
that detect the angle of incidence of the incident light and
provide a signal to the controller.
10. A method of manufacturing a solar energy collection system,
comprising: providing a first lens array; providing a second lens
array with a mirrored coating; molding a prism array from an
ultra-violet-curable polymer film; positioning the prism array
between the first and second lens arrays.
11. The method of claim 10, further comprising: providing one or
more cladding films having lower refractive index than the first
lens array or the second lens array; and placing the one or more
cladding films between the prism array and the first lens array or
the second lens array to form a light-guide.
12. The method of claim 10, further comprising: depositing a
reflective coating on facets of the prism array.
13. The method of claim 10, further comprising: crosslinking
specific portions of the ultraviolet-curable polymer film by
exposing the first or the second lens arrays and the prism array to
ultraviolet light and subsequently rinsing the first or the second
lens arrays and the prism array in a solvent to remove uncured
polymer material.
14. The method of claim 13, wherein the ultraviolet light is
directed through the first lens array or the second lens array.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent document is a continuation of U.S. patent
application Ser. No. 13/119,955, having a filing date of Jun. 3,
2011, which is a 35 U.S.C. .sctn.371 National Stage Application of
International Application No. PCT/US2009/057567, filed on Sep. 18,
2009, which claims the benefit of U.S. Provisional Patent
Application No. 61/098,279 entitled "System and Method for Solar
Energy Capture and Related Method of Manufacturing" and filed on
Sep. 19, 2008, all of which are incorporated by reference
herein.
FIELD OF THE INVENTION
[0002] The present invention relates to solar energy systems and
methods and, more particularly, to systems and methods for
capturing solar energy that operate at least in part by way of
concentrating received light prior to conversion of the light into
electrical or other power, as well as to methods of manufacturing
such systems.
BACKGROUND OF THE INVENTION
[0003] Solar energy systems are of greatly increased interest due
to rising energy demands worldwide and consequent rising prices for
existing energy resources, especially petroleum resources. While
much effort is being focused upon developing more efficient
photovoltaic (PV) cells that can generate ever greater amounts of
electrical energy based upon a given amount of solar radiation
directed upon those cells, high efficiency PV cells nevertheless
remain expensive. A less-expensive alternative to employing high
efficiency PV cells is to employ low (or lower) efficiency PV
cells. However, such PV cells need to be implemented across larger
surface areas in order to collect sufficient solar radiation so as
to generate the same amount of energy as can developed using high
efficiency PV cells having a smaller surface area.
[0004] Although the efficiency of a PV-based solar energy system
depends upon the efficiency of the PV cell(s) employed in that
system, the amount of energy generated by such a system can also be
enhanced without increasing the efficiency of the PV cell(s) or
larger area PV cell(s) by combining the use of PV cell(s) with
additional devices that concentrate the solar radiation prior to
directing it upon the PV cell(s). Because such solar concentration
devices can employ components that are less expensive than the PV
cell(s) themselves, a solar energy system employing such a solar
concentration device in combination with PV cell(s) covering a
relatively small surface area can potentially produce, at a lower
cost, the same high level of energy output as that achieved by a
solar energy system employing only PV cell(s) of the same or
greater area. Also, a solar energy system employing such a solar
concentration device in addition to high efficiency PV cell(s)
covering a relatively small area can achieve higher levels of
energy output than would be possible using those PV cell(s) alone,
even if those cells covered a large area.
[0005] While potentially providing such advantages, existing solar
energy systems employing both PV cell(s) and solar concentration
devices have certain disadvantages as well. In particular, some
stationary solar concentration devices tend to be not very
efficient. For example, one particular type of existing solar
energy system employing both PV cell(s) and solar concentration
devices is a system employing one or more fluorescent solar
concentrators (FSCs). In such a device, light incident on the
surface of a slab waveguide is absorbed by an atomic or molecular
transition of material embedded in the slab. Upon absorption, some
of the energy is then emitted as fluorescence uniformly in all
directions, and this fluorescent light is emitted at a longer
wavelength with less energy than the incident light. While a
fraction of the emitted fluorescence is trapped within the slab,
and guided to an edge of the waveguide for illumination of a PV
cell, a large fraction of the fluorescent light is re-absorbed and
re-emitted into a non-guided direction, thus resulting in
substantial inefficiency.
[0006] An additional problem associated with some conventional
solar concentrators (e.g., imaging lens or mirror-based
concentrators) is that, for proper operation, such solar
concentrators require sunlight that is incident from a particular
direction relative to the concentrator. That is, while such solar
concentrators are able to condense/magnify light incident over a
large area onto a smaller area PV cell, such large magnifications
require precise alignment that must be maintained as the sun moves
through the sky through the daily arc, and through the seasonal
variation of elevation. Although it is possible to achieve such
alignment by way of an "active" system that uses tracking (with or
without positional feedback), such active systems are expensive and
often complicated to implement. The alternative, "passive" systems,
which do not use active alignment, can achieve only a relatively
small concentration factor (e.g., of approximately 10 suns),
depending on the range of angles over which the concentrator is
designed to maintain relatively high throughput efficiency.
[0007] Still another disadvantage associated with at least some
conventional solar energy systems employing solar concentrators is
that they are complicated and/or expensive to manufacture.
[0008] It would therefore be advantageous if an improved design for
a solar energy system employing both PV cell(s) and solar
concentration devices could be developed. More particularly, it
would be advantageous if such an improved design allowed for one to
achieve one or more of the benefits of conventional solar energy
systems employing both PV cell(s) and solar concentration devices,
while not suffering from (or suffering as much from) one or more of
the above-described disadvantages of such systems.
SUMMARY OF THE INVENTION
[0009] The present inventors have recognized the desirability solar
energy systems employing PV cells in addition to solar
concentrators, and further recognized that existing systems
employing fluorescent solar concentrators (FSCs) are advantageous
in that, insofar as they employ slab waveguides, such systems can
be more compact than many other forms of solar energy systems that
employ other forms of solar concentrators. Additionally, however,
the inventors have further recognized that a new form of solar
energy system employing slab waveguides can be achieved having
higher efficiency than existing systems if, instead of employing
FSCs, the solar concentrators instead are built by placing a lens
array adjacent to a slab waveguide formed between a low index
cladding layer and an additional layer having prism facets, with
the lens array being along the cladding layer opposite the
additional layer of the slab waveguide having the prism facets. By
appropriate design of the prism facets, total internal reflection
can be achieved within the slab waveguide with respect to much if
not all incoming light directed into the slab waveguide, and this
light can in turn be directed to one or more PV cells positioned at
one more ends/edges of the slab waveguide.
[0010] Additionally, the present inventors have also recognized the
desirability of solar energy systems that are capable of receiving
light from changing angles of incidence. Consequently, while prism
facets with constant optical properties can be employed in at least
some embodiments of the present invention, the present inventors
have further recognized that in at least some other embodiments of
the present invention the prism facets can be formed or revealed by
way of one or more materials and/or processes that allow for the
prism facet characteristics to vary, including location relative to
the microlens, depending upon the light incident upon those prism
facets. Also, in at least some other embodiments, components of the
solar energy systems can be shifted slightly in various manners to
also allow light of various angles of incidence to be received and
directed to PV cells. In some such embodiments, the waveguide with
the prism facets can be shifted relative to one or more lens
devices. Further, the present inventors have also recognized the
desirability of increasing the degree to which light is
concentrated onto less numbers of (or smaller) PV cells, as well as
the desirability of being able to receive multiple light components
rather than merely a single light component or single range of
light components, and have further developed various arrangements
that facilitate achieving such objectives.
[0011] In at least one embodiment, the present invention relates to
a system for capturing solar energy. The system includes a first
lens array having a plurality of lenses, and a first waveguide
component adjacent to the lens array, where the waveguide component
receives light, and where the waveguide component includes an array
of prism or mirrored facets (or other light-directing feature)
arranged along at least one surface of the waveguide component. The
system further includes at least one photovoltaic cell positioned
so as to receive at least a portion of the light that is directed
out of the waveguide. At least some of the light passing into the
waveguide component is restricted from leaving the waveguide
component upon being reflected by at least one of the prism or
mirrored facets, whereby the at least some light restricted from
leaving the waveguide component is directed by the waveguide toward
the at least one photovoltaic cell.
[0012] Further, in at least one embodiment, the present invention
relates to a method of manufacturing a solar energy collection
system. The method includes providing a waveguide layer, providing
a lens array in combination with the waveguide layer, and forming
prism or mirrored facets on the waveguide layer by exposing the
waveguide layer and at least one additional layer to light.
[0013] Additionally, in at least one embodiment, the present
invention relates to a method of capturing solar energy. The method
includes receiving light at a waveguide component, and reflecting
at least a portion of the received light at a plurality of prism or
mirrored facets formed along a surface of the waveguide component,
where substantially all of the reflected light experiences total
internal reflection within the waveguide component subsequent to
being reflected by the prism or mirrored facets. The method also
includes communicating the reflected light within the waveguide
component toward an edge surface of the waveguide layer, and
receiving the communicated reflected light at a photovoltaic cell
upon the communicated reflected light being transmitted through the
edge surface.
[0014] Further, in at least one embodiment, the present invention
relates to a system for capturing solar energy. The system includes
an optical waveguide layer, having an upper and lower cladding
layer, and a lens array having a plurality of lenses, disposed
above the upper cladding layer, and upon which sunlight is
incident. The system also includes an array of injection features
formed on the optical waveguide layer and arranged so that each
injection feature is located at or near the focus of a respective
one of the lenses, wherein each of the injection features is
oriented so that light focused from the lens onto the respective
injection feature is coupled into the optical waveguide layer. The
system further includes at least one photovoltaic cell positioned
along at least one edge surface of the optical waveguide, wherein
the light coupled into the optical waveguide layer is guided by the
waveguide toward and absorbed by the at least one photovoltaic
cell.
[0015] In at least one further embodiment, the present invention
relates to a solar photovoltaic system that includes a solar
concentrator that collects direct sunlight into a small-area PV
cell, overlapping in light collection area with a non-concentrated
solar panel that collects indirect sunlight into a large-area PV or
solar-thermal panel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic, perspective, exploded view of an
additional exemplary solar energy device employing components
allowing for concentration and collection of incoming light, in
accordance with at least one embodiment of the present
invention;
[0017] FIG. 2 is a schematic, cross-sectional elevation view of the
solar energy device of FIG. 1 that particularly illustrates
exemplary light paths that occur within that device;
[0018] FIG. 3 is a schematic diagram illustrating exemplary steps
of a manufacturing process that can be employed to produce a device
such as that shown in FIGS. 1-2;
[0019] FIGS. 4A-4C are flow diagrams further illustrating other
exemplary manufacturing processes that can be employed in producing
a device such as that shown in FIGS. 1-2;
[0020] FIG. 5 is an additional schematic, cross-sectional elevation
view of the solar energy device of FIG. 1 illustrating possible
exemplary operation when incident light impinging the solar energy
device is tilted relative to an axis normal to the solar energy
device;
[0021] FIG. 6 is a schematic, cross-sectional elevation view of a
further exemplary solar energy device differing from that of FIGS.
1-2, particularly in that it employs materials that react to
sunlight to efficiently couple light which is incident from a range
of angles into a waveguide layer, in accordance with another
embodiment of the present invention;
[0022] FIG. 7 is a flow chart illustrating exemplary steps of
operation of the solar energy device of FIG. 6, particularly in
terms of its reaction to positional variation of sunlight;
[0023] FIGS. 8-14 are additional schematic, cross-sectional
elevation views of additional exemplary embodiments of solar energy
devices that employ various forms of micro-tracking;
[0024] FIGS. 15-17 are additional schematic, cross-sectional
elevation views of further exemplary embodiments of solar energy
devices that allow for differ manners of extraction of light from
waveguides of the solar energy devices;
[0025] FIG. 18 is an additional schematic, perspective view of a
solar energy system in the form of a planar concentrator array;
[0026] FIGS. 19-23 are additional schematic, cross-sectional
elevation views of further exemplary embodiments of solar energy
devices that allow for various spectral components of light to be
directed to different PV cells;
[0027] FIGS. 24A-24D, 26A-C and 28B are further schematic
perspective views of portions of additional exemplary embodiments
of solar energy devices that are arranged to facilitate various
manners of concentration of light; and
[0028] FIGS. 25, 27 and 28A are additional schematic views
illustrating manners of concentration employed by some of the solar
energy devices shown in FIGS. 24A-24D, 26A-C and 28B as well as at
least one other type of solar energy device.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0029] Referring to FIG. 1, an exploded view of a solar energy
system 2 in accordance with one embodiment of the present invention
is provided. As shown, the solar energy system 2 includes a solar
concentration section 4 and multiple PV cells 6. The solar
concentration section 4 can also be referred to as a micro-optic
concentration section in view of the small size of the structure
and its components relative to the overall physical structure of
the system 2. More particularly as shown, the solar concentration
section 4 includes a lens array 8 having multiple lenses 10
arranged substantially along a plane. The lenses 10, which in the
present embodiment are formed by embossing the lenses on a surface
of glass or plastic superstrate, can be referred to as microlenses,
again in view of their size relative to the overall physical
structure of the system 2. Sunlight (or possibly other incoming
light) is incident upon an outer surface 12 of the lens array 8,
and exits the lens array by way of an inner surface 14 on the
opposite side of the lens array relative to the outer surface.
[0030] In addition, the solar concentration section 4 further
includes additional waveguide portions 19 (aside from the lens
array 8). The additional waveguide portions 19 include a low index
cladding layer 16 and a slab waveguide 18. When the solar energy
system 2 is assembled, the low index cladding layer 16 is
positioned in between the lens array 8 and the slab waveguide 18.
The low index cladding layer 16 can be, for example, a Teflon AF or
related fluoropolymer material, while the slab waveguide 18 can be
made from glass (e.g., F2 flint glass) or an acrylic polymer. The
slab waveguide 18 has a thickness 20, an inner surface 22 that is
in contact with the low index cladding layer 16 (when the section 4
is assembled) and an outer surface 24 opposite the inner surface
and separated from the inner surface by the thickness 20. A
plurality of prism facets 26 are formed along the outer surface 24.
The respective prism facets 26 are aligned with the respective
lenses 10 and the thickness 20 is determined so that a respective
focal point of each of the lenses occurs at a respective one of the
prism facets. As discussed below, in practice, the prism facets 26
are much smaller in extent relative to the lenses (for descriptive
purposes, the prism facets are not drawn to scale in FIG. 1).
[0031] The prism facets 26 are intended to be representative of a
variety of different types of injection facets or injection
features that are configured to refract, reflect, diffract,
scatter, and/or otherwise direct light incident thereon so that the
light entirely or substantially remains within the slab waveguide
18 or at least is partly restricted from exiting the waveguide, any
and all of which are encompassed by the present invention. While
the prism facets 26 particularly can be considered as injection
features that are largely or entirely refractive in their
operation, other forms of injection features also encompassed by
the present invention such as mirrored facets can be considered
largely or entirely reflective. In some embodiments, the injection
features employed will provide any one or more of refraction,
reflection, diffraction (e.g., in the form of a diffraction
grating) or scattering. As discussed further below, the prism
facets 26 (or other light directing/injection features) can be
formed using any of a variety of techniques that can involve, for
example, embossing, molding, ruling, lithography, or
photolithography. In some embodiments, the outer surface 24 of the
waveguide 18 includes an additional cladding layer in addition to
the prism facets 26.
[0032] The overall optical collection efficiency of the
concentrator/solar energy system depends upon, among other things,
the exact lateral and vertical position of the injection features
(e.g., the positions of the injection features relative to lenses),
as well as on the physical profile of the injection features (the
shapes and orientations of one or more particular facet surfaces of
the injection features). Among other things, the angle(s) of the
injection features (e.g., the angles of surfaces of the injection
features relative to the outer surface 24 of the waveguide 18 on
which those injection features are mounted) can be of significance.
Often these angles are determined in a manner that takes into
account the angle(s) at which light is expected to impinge the
injection features. For example, where light is expected to impact
one of the prism facets 26 at smaller angles (e.g., 0 to 15 degrees
relative to an axis normal to the outer surface 24 of the waveguide
18), the angle of the facet surface relative to the outer surface
24 can be 30 degrees, while where light is expected to impact one
of the prism facets at larger angles, the angle of the facet
surface relative to the outer surface 24 can be 45 degrees. It will
be understood that, in developing any given solar concentrator, one
can employ optical design software to generate injection feature
profiles that are appropriate given the combination of material
properties and physical and fabrication constraints (and expected
operational constraints) that apply to that particular
embodiment.
[0033] Further as shown, the inner and outer surfaces 22, 24 of the
slab waveguide 18 are each rectangular, such that the slab
waveguide 18 has first, second, third and fourth edge surfaces 28,
30, 32 and 34, respectively, extending between the inner and outer
surfaces, where the first and second edge surfaces oppose one
another and the third and fourth edge surfaces oppose one another.
While fully-reflecting coatings can optionally be applied to the
third and fourth edge surfaces 32, 34, the PV cells 6 are arranged
along the first and second edge surfaces 28 and 30. As shown, each
of the PV cells 6 more particularly in the present embodiment has a
width equaling the thickness 20, and extends along the entire
respective one of the oppositely-oriented edge surfaces 28, 30. The
first and second edge surfaces 28, 30 at which the PV cells 6 are
located can also be referred to as longitudinal edge surfaces since
they are at opposite ends of the length of the slab waveguide 18
and are the edges toward which light is being directed by the
waveguide.
[0034] Referring additionally to FIG. 2, a cross-sectional view of
the solar concentration section 4 of the solar energy device 2
(that is, the solar energy device 2 with the PV cells 6 removed) in
assembled (rather than exploded form) is provided, which
particularly illustrates exemplary operation of the solar
concentration section 4 in channeling light to the first and second
edge surfaces 28, 30 along which the PV cells 6 are to be mounted.
As shown, light rays (e.g., sunlight) 36 enter an exemplary one of
the lenses 10 of the lens array 8 and, upon doing so, are focused
by that lens. The focused light proceeds through the low index
cladding layer 16 and subsequently into the slab waveguide 18, with
the light then proceeding from the inner surface 22 of the slab
waveguide to the outer surface 24 of the slab waveguide.
Ultimately, the focused light reaches an exemplary one of the prism
facets 26 located along the outer surface 24 of the slab waveguide,
with the focal point of the focused light occurring at that prism
facet.
[0035] Although the incident light rays 36 focused by the exemplary
one of the lenses 10 and received by the exemplary one of the prism
facets 26 is particularly shown in FIG. 2, it will be understood
that other light rays (not shown) incident upon each of the other
lenses 10 would similarly be focused by the respective lenses and
proceed through the low index cladding layer 16 and through the
slab waveguide 18 to other respective ones of the prism facets (not
shown).
[0036] The prism facets 26 in particular are reflective facets that
are configured to reflect (or "inject") the focused light back into
the slab waveguide 18 at sharp angles such that when the light
reencounters the low index cladding layer 16 it is again reflected
into the slab waveguide. That is, once the prism facets 26 have
acted upon the focused light, the light reflected off of the prism
facets experiences total internal reflection (TIR) or at least
substantially experiences TIR within the slab waveguide 18 as far
as the light's interaction with the low index cladding layer 16,
the outer surface 24 and the third and fourth edge surfaces 32, 34
(due to the reflective coating applied thereto) is concerned. To
the extent that TIR is only substantially (but not exactly)
achieved, a small portion of the light still escapes the slab
waveguide 18 as a decoupling loss 31. Regardless, once light has
entered the slab waveguide 18 by way of the low index cladding
layer 16, all or substantially all of the light continues to
reflect repeatedly within the waveguide until it reaches either of
the first or second edge surfaces 28, 30. In the absence of the PV
cells 6, light reaching the edge surfaces 28, 30 would escape from
the slab waveguide 18 as illustrated in FIG. 2; however, in the
presence of the PV cells 6, the light reaching the edge surfaces
28, 30 enters the PV cells and is converted into electricity.
[0037] The TIR experienced by light within the slab waveguide 18 is
completely independent of wavelength and polarization over a wide
range of angles that are steeper than the critical angle. The angle
of incidence at the first and second edge surfaces 28, 30 is less
than the critical angle, so the light can be emitted through those
surfaces. In order to ensure that all (or substantially all) of the
light trapped within the slab waveguide 18 is coupled into the PV
cells 6, the PV cells typically have an anti-reflection coating (or
an index-matching layer between the waveguide and the surface of
the PV cell). It should be noted that the operation of the slab
waveguide 18 is not perfectly efficient, since the prism facets 26
that reflect the light so that TIR occurs can also act to strip the
light from the waveguide. However, in this regard, it is
significant that the diameter of the focal spots occurring at the
prism facets 26 is roughly one percent of the diameters of the
lenses 10 (e.g., a 1 mm diameter lens can produce approximately a
10 micron spot), such that the total area of the focal spot is
0.01% of the area of the lens, and such that the surface of the
waveguide is 99.99% reflective (thus light can propagate for
hundreds of lens diameters before significant amounts of light are
lost).
[0038] The shape and sizes of the prism facets 26 or other
injection features such as mirrored facets employed in any given
embodiment can vary depending upon the embodiment (indeed,
different ones of the prism facets along the same waveguide can
have different shapes/sizes). Often, the particular injection
features employed will desirably be tailored specifically for the
application. In at least one embodiment, the prism (or mirrored)
facets 26 are symmetric, triangular in cross section, and couple
light equally to the left and the right as illustrated in FIG. 2.
Such a shape is easy to fabricate due to the lack of sharp
transitions. In another embodiment, the prism (or mirrored) facets
26 can have sawtooth-shaped features that reflect light primarily
or entirely toward one or the other of the PV cells 6 at the
opposite edge surfaces 28, 30. In some other embodiments, more than
two PV cells are employed along more than two of the edge surfaces,
or only one PV cell is employed along only one of the edge
surfaces. Further, while in some embodiments there is only a single
PV cell along any given one of the edge surfaces, in other
embodiments there are more than one PV cell along one or more of
the edge surfaces.
[0039] As noted above, it is desirable that a solar energy system
employing both solar concentrators and PV cells such as the solar
energy system 2 be easily manufactured so as to reduce
manufacturing costs. Further, with respect to the present solar
energy system 2, accurate alignment of the respective prism facets
26 relative to the respective lenses 10 is an important
consideration in obtaining effective performance of the solar
energy system. While manual alignment of the prism facets 26
relative to the lenses 10 is possible, this becomes more difficult
as the prism facets 26 become smaller, which (as discussed above)
is desirable to minimize the amount of light that escapes from the
slab waveguide 18. In view of these considerations, referring to
FIG. 3, in at least some embodiments of the present invention, an
automated manufacturing process 40 is employed to create the solar
energy system 2. In particular, as will be discussed below, in at
least some such embodiments, a "self-alignment" process is employed
to form the prism facets 26. Also as will be noted, by employing
the manufacturing process 40, it is particularly possible to
manufacture solar energy systems 2 (especially the solar
concentration sections of those systems) in sheets in a batch
manner using rollers and other conventional mass-production
technologies, that is, in a roll-processing manufacturing
process.
[0040] As shown in FIG. 3, the process 40 begins at a step 41 by
applying a superstrate 42, composed of acrylic or similar material,
upon an assembly 44 including a slab waveguide portion upon which a
low index cladding layer has already been applied (such that the
cladding layer is ultimately positioned between the superstrate and
the slab waveguide portion). This can be performed by way of
rollers 46 as shown. Next, at a step 48, lens array embossing is
performed by way of an embossment roller 52 so as to form lenses 49
in the acrylic superstrate 42. Next, at a step 50, an
ultraviolet-curable epoxy serving as a molding film/photoresist 54
is further applied by a molding film application roller 56 along
the outer surface of the assembly 44 (that is, the surface which
does not face the acrylic superstrate 42). Upon being applied, the
molding film/photoresist 54 can be considered to be part of the
slab waveguide portion of the assembly 44. Additionally, at a step
58, a ruled prism master roller 60 is employed to emboss/stamp
intermediate prism facet formations 62 within the molding
film/photoresist 54.
[0041] Next, at a step 64, localized prism facets 66 are
particularly formed in the intermediate prism facet formations 62.
The localized prism facets 66 are formed in particular by shining
light from a light source (or multiple light sources) 68 through
the lenses 49, where the light in particular serves to expose
apertures in the molding film/photoresist 54. That is, the light
causes certain portions of the molding film/photoresist 54 that are
desired as the localized prism facets 66 to be cured. The light
source 68 can be a deep blue 420 nm light, for example, since this
is the longest wavelength that will crosslink the epoxy and
minimize the effect of chromatic aberrations from the lenses.
Subsequently at a step 70, a solvent bath 72 removes excess uncured
facet material (e.g., removes unused, uncured material of the
molding film/photoresist 54) such that only the localized prism
facets remained (approximately 99.99 of the molding
film/photoresist 54 is removed).
[0042] Subsequent to the step 70, additional steps (not shown)
involve spraying the bottom surface of the waveguide (that is, the
outer surface of the assembly 44 including the localized prism
facets 66 with another thin layer of low-index cladding material,
depositing a metal mirror on certain edge surface(s) of the
waveguide (e.g., the edge surface corresponding to the surfaces 32,
34 mentioned above), and then mounting the PV cells upon the
overall assembly, particularly along the remaining (unmirrored)
edge surface(s) of the slab waveguide (e.g., the edge surfaces
corresponding to the surfaces 28, 30 mentioned above).
[0043] By using the above-described process (or similar processes)
of manufacture, it is possible to create solar energy systems such
as the solar energy system 2 of FIGS. 1-2 having a variety of
dimensions and optical characteristics. For example, the
concentration power of the lenses 10 can vary depending upon the
embodiment and, in one exemplary embodiment, the concentration
power of the lenses is 500 suns. Also, in one embodiment, the
length of the slab waveguide 18 (that is, the distance along which
light is intended to flow through the waveguide, e.g., the distance
between the two PV cells 6 at the edges 28, 30) can be of any
arbitrary length, for example, several meters long. Likewise, the
width of the slab waveguide 18 (that is, the distance across the
slab waveguide perpendicular to the distance along which light is
intended to flow, and perpendicular to the thickness 20) can be
arbitrarily large or small, for example, 500 millimeters or
alternatively 1 meter. Also, the thickness 20 can be arbitrarily
large or small. Typically, it is desired that the thickness 20 be
small, and/or that the thickness at least in part be determined by
the characteristics (e.g., the concentration powers/focal lengths)
of the lenses 10. In one embodiment, further for example, the
lenses 10 are F/2.9 lenses and the thickness is only 6 mm (which is
far less than for thin parabolic reflector optics).
[0044] A variety of other operational processes are also intended
to be encompassed within the present invention in addition to that
described above with respect to FIG. 3. For example, with respect
to FIG. 4A, a modified version of the process 40, shown as a
process 80, includes a slightly-different set of operational steps
than those shown in FIG. 3. More particularly as shown, the process
80 begins at a step 82 with the addition of low index cladding to
one (e.g., the top) surface of a slab waveguide, continues at a
step 84 with the addition of a photosensitive polymer molding agent
to an additional, opposed (e.g., the bottom) surface of the slab
waveguide, and further continues at a step 86 with the formation of
prism facets in the molding agent using a ruled master. Next, at a
step 88, a reflective coating is placed onto the prism facets, at a
step 90 a lens array is attached to the low index cladding (e.g.,
attached to the top surface of the slab waveguide as modified to
include the cladding), and at a step 92 the molding agent is
exposed to light via the lens array. Finally, at a step 94 mold
development occurs, followed by remaining actions (e.g., attachment
of the PV cells) that result in a final device at a step 96.
[0045] Further for example, FIG. 4B shows an additionally modified
version of the process 80, shown as a process 100. As shown, the
process 100 includes steps 102-108 that respectively correspond to
the steps 82-96 of the process 80 respectively (except insofar as
no step corresponding to the step 88 is included). Additionally, a
step 101 shown to precede the step 102 merely is indicative of the
fact that a slab waveguide is provided prior to the application of
the low index cladding layer in the step 102, and a step 98 is
shown to be performed between the steps 106 and 107, in which a
reflective coating is applied to the slab waveguide (e.g., along
edge surfaces such as the edge surfaces 32, 34 of FIG. 1). (The
step 98 can be considered a substitute for the step 88 of FIG.
4A).
[0046] Additionally for example, FIG. 4C shows in yet another form
a process 110 for manufacturing a solar energy system such as the
system 2 of FIGS. 1-2. As shown, the process 110 begins at a step
111 by applying an un-crosslinked photopolymer coating onto a
waveguide. Next, at a step 111, a mold is applied to the
photopolymer coating and additionally a pull vacuum is applied.
Subsequently, at a step 112, the waveguide, photopolymer coating
and mold are baked with weight (pressure) applied, particularly
pressure upon the mold tending to compress the assembly as shown.
Further, at a step 113, the mold is removed. The removal of the
mold leaves the molded photopolymer coating exposed. The waveguide
and molded photopolymer coating is at this time then inverted.
[0047] Next, at a step 114, a lens array is attached to the
waveguide along its side that is opposite the side on which the
photopolymer coating is attached. As illustrated particularly in
FIG. 4C, ultraviolet light is further directed so as to be incident
upon the lens array. The ultraviolet light in turn passes through
the lens array and the waveguide and then reaches the molded
photopolymer coating. Due to the focusing of the lenses of the lens
array, the ultraviolet in particular only reaches (is focused upon)
specific portions of the molded photopolymer coating, and these
specific portions of the coating in turn become crosslinked
photopolymer. Next, at a step 115, a reflective coating is
deposited upon the exposed outer surface (that is, the surface not
in contact with the waveguide) of the molded photopolymer coating,
including the un-crosslinked and crosslinked portions. Finally, at
a step 116, the overall assembly is heated above a glass transition
temperature (Tg), the un-crosslinked portions of the photopolymer
coating are removed (so as to complete formation of prism facets)
and a completed solar concentration section, suitable for
implementation in a solar energy system such as the system 2,
results.
[0048] The solar concentrators within the above-described solar
energy systems including the solar concentrator section 4 of the
solar energy system 2 can be referred to as passive solar
concentrators. In such solar concentrators, the
refractive/reflective properties of the lenses and prism facets 10
are fixed such that variation in the angle of incidence of incoming
sunlight (or other light) as a function of movement of the sun (or
otherwise) alters the degree of concentration and efficiency of the
device. Referring to FIG. 5 in this regard, for example, a
cross-sectional view of the solar energy system 2 of FIG. 1 is
provided with respect to which incident light 157 is shown to be
tilted relative to a normal axis 159 (which is perpendicular to the
surfaces 22, 24 of the slab waveguide 18). As shown, if the
incident light 157 is tilted in this manner, the light after
passing through a given one of the lenses 10 of the lens array 8 is
no longer directed toward the respective one of the prism facets
26, but rather misses the prism facets. Assuming that the outer
surface 24 is transparent generally, that light can pass completely
out of the slab waveguide 18 such that it is no longer directed to
the PV cells 6 but rather simply is uncoupled light.
[0049] To reduce or minimize the amount of incident light that is
lost due to the light being imperfectly aligned with the solar
energy system 2 as illustrated by FIG. 5, in at least some
embodiments the solar energy is modified, takes other forms, or can
be operated in particular manners than as discussed above. In at
least some embodiments, for example, to allow for enhanced
performance of such a solar energy system notwithstanding variation
in the angle of incidence of incoming light, the system in at least
some embodiments is mounted upon (or otherwise implemented in
conjunction with) an active alignment system.
[0050] Further, as noted above, in at least some additional
embodiments, the prism facets 26 are configured to be more tolerant
of variations in the incidence angles of light impinging the solar
energy system. Indeed, in at least some such embodiments, the
upward-facing lenses 10 can themselves be used during the
construction of the system 2 to identify and form the location of
the coupling prism facets 26, for example, as shown in the step 68
of FIG. 3. That is, while in some embodiments, the prism facets 26
that are formed in the manner described with respect to FIG. 3 are
localized discrete facets intended to receive incoming light along
a particular path, it is possible in other embodiments for the
angle and intensity properties of the light used during the
exposure to be altered so as to form prism facets that are arcs or
other structures instead of merely localized discrete facets. When
appropriately configured, such arcs or other structures can direct
light into the waveguide 18 even though the incoming path of the
light varies with the path taken by the sun over the course of a
day. Thus, through a customized facet exposure/formation process to
form such arcs and other structures, and the subsequent use of such
arcs or other structures, which can be said to "mimic" the path of
the sun, the daily collection efficiency of the system 2 can be
enhanced even when only using relaxed or no active solar
tracking.
[0051] Further, in other embodiments of the present invention, it
is envisioned that certain physical characteristics of the solar
concentrators, and particularly the prism facets/injection features
or coupling medium, will actively respond to variations in the
angle of incidence of incoming sunlight (or other light) and thus
performance of the solar concentrators will be enhanced, in the
absence of (or in addition to) any active alignment system. The
solar concentrators of such embodiments, which can be referred to
as reactive concentrators, operate by providing a large area region
that can temporarily form (or reveal) prism facets/injection
structures using a material that reacts to bright light at or near
the focus of a lens. This creates a local change in the optical
properties, which covers only a small fraction of the total area
within the total guiding structure. As the sun's illumination angle
changes, the positions of these prism facets/injection
features/defects passively react and move along with it. Thus, such
reactive concentrators do not require active alignment or tracking
to capture and convert specular sunlight into electricity.
[0052] Various embodiments of solar energy systems with reactive
solar concentrators are possible. As noted, in some embodiments,
the sun's heat and/or illumination is used to form the locations of
the prism facets. This can be in the form of thermal expansion or
other mechanical motion to bring the prisms in close contact with
the guiding layer. In other embodiments, the prism facets are
positioned just along the outer surface of the slab waveguide, just
outside of that surface (that is, outside of the waveguide). An
intermediate medium that responds to the location/intensity of the
sun causes a localized physical change in the refractive index at
the point of focus allowing light that is reflected off the prisms
to be coupled into the high-index guiding slab waveguide. A
localized high refractive index surrounded by a low-index cladding
is desirable (or necessary) for the purpose of allowing the prism
to encounter incoming light once and not adversely strip already
guided light.
[0053] Depending upon the embodiment, several potential phenomena
are available to generate the necessary localized index change. In
at least some embodiments, a colloidal suspension of high index
nanoparticles in a lower index fluid similar in optical properties
to the outer cladding is provided. The particles can be smaller in
size than the wavelength of light, and therefore seen as average
and not individual scattering particles. An accumulation of high
index particles causes the perceived index of refraction to rise
creating the coupling window between the reflective prism facets
and the guiding slab while still maintaining the lower-index
cladding surround. One method for initiating this perceived index
increase is using optical trapping forces inherent to high
illumination flux. Other embodiments can incorporate
photoconductive or weakly photovoltaic polymers which generate an
electric field in the presence of intense illumination. The
resulting field can exert forces on the high index particles
causing them to migrate towards the areas of maximum flux,
generally occurring at the point focus of each lens in the array.
The system is still reactive in that the polymer can be placed
everywhere behind the guiding slab and not require individually
patterned electrodes. Other optically induced physical changes may
aid in the coupling of light such as photochromic, photothermal or
phase change materials.
[0054] Referring to FIG. 6, a side elevation view of one exemplary
embodiment of a solar energy system 122 employing a reactive
concentrator section 124 in addition to PV cells 126 is shown. As
shown, the reactive concentrator section 124 in particular is
formed from several layers stacked together. On the top is an array
128 of lenses 130 used to form focal points from the incident
sunlight. A low index cladding layer 136 exists just below the
lenses 130 followed by a high index guiding layer (e.g., a slab
waveguide layer or core) 138. Lastly a mirror (reflective)
microstructure 140 sits below the high index guiding layer 138 with
a gap 142 filled by a colloid in suspension (colloidal suspension)
144. The colloid 144 contains a low index fluid or gel with high
index particles evenly dispersed within, achieving an average index
similar to the cladding layer 136 found above the guiding layer
138. The PV cells 126 are placed at edge surfaces of the high index
guiding layer 138 as with the solar energy system 2 of FIGS. 1-2
(also, while not shown, reflective coatings are placed on the other
edge surfaces).
[0055] Turning to FIG. 7, the solar energy system 122 can be
understood as operating generally according to a process 150. Upon
providing of the solar energy system 122 at a step 152, sunlight is
incident upon the solar concentrator section 124 at a step 154.
Upon illumination, the lenses 130 focus the light so that it passes
through all of the layers of the solar concentrator section 124
(e.g., the layers 136 and 138) so as to be incident on the mirror
microstructure 140 as spots. Next, at a step 156, with high
illumination flux, significant optical trapping forces are exerted
on the particles suspended in the colloid 144. Particles outside
the illumination cone undergo Brownian motion causing them to
constantly migrate. Over time, more particles can be trapped by the
illumination causing a local grouping of high index particles.
Since each is significantly smaller than the wavelength of light,
the sunlight only sees the average index of refraction which is
increased by the accumulation of particles. Thus, a high index
channel is created for light to couple into the guiding layer, and
prism facets are created within the colloid 144. As discussed in
relation to the solar energy system 2 of FIGS. 1-2, the coupling
windows should remain small to reduce the probability of a ray
which is already guided from seeing the microstructure beneath and
scattering out of the core (this will ultimately limit the distance
light can be guided and can be a main consideration of design).
[0056] Upon completion of the step 156, the angled prism facets of
the mirror or grating reflect light at angles necessary to achieve
TIR, such that the light reflected by the prism facets couple
directly into the layer 138 (rather than refracting into and out of
the various layers), and eventually then are channeled toward the
PV cells 126, at which electrical power is then generated, as
indicated by a step 158. Further, since the colloidal accumulation
is optically induced and occurs locally, the system is able to
react to the position of the sun. That is, as indicated by a step
160, over time the angle of incidence of the sunlight upon the
lenses 130 changes. When this occurs, the colloid 144 further
responds so as to result in modified prism facets at the step 156.
Thus, continued movement of the sunlight results in repeated
performance of the steps 156, 158 and 160 (on a continuous
basis).
[0057] In at least some embodiments, the colloid 144 can involve
the suspension of titanium dioxide (TiO.sub.2) particles. These are
subwavelength particles with a very high index of refraction and
have potential to be easily trapped and manipulated with sunlight.
The particles will likely be coated with silica, etc., to avoid
clumping due to Vander Waals forces. In at least one such
embodiment, the colloid 144 includes both the titanium dioxide
particles, which are nanoscale, high dielectric index particles,
and also dense but low index of refraction fluoropolymer material,
within which the particles are contained. During operation, the
photosensitive material repeatedly senses and responds to changes
in electric fields of portions of the light, by drawing in some of
the high dielectric index particles (that is, due to the light
exposure, some of the particles move from one location to another
within the overall colloid) so as to achieve optical trapping. In
other embodiments, other materials can be used as the colloid.
Also, the colloidal solution is only one of many potential methods
for creating a high index window to couple to the waveguide/core.
Other static and mechanical possibilities exist as well as active
electrical addressing. Phenomena including dielectrophoresis can
also be utilized to manipulate the location of particles. It will
be further understood that the solar energy system 122 of FIG. 6
employing the reactive solar concentrator 124 can be manufactured
using processes similar to (albeit not identical to) the processes
described above with respect to FIGS. 3-4B.
[0058] Notwithstanding the above discussion, in still additional
embodiments of the present invention various techniques can be
employed by which the solar energy system, rather than using full
active tracking, instead employs micro-tracking features in which
one or more components of the solar energy system are moved
slightly relative to other components so as to achieve improved
performance by the solar energy system in terms of its ability to
receive and couple light to the PV cells 6 even when that light is
incident in a tilted manner and/or varies in its angle of incidence
over time. These slight movements can involve, for example, both
lateral movements (that is, movements of the waveguide side-to-side
but not toward or away from a lens array), as well as vertical
movements (that is, movements of the waveguide toward or away from
a lens array). Turning to FIG. 8 in particular, in one such
embodiment a solar energy system 162 includes not only one or more
PV cells 6 and the solar concentration section 4 with the slab
waveguide 18 and the lens array 8 with the lenses 10 (as well as
the prism facets 26) of the solar energy system 2 discussed above,
but also includes first, second and third additional lens arrays
166, 167 and 168.
[0059] As shown, each of the lens arrays 166-168 includes a
plurality of individual lenses 169. More particularly, the lenses
of the first, second and third lens arrays 166, 167 and 168 are
respectively arranged along first, second and third planes parallel
to the plane along which the lens array 8 is arranged, with the
third, second and first planes being positioned successively
outwardly away from the lens array 8. In the present embodiment,
each of the lenses 169 of each of the lens arrays 166-168 is
identical. However, in other embodiments the lenses of the
different lens arrays 166-168 can be different from one another
and, indeed, in at least some embodiments different lenses of a
given one of the lens arrays 166, 167 and/or 168 can also differ
from one another. In the present embodiment, the lenses 169 of the
different lens arrays 166-168 can be considered micro-lens arrays
since the lenses are typically small in diameter (and equal in
diameter to the lenses 10 of the lens array 8).
[0060] The lenses 169 of the lens arrays 166-168 are intended to be
moveable relative to one another and/or the lenses 10 of the lens
array 8 such that incident light that is incident upon the solar
energy system 162 (and particularly incident upon the lenses of the
lens array 166) at a variety of angles can still be ultimately
directed in a manner so that the light is normally incident upon
the lenses 10 of the lens array 8, that is, parallel or
substantially parallel to the normal axis 159. In the present
embodiment, the lens array 167 in particular is moveable along an
axis of movement represented by an arrow 170 that is parallel to
the inner and outer surfaces 22, 24 of the slab waveguide 18 and
thus perpendicular to the normal axis 159. By appropriately
adjusting the second lens array 167 relative to the other lens
arrays 166, 168 (and 8), incident light 171 that is tilted relative
to the normal axis 159 thus can be redirected so as to be normal
upon the lens array 8 in a manner that is parallel or substantially
parallel to the normal axis 159. Thus, even though the incident
light 171 is tilted, light is effectively received and coupled by
the solar concentration section 4 as if it were normally received
and thus the solar concentration section is able achieve effective
coupling of the light to the PV cells 6.
[0061] The embodiment shown in FIG. 8 employs a triplet of
micro-lens arrays where the second lens array 167 in particular
serves as a field lens that increases the fill factor at the output
of the lens arrays (that is, the light as it proceeds toward the
lens array 8). Nevertheless, in other embodiments other lens
arrangements can also be employed. For example, in one other
embodiment, only two lens arrays are employed (albeit such an
embodiment can suffer from somewhat limited steering range and
increased number of surface reflections, with the limited steering
range being partly the result of the arising of spurious rays). In
other embodiments, more than two lens arrays are present. Also,
depending upon the embodiment, not merely the second lens array 167
but also (or instead) one or another of the lens arrays 166, 168
(and/or 8) can be moved. By appropriately moving such one or more
lens arrays over time, changes in the direction of incident light
as can be associated with movement of the sun over the course of a
day (or as may occur for other reasons as well) can be largely
compensated for, and thus, operation of the solar energy system 162
can continue unimpeded or largely unimpeded throughout the day.
[0062] Turning next to FIG. 9, an additional embodiment of a solar
energy system 172 also employs a micro-tracking capability that
differs from that of FIG. 8. In the embodiment of FIG. 9, the solar
energy system 172 can be understood to include the both the lens
array 8 as well as the additional waveguide portions 19 of the
solar concentration section 4 of the solar energy system 2 of FIG.
1 (e.g., the waveguide 18 with the prism facets 26, as well as
possibly the cladding layer 16). However, in contrast to the solar
concentration section 4, in this embodiment the lens array 8 is
moveable relative to the additional waveguide portions 19 of the
solar concentration section, such that the additional waveguide
portions can be moved relative to the lens array 8 back and forth
along a direction indicated by an arrow 177, the direction
represented by the arrow 177 being parallel to the outer and inner
surfaces 22 and 24 of the waveguide 18. In at least some such
embodiments, a space 178 between the lens array 8 and the
additional waveguide portions 19 can exist to facilitate such
movement (such space can be filled with air or other cladding). By
appropriately moving the additional waveguide portions 19 (this
movement can involve a sliding movement along a bottom surface of
the lens array 8) the additional waveguide portions can be
positioned relative to the lenses 10 such that incident light 174
that is incident upon the lenses in a tilted manner relative to the
normal axis 159 still is focused upon appropriate ones (in this
example, an appropriate one) of the prism facets 26. Thus, even
thought the incident light 174 is tilted, the light ultimate
experiences TIR within the waveguide 18 and is directed to the PV
cells 6.
[0063] As discussed with respect to FIG. 8, it will be understood
that the appropriate positioning of the additional waveguide
portions 19 relative to the lens array 8 will vary depending upon
the particular angle of incidence of the incident light 174
relative to the normal axis 159 and thus, as that angle of
incidence changes (e.g., again due to movement of the sun during
the course of the day or for some other reason) the relative
positioning of the additional waveguide portions relative to the
lens array 8 will need to be appropriately modified so that the
incident light continues to be directed towards one or more of the
prism facets 26. Such appropriate positioning can be governed by a
controller such as a microprocessor (not shown) that receives
signals from one or more light sensors (also not shown) that detect
the angle(s) of incidence of the incident light 174 (or at least
predominant or substantial component(s) of that light) and based
upon such received signals in turn adjusts the relative positioning
of the additional waveguide portions 19 vis-a-vis the lens array 8.
In general, the amount of shifting of the additional waveguide
portions 19 relative to the lens array 8 will correspond to the
degree of tilt in the incident light; increased tilt will typically
require increased amount of shifting. Although the amount of
shifting required to achieve a desired effect will vary depending
upon the embodiment, the amount of shifting will often be quite
small (e.g., on the order of 1 millimeter or less).
[0064] Turning to FIG. 10, in yet another embodiment of a solar
energy device 182, not only is there present the lens array 8 as
well as the additional waveguide portions 19 (and possibly the
space 178 separating the two), but also there is an additional
diffuse light collector 184 (which for example can be a large area
PV cell panel or solar-thermal panel) that is positioned outside of
the additional waveguide portions alongside the outer surface 24.
Given such an arrangement, incident light 186 that is
well-collimated is directed towards the prism facets 26
(particularly assuming that the additional waveguide portions 19
are appropriately aligned relative to the lens array 8), while
other diffused light 188 that is incident upon the lens array 8 is
not directed towards the prism facets 26 but instead is allowed to
pass through the slab waveguide 18 completely and so is received at
the diffuse light collector 184. Thus, well-collimated incident
light is provided to the PV cells 6 while diffuse light is received
at the diffuse light collector 184. Notwithstanding the
effectiveness of the solar energy systems 162, 172, 182 discussed
above with respect to FIGS. 8-10, the effectiveness of such solar
energy systems can still be somewhat limited depending upon field
curvature 178 as illustrated in FIG. 9.
[0065] Next, referring to FIGS. 11-12, a further exemplary solar
energy system 192 is shown in two different operational positions.
As shown, the solar energy system 192 includes waveguide portions
194 that are similar to the additional waveguide portions 19
discussed above, and that particularly include a slab waveguide 195
and prism facets 193 by which light is directed to PV cells 196 at
opposite ends of the slab waveguide. Additionally, the solar energy
system 192 further includes a lens array 198 having a plurality of
lenses 199. Further, as in the solar energy system 172, the
waveguide portions 194 (and PV cells 196 mounted in relation
thereto) are laterally shiftable relative to the lens array 198.
However, in contrast with the solar energy system 172, the solar
energy system 192 is configured to receive incident light that
first impinges the system at an outer surface 191 of the waveguide
portions 194 along which the prism facets 193 are located rather
than at the lenses 199 of the lens array 198. More particularly as
shown, incident light 200 passes through the outer surface 191,
proceeds through the slab waveguide 195 and through an inner
surface 201 of the slab waveguide (again at which can be provided a
cladding layer), then through an air gap (or other possible
cladding) 203 between the waveguide portions 194 and the lens array
198, and then through the lens array to the lenses 199. Upon
reaching the lenses 199, the light is then reflected by the lenses
back generally in the opposite direction toward appropriate ones of
(in this case, one of) the prism facets 193, at which point the
light experiences TIR and is directed to the PV cells 196. It will
be understood that, to achieve operation in the above-described
manner, the outer surface 191 of the slab waveguide 195 is
substantially transparent, while the lenses 199 are mirrors (or are
lenses with a mirror coating applied thereto). The lenses 199 in
the present embodiment can be more appropriately termed
micro-mirrors given their small size.
[0066] While FIG. 11 shows the light 200 incident upon the
waveguide portions 194 to be normal to waveguide portions (that is,
perpendicular to the outer and inner surfaces 191, 201), the solar
energy system 192 again allows for incident light that is tilted to
also be captured and directed towards the PV cells 196. In
particularly referring to FIG. 12, tilted incident light 189 also
can be successfully directed to the PV cells 196 by laterally
shifting the waveguide portions 194 relative to the lens array 198
by an appropriate amount along a direction (back and forth along
the direction) represented by an arrow 187. It should further be
noted that the use of the solar energy system 192 is particularly
advantageous insofar as, due to the flatness (and typically
robustness) of the outer surface 191, the solar energy system
realizes improved durability of packaging and ease of cleaning.
[0067] Turning to FIGS. 13 and 14, a further solar energy system
202 is shown in accordance with another exemplary embodiment of the
present invention in which slight movements of system components
allow for tilted incident light to be captured at the PV cells of
the device. The solar energy system 202, for reasons that will be
understood in view of the discussion below, can be particularly
termed a micro-catadioptric concentrator system. Referring
particularly to FIG. 13, the solar energy system 202 among other
things includes waveguide portions 204 including a slab waveguide
205 having first and second surfaces 206 and 208 that are opposed
to one another on opposite sides of the waveguide, and further
having prism facets 210 that are positioned along the surface 206.
PV cells (one of which is shown) 212 are positioned at one (as
shown) or more edge surfaces of the waveguide 205. Additionally,
the system 202 also includes a lenslet array 214 that is positioned
along (and spaced apart from) the first surface 206 of the
waveguide 205 and a micro-mirror array 216 that is positioned along
(and spaced apart from) the second surface 208 of the waveguide. In
the present embodiment, air gaps (or other cladding) 218 are
provided between the lenslet array 214 and the first surface 206 as
well as between the micro-mirror array 216 and the second surface
208.
[0068] As with the solar energy system 192 and 172 discussed above,
the waveguide portions 204 and associated components (e.g., the PV
cells 212) can be laterally shifted relative to the lens components
of the device, namely, laterally shifted relative to both the
lenslet array 214 and the micro-mirror array 216 back and forth
along a direction represented by an arrow 220. When in the position
shown in FIG. 13, incident light 222 that is parallel to an axis
230 normal to the slab waveguide 205 (that is, perpendicular to the
surfaces 206, 208) initially impinges the solar energy system 202
at the outer surface of the lenslet array 214, after which it
passes through the lenslet array (which causes some focusing of the
light), through the air gap 218 between that lenslet array and the
waveguide portions 204, through the waveguide portions including
the slab waveguide 205, through the additional air gap 218 between
the waveguide portions and the micro-mirror array 216 and up to an
outer surface 224 of the micro-mirror array. As with the solar
energy system 192, at this point the light is reflected by the
micro-mirror array 216 back inward towards the slab waveguide 205
and eventually passes through the slab waveguide and to appropriate
ones of (in this example, one of) the prism facets 210, as a result
of which the light experiences TIR and proceeds to the PV cells
212.
[0069] Further, as shown in FIG. 14, with an appropriate lateral
shifting of the waveguide portions 204 (and PV cells 212) relative
to the lenslet array 214 and the micro-mirror array 216, incident
light 226 that is tilted relative to the axis 230 is largely also
directed eventually to the PV cells 212. As illustrated, while most
of the tilted incident light 226 eventually finds its way to the PV
cells 212, a small amount of light is vignetted light 231 and
escapes the system 202. As with respect to the embodiments
discussed with reference to FIGS. 8-12, the solar energy system 202
can achieve successful coupling of incident light to the PV cells
212 for incident light that is tilted at a variety of angles, it
being understood that as the degree of tilting increases the degree
of shifting will also need to increase. It will be understood that,
in any given embodiment, it is possible for one or more actuators
to be controlled to move waveguide portions relative to lens array
structures (including multiple structures such as both the lenslet
array 214 and the micro-mirror array 216), with those lens array
structures being stationary, or vice-versa, or to move all of the
different components in various directions.
[0070] In the above-described embodiments of solar energy systems,
PV cells are positioned along edges of slab waveguides so as to
receive light directed by the slab waveguides to and outward form
those edges. However, the confining of light at known angles within
waveguides as is achieved in such solar energy systems does not
mandate that PV cells be oriented in such manners to receive that
light. Rather, depending upon the embodiment, additional
arrangements are possible that allow for repositioning of PV cells
or light extraction in a manner that achieves additional
concentration. More particularly, referring now to FIGS. 15-18,
solar energy systems such as those discussed above can be modified
in additional manners that facilitate the communication of light
within the slab waveguides to PV cells that are intended to receive
that light that are positioned in a variety of manners, and/or
facilitate light extraction in a manner by which greater
concentration is achieved.
[0071] For example, with respect to FIG. 15, a modified version of
the solar concentration section 4 of the solar energy system 2 of
FIG. 2, referred to as a solar concentration section 234, is shown.
The solar concentration section 234 in particular has, as shown,
not only a lens array 232 with a plurality of lenses 236 as well as
a slab (uniform-thickness) waveguide 238 having an outer surface
240, an inner surface 242 and a plurality of prism facets 246, but
also additionally a fold prism 248 positioned at an edge 250. The
fold prism 248 serves to rotate the light emanating from the
waveguide 238 from lateral to downward propagation (e.g., a 90
degree rotation), which allows a PV cell (not shown) to be placed
underneath the waveguide so as to be parallel to the outer surface
240 of the waveguide 238 rather than along the edge 250 of the
waveguide.
[0072] Referring additionally to FIG. 16, through the use of
multiple solar energy systems each employing the solar
concentration section 234 of FIG. 15, further concentration of
light for reception by a PV cell (and/or ease of manufacture of the
overall solar energy system) can be achieved. For example, by
positioning two of the solar concentration sections 234 end-to-end,
where the sections include respective fold prisms 248, light from
the two solar concentration sections 234 can be directed to a
single PV cell 251. Thus, only the single PV cell 251 is necessary
for capturing light from two of the sections 234. It will be
understood that, in further embodiments, more than two (e.g., 4)
solar concentration sections can effectively share the same PV cell
in a similar manner.
[0073] Referring to FIG. 17, portions of another exemplary solar
energy system are shown. In this embodiment, the solar energy
system includes two solar concentration sections 254. As with the
solar concentration section 234, each of the solar concentration
sections 254 again includes a respective lens array 252 with a
respective plurality of lenses 256 as well as a respective slab
(uniform-thickness) waveguide 258 having a respective inner surface
262 and a respective outer surface 260, along which are formed a
respective plurality of prism facets (not shown). Each solar
concentration section 234 can have a thickness (that is, as
measured between the outer surface of the lens array 252 and the
outer surface 260) of, for example, 2 millimeters. Additionally,
positioned between the solar concentration sections 254 is a curved
mirror reflector 268 oriented so as to be concave toward the plane
of the inner surfaces 262. The curved mirror reflector 268 can be
any of a variety of different curved shapes depending upon the
embodiment and, for example, can be an aspheric mirror reflector or
a curved mirror reflector. In the present embodiment, the curved
mirror reflector 268 extends outward away from the waveguides 258
farther than do the lens arrays 252, although this need not be the
case in all embodiments.
[0074] The curved mirror reflector 268 receives light provided to
it from the waveguides 258 as that light proceeds out of the ends
of the waveguides, and in turn focuses the light toward a central
location 266 between the solar concentration sections 254 generally
along the plane determined the outer surfaces 260. Again, as with
respect to the system of FIG. 16, a packaged PV cell 270 can be
positioned at this central location as shown so as to receive the
focused, concentrated light. Thus, both the fold prisms 248 and the
curved mirror reflector 268 of FIGS. 16 and 17, respectively, serve
to rotate the light emanating from the waveguides 238, 258, from
lateral to downward propagation, albeit the curved mirror reflector
provides the added benefit of further concentrating the light for
receipt by the PV cell 270. Such concentration not only allows
potentially the use of a smaller PV cell (which is desirable, due
to the cost of larger PV cells), but also allows the PV cell to be
more effectively operated (typically, PV cells achieve greater
efficiency of operation upon receiving more intense light).
[0075] In view of the embodiments of FIGS. 15-17, it should be
further evident that, depending upon the embodiment, two (or more)
opposing solar concentration sections can be joined so as to couple
bi-directional (or multi-directional) light into a common PV cell.
Also, symmetric couplers enable linear arrays of micro-optic
concentrators. Referring additionally to FIG. 18, for example, a
planar concentrator array 272 is shown in cutaway to include six
solar concentration sections 254 of the type shown in FIG. 17 (the
waveguides 258 being shown in particular) and four of the curved
mirror reflectors 268, with each of the reflectors being positioned
between two corresponding ones of the solar concentration sections
(where to of those sections are between two of the reflectors). The
curved mirror reflectors 268 direct light received from the solar
concentration sections to PV cells (not shown) positioned beneath
four different curved mirror reflectors 268. Notwithstanding the
particular structure shown, it will be understood that any
arbitrary number of solar concentration sections and curved mirror
reflectors of this type can be assembled into a larger structure in
this manner. Such a structure is not only easy and convenient to
fabricate but also in some cases can be easily stored (e.g., the
planar array can potentially be rolled up).
[0076] As already mentioned, increased concentration of light onto
a given PV cell can improve the performance of the PV cell. Output
coupler designs such as those discussed above using curved (e.g.,
aspheric or parabolic) mirrors (instead of planar fold prisms) are
particularly capable of remapping guided ray angles and focusing
light onto a given PV cell. Additionally it can be noted that
reflective surfaces with optical power enable another stage of
concentration in addition to the increased flux gained from
coupling light into the waveguide. Combining two methods of
concentration allows the system to efficiently reach high levels of
flux needed for multi junction PV cells. Many potential designs
have been explored and vary based on the waveguide modes, yet most
embodiments utilize at least one curved mirror to collect diverging
light as it leaves the waveguide.
[0077] An additional factor influencing the performance of a PV
cell is the degree to which the PV cell is suited to receiving the
particular light spectra that are provided to it. Turning next to
FIGS. 19-23, in at least some embodiments of the present invention,
solar energy systems are configured to differentiate between/among
different light spectra and to direct different light components to
different PV cells that are particularly well-suited for receiving
those respective light components. In at least some such
embodiments, dielectric mirrors are incorporated into the solar
concentrator design to split broad spectrum illumination into
multiple bands for collection using specialized PV cells.
[0078] Referring to FIG. 19, in one such embodiment a solar
concentration section 274 is employed. The solar concentration
section 274, as shown, is similar to the solar concentration
section 4 of FIG. 2 insofar as it employs a lens array 276 having
multiple lenses 278 placed adjacent to a slab waveguide 280. The
slab waveguide 280 can, as was the case with the slab waveguide 18
of FIG. 2, include inner and outer surfaces 282 and 284,
respectively, with the inner surface 282 being adjacent to the lens
array 276 (it being further understood that a low index cladding
layer such as the layer 16 of FIG. 1 serves as this inner surface
282), and prism facets 286 (two of which are shown) being formed
along the outer surface 284. PV cells (not shown) can be provided
along outer edges 288 and 289 of the slab waveguide 280. In
contrast to the solar concentration section 4, however, the solar
concentration section 274 additionally includes first and second
dichroic mirrors 290 and 291 that are respectively positioned along
the first and second edges 288 and 289, respectively (and which
would therefore be positioned between those edges and any PV cells
intended to receive light emanating through those edges).
[0079] As shown in FIG. 19, the dichroic mirrors 290, 291 are
particularly configured to pass certain way lengths of light and to
reflect other wavelengths of light. In the present example, first
incident light 292 of wavelength .lamda..sub.1 (shown in dashed
lines), upon impinging the lenses 278 and passing into the slab
waveguide 280 and being reflected by a respective one of the prism
facets 286, experiences TIR within the slab waveguide 280 and can
proceed in either direction towards the first edge 288 or the
second edge 289. However, assuming that the dichroic mirror 291 is
reflective with respect to light of wavelength .lamda..sub.1, any
such light that arrives at the second edge 289 is consequently
reflected by the dichroic mirror 291 and thus proceeds in the
opposite direction toward the first edge 288. Assuming that the
first dichroic mirror 290 is configured to allow light of
wavelength .lamda..sub.1 to pass through that dichroic mirror, all
of the light of that wavelength then proceeds out of the first edge
288 and through that dichroic mirror 290. To the extent that a PV
cell (not shown) is positioned on the opposite side of that
dichroic mirror 290, that PV cell only receives light of the
wavelength .lamda..sub.1. Assuming that such PV cell is selected so
as to be particularly suited for receiving light of this
wavelength, the efficiency of operation of the PV cell can be
maximized.
[0080] In contrast, with respect to second light 293 of wavelength
.lamda..sub.2 that is incident upon the lenses 278 (shown in solid
lines), that light also can proceed in through the lenses and into
the slab waveguide where it experiences TIR due to interaction with
the prism facets 286. However, in this case, the first dichroic
mirror 290 is configured to reflect light of the wavelength of the
second light (.lamda..sub.2) while the second dichroic mirror 291
is configured to pass such light. Thus, all of the second light of
the wavelength .lamda..sub.2 only passes out of the waveguide
through the edge 289 through the dichroic mirror 291 and, upon
making such passage, can be received by a PV cell that desirably is
suited for receiving light of that frequency.
[0081] The above-described features of the solar concentration
section 274 of FIG. 19, in which light is selectively reflected or
passed at the edges (exit apertures) of a waveguide depending upon
the wavelength of the light, can be further combined with
additional light-selective operation as shown in FIG. 20. More
particularly, as shown in FIG. 20, an additional embodiment of a
solar concentrations section 294 includes not only a lens array 296
with lenses 298 but also a first waveguide 300 and a second
waveguide 301. The first waveguide 300 has a first surface 302 and
a second surface 304, where the first surface 302 is in contact
with the lens array 296 and the second surface 304 is in contact
with the second waveguide 301. The second waveguide 301 includes a
first surface 305 that is an outermost surface of the solar
concentration section 294 and additionally a second surface 306
that is in contact with the second surface 304. The first surface
302 of the first waveguide 300 can be formed by a low index
cladding layer such as the cladding layer 16 of FIG. 1. However, in
contrast to the embodiment of FIGS. 1-2, prism facets 308 (two of
which are shown) are formed not along the second surface 304 of the
waveguide 300 but rather along the first surface 302 that is in
contact with the lens array 296.
[0082] Instead of placing prism facets at the second surface 304,
that surface instead is where a dichroic mirror (as well as
possibly another cladding layer) is formed and, for purposes of the
description below, the second surface 304 is considered to be such
a dichroic mirror (albeit the second surface 306 of the second
waveguide 301 or both of the surfaces 304, 306, can also be
considered to be or include such a mirror). As for the second
waveguide 301, it also has prism facets 310, two of which are
shown, formed along the first (outer) surface 305. Additionally as
shown, at each of the longitudinal edges of the first and second
waveguides 300, 301, further dichroic mirrors are placed in the
same manner as was described with respect to FIG. 19. Thus, at a
right edge (as shown in FIG. 1) of the first waveguide 300 a first
dichroic mirror 311 is positioned while at a left edge of that same
waveguide a second dichroic mirror 312 is positioned. Likewise, at
a right edge of the second waveguide 301 a third dichroic mirror
313 is positioned while at a left edge of that waveguide a fourth
dichroic mirror 314 is positioned.
[0083] Given the above-described arrangement, the solar
concentration section 294 is capable of differentiating among four
different types of light and directing those respective types of
light to four different PV cells respectively. More particularly,
first light 315 of wavelength .lamda..sub.1 that is incident upon
the lenses 298, upon passing through the lens array 296 and passing
into the first waveguide 300, is reflected by the dichroic mirror
304 and consequently reflected back up to appropriate ones of (in
this example, one of) the prism facets 308 associated with that
first waveguide. Likewise, second light 316 of wavelength
.lamda..sub.2 (shown in dashed lines) upon passing into and through
the lens array 296 and into the first waveguide 300 similarly is
reflected by the dichroic mirror 304 and received at the prism
facets 308. Upon reaching the prism facets 308, each of the first
and second light 315, 316, experiences TIR and is reflected within
the first waveguide 300. Due to the additional operation of the
first and second dichroic mirrors 311, 312 (in substantially the
same manner as was discussed with respect to FIG. 19), however, the
first light of wavelength .lamda..sub.1 is reflected by the second
dichroic mirror 312 so that it cannot pass out of the waveguide 300
at its left edge, but instead all of the first light passes through
the first dichroic mirror 311 and thus exits the waveguide through
its right edge. Conversely, the second light 316 of wavelength
.lamda..sub.2 is precluded from exiting the first waveguide 300 at
its right edge associated with the first dichroic mirror 311, at
which such light is reflected, but is instead able to exit the
first waveguide at its left edge at which is located the second
dichroic mirror 312, which passes that light.
[0084] In contrast to the first and second light 315, 316 that is
reflected by the dichroic mirror 304, both third light 317 of
wavelength .lamda..sub.3 and fourth light 318 of wavelength
.lamda..sub.4, upon entering the lens array 296 and passing through
the first waveguide 300, are able to pass through that dichroic
mirror and into the second waveguide 301. Upon passing into the
second waveguide 301, the focused light 317, 318 reaches the prism
facets 310, at which that light experiences TIR. Due to the
presence of the dichroic mirror 304 (and possibly due to any
further effect of any other layer such as a low index cladding
layer at the second surface 306, etc.), the third and fourth light
cannot re-enter the first waveguide 300. Rather, due to the
operation of the third and fourth dichroic mirrors 313, 314, the
third light 317 is reflected at the left edge of the waveguide 301
and only passes out of that waveguide at its right edge by way of
the third dichroic mirror 313, while the fourth light 318 is
reflected at the right edge of the waveguide 301 and only passes
out of that waveguide at the left edge by way of the fourth
dichroic mirror 314. Thus, given the embodiment shown in FIG. 20,
incident light can be separated successfully into four different
light components .lamda..sub.1, .lamda..sub.2, .lamda..sub.3 and
.lamda..sub.4, which respectively exit the solar concentration
section at four different locations. Assuming that respective PV
cells are placed adjacent to the respective dichroic mirrors
311-314 (or otherwise in position so as to receive light emanating
through those respective dichroic mirrors) that are suited for
receiving the particular light components emanating from those
respective dichroic mirrors, enhanced operation of the PV cells and
thus of the entire solar energy system 294 can be achieved.
[0085] Referring next to FIG. 21, another exemplary solar
concentration section 324 is shown in which incident light 332 is
separated into different light components suitable for receipt by
different PV cells. As shown, the solar concentration section 324
of FIG. 21 like the solar concentration section 294 of FIG. 20
includes a first waveguide 320, which can be, for example, an
infrared waveguide, and a second waveguide 321, which can for
example be a visible waveguide (again, each of the waveguides can
include appropriate cladding along its outer surfaces so as to form
the waveguides; also, there can be in some cases a planar first
surface anti-reflective coating applied to various surfaces of the
solar concentration section 324). In this embodiment, however,
rather than employing a lens array that receives incident light
prior to that incident light being transmitted to the waveguides,
the solar concentration section 324 instead employs a lens array
322 that is positioned in between the first and second waveguides
320, 321. More particularly as shown, the lens array 322 includes a
first lens subarray 326 that includes a plurality of lenses 328
that are directed concave up toward the first waveguide 320 and a
second lens subarray 327 having a plurality of lenses 329 that are
directed concave down towards the second waveguide 321. As shown,
the second lens subarray 327 is thus closer to the second waveguide
321 than the first waveguide 320, and the first lens subarray 326
is thus closer to the first waveguide 320 than the second waveguide
321, where a space 330 exists between the first and second lens
subarrays.
[0086] Further as shown, the first lens subarray 326 more
particularly is coated with a dichroic coating such that the lenses
328 of that subarray serve as reflective lenses (or mirrors) in
relation to infrared light while passing non-infrared (and in
particular visible) light. In contrast, the lenses 329 of the
second lens subarray 327 are not coated with any dichroic coating
but merely serve as refractive lenses for any light (and
particularly visible light) that reaches those lenses after passing
through the reflective lenses of the first lens subarray 326. Given
this arrangement, upon incident light 332 impinging the solar
concentration section 324 via an outer surface of 334 of the first
waveguide 320, that light proceeds through the first waveguide 320
and into the lens array 322. Infrared light components of the
incident light 332 are reflected by the lenses 328 of the first
lens subarray 326 and, due to the focusing of those lenses, arrive
at prism facets 336 formed along the outer surface 334 of the first
waveguide. Upon being reflected at those prism facets 336, the
infrared light experiences TIR and proceeds to the edges of the
waveguide where the light can then proceed to PV cells (not
shown).
[0087] By comparison, other light and particularly visible light
entering into and passing through the first waveguide 320 passes
through the lenses 328 of the first lens subarray 326 and into the
lenses 329 of the second lens subarray 327. This light is then
focused so as to reach prism facets 338 along an outer surface 340
of the second waveguide 321. Upon reaching the prism facets 338,
the visible light experiences TIR and thus proceeds within the
waveguide 321 to edges at which the light can exit the waveguide
and be received by PV cells (again not shown). It should be noted
that the embodiment of FIG. 21 is capable of achieving a unique
lens power and concentration for each light band provided, assuming
that there is normal incidence upon the dichotic reflectors.
[0088] Various combinations of two or more of the features
described above can also be encompassed in additional embodiments
of the present invention. For example, as shown in FIG. 22, in one
embodiment a solar concentration section 344 is substantially
identical to the solar concentration section 294 of FIG. 20 insofar
as it includes a lens array 346, a first waveguide 350 and a second
waveguide 351, along with a dichroic mirror 348 positioned in
between the two waveguides. Again, given this design, when incident
light 349 impinges the solar concentration section 344, certain
light components (e.g., infrared light) are reflected back into the
first waveguide 350 and experience TIR within that waveguide while
other wavelength components are passed through the dichroic mirror
into the second waveguide 351 and experience TIR in that waveguide.
Although not shown, it will be understood that dichroic mirrors can
also be positioned along the edges of the waveguides 350, 351 to
further determine whether particular light components within the
respective waveguides exit the waveguides at any particular
longitudinal edges, although this need not be the case in all
embodiments.
[0089] Unlike the solar concentration section 294 of FIG. 20,
however, each of the waveguides 350, 351 of the solar concentration
section 344 is shown to include a respective longitudinal edge 352,
353, respectively, at which is positioned a respective folding
prism 354, 355, respectively, as discussed above in relation to
FIG. 15 (in alternate embodiments, reflectors can be employed in
place of the folding prisms). As a result of the folding prism 354,
the light emanating from the first waveguide 350, which can be
infrared light, is directed to a first PV cell 356 that is suited
for receiving such light and that is positioned so as to extend
parallel to the dichroic mirror 348 (that is, parallel to the
waveguides 350, 351) while light emanating from the second
waveguide 351 is directed to a second PV cell 357 that is suited
for receiving such light (e.g., visible light). Thus, in the
embodiment of FIG. 22, the solar concentration section 344 achieves
some of the same benefits of each of the solar concentration
sections of FIG. 20 and FIG. 15, both in terms of concentrating
light and directing certain light components to suitable PV cells,
as well as arranging PV cells so as to be positioned in a desirable
manner (and a manner in which the different PV cells are positioned
apart from one another). This embodiment can further allow for the
development of thin/small volume solar energy systems, and systems
with improved polarization performance.
[0090] Referring further to FIG. 23, an additional solar
concentration section 364 includes both the solar concentration
section 4 of FIGS. 1-2 as well as additional components that allow
for the separation of different light components and direction of
those respective light components to different PV cells. More
particularly as shown, in the embodiment of FIG. 23, a reflective
output coupler 362 is positioned at an edge 28 of the waveguide 18
as shown and in turn directs the received light 360 to a dichroic
reflector 366 that is located outside of the solar concentration
section 4. Due to the external dichroic reflector 366, certain
light components (e.g., infrared light) are further reflected in a
first direction toward a first PV cell 368 suitable for receiving
that light while other light components are passed through the
dichroic reflector and received by a second PV cell 370 suitable
for receiving those light components. This embodiment thus provides
a simple concentrated design, where a concentration ratio can be
reduced by a factor of z. To the extent that a common output angle
from multimode waveguide is desired, this can require additional
reflection.
[0091] In view of the above, it should be noted that at least some
embodiments of the present invention achieve primary concentration
of light by collecting light over an entire lens array aperture and
confining the energy within a waveguide of constant thickness. The
geometric concentration is therefore the waveguide length divided
by the waveguide slab thickness (or twice the thickness where there
exists symmetric coupling). Yet the aforementioned analysis of the
concentration value assumes no focusing in the orthogonal
direction, that is, the direction perpendicular to the thickness of
the waveguide (e.g., as measured along the normal axis 159
discussed above) and also perpendicular to the length of the
waveguide along which captured light generally proceeds toward one
or more PV cells. Nevertheless, focusing in the orthogonal
direction can also be achieved in various manners and can result in
additional light concentration.
[0092] Referring to FIGS. 24A-24D for example, in at least some
embodiments the PV cells need not occupy the entire widths of the
edges of the waveguides along which those PV cells are positioned.
That is, the exit apertures (the portions of the edges of the
waveguides along which PV cells are positioned) need not be
coextensive with the edges of the waveguides. For example, with
respect to FIG. 24A, the slab waveguide 18 of FIG. 1 having the
first and second edges 28 and 30 need not be employed in
conjunction with PV cells that extend the full width of the
waveguide as do the PV cells 6 of FIG. 1. Rather, as shown in FIG.
24A, PV cells 372 can instead be employed that only extend
approximately one-third of the width of the waveguide 18. Assuming
that mirrors 374 are positioned along the remaining portions of the
edges 28, 30 that are not covered by the PV cells 372, light within
the waveguide 18 that is not incident upon the PV cells continues
to reflect back and forth within the waveguide 18 as represented by
arrows 376 until such time as the light enters into one of the PV
cells 372. (A similar arrangement can be employed to achieve
separation of different light components from one another). By
reducing the size of the PV cells (and exit apertures) relative to
the longitudinal waveguide edges in this manner, the geometric
concentration ratio is increased.
[0093] By comparison, FIG. 24 B also includes a waveguide 378 that
has first and second edges 379 and 380, respectively, and PV cells
382 and mirrors 384 along each of the respective edges, where the
PV cells occupy about one-third of the widths of each of those
respective edges and the mirrors along those edges occupy the
remainders of the widths of those respective edges. In contrast to
the embodiment of FIG. 24A, however, the edges 379, 380 of the
waveguide 378 are not parallel to one another but rather are
tapered such that the overall waveguide has a trapezoidal shape as
viewed normal to the waveguide (that is as viewed along the axis
159 discussed above). By properly selecting the angles of such
tapered edges, reflection of the light (again as represented by
arrows 386) can be achieved that more rapidly results in arrival in
the light at the PV cells 382 than in the case of FIG. 24A.
Although a trapezoidal arrangement is shown in FIG. 24B, it will be
understood that other shapes are also possible including, for
example, parallelogram arrangements or arrangements in which the
edges of the waveguide are curved. In each case, the edge
configurations are selected so as to alter the reflection angles of
the light being reflected off of the mirrors along the edges of the
waveguide so as to increase the likelihood of reflections toward
the PV cells and thus the likelihood of capture of that light by
the PV cells.
[0094] FIG. 24C shows yet another waveguide 388. In this
embodiment, a PV cell 392 is only located along a first edge 390 of
the waveguide while an opposite edge 391 of the waveguide is a
mirror such that no light exits the waveguide at that edge. Thus,
in such an embodiment, light is reflected not only by mirrored
surfaces 394 existing along the first edge 390 at which is located
the PV cell 392 (which as in the cases of FIGS. 24A-24B does not
occupy the entire width of the edge) but also is reflected at the
mirrored edge 391, as indicated by arrows 396. As for FIG. 24D,
still an additional waveguide 398 is shown that also has the PV
cell 392 and mirrored portions 394 along a first edge 397 but,
instead of having a mirrored edge 391 as in FIG. 24C instead has an
edge 399 that is a Fresnel reflector or retroreflector (in the
present embodiment, the Fresnel reflector is a planar Fresnel
reflector). Again, in the embodiments of FIGS. 24C and 24D,
increased concentration of light upon the PV cell 392 results.
Further, from FIG. 24C it is apparent that a single PV cell can be
used with symmetric coupling by mirroring one entire exit aperture
of the slab waveguide, while as evidenced by FIG. 24D the use of
other types of mirrors/prisms at one edge of the waveguide can also
be provided in some embodiments, for example, where it is desired
to achieve effectively the effect of a curved mirror on a planar
surface.
[0095] Turning to FIGS. 25-28B, control or influence over the
direction of light proceeding within a waveguide such as the
waveguides discussed above can be achieved not only through the use
of mirrors and lenses but also by appropriate
selection/configuration of the prism facets as well. In particular,
every given prism facet can be configured to tend to direct/reflect
light in a particular direction. Referring to FIG. 25, a schematic
diagram illustrates one exemplary waveguide 400 within which are
positioned numerous prism facets 402. As shown, each of the prism
facets 402 is configured to direct/reflect light predominately in a
direction indicated by a respective arrow emanating from that prism
facet. Further, as can be seen from FIG. 25, given appropriate
selection of such directional orientations of the prism facets 402,
light from all of the prism facets can be directed generally
towards a PV cell 404 located at a given edge 406 of the waveguide
400.
[0096] Additionally, given the ability of prism facets to not only
tilt rays for the purpose of achieving TIR but also for the purpose
of orientating/directing light towards a given region a waveguide
(e.g., toward a given edge or exit aperture of a waveguide, FIGS.
26A-26C show how appropriate selection of the prism facets can be
used to achieve direction of light towards any arbitrarily-located
PV cell positioned along an edge of a waveguide. More particularly,
each of FIGS. 26A-26C show respective exemplary waveguides 410,
420, and 430, respectively, at which first, second and third PV
cells 415, 425 and 435, respectively, are located at first, second
and third positions along respective edges of the respective
waveguide. Although prism facets are not shown with particularity
in FIGS. 26A-26C, it will be noted that tangent curves 417, 427 and
437 are shown instead. So long as the prism facets are configured
to direct light in directions perpendicular to the respective
tangent curves 417, 427, and 437, in respective FIGS. 26A, 26B and
26C (and within the longitudinal plane of the waveguides), light
will be generally directed towards the respective PV cells 415,
425, 435 along directions generally indicated by respective arrows
419, 429 and 439, respectively. With respect to FIG. 26C
specifically, it should be further noted that two PV cells 435 are
positioned along both opposite edges of the waveguide 430, and it
will be noted that there exists symmetry in the tangent curves 437
shown with respect to opposite halves of the waveguide. Layout of
the prism facets in the manner shown in FIG. 26C can facilitate the
manufacturing of numerous waveguides since the prism facet pattern
is repetitive/cyclic (and thus the numerous waveguides can be
manufactured in any role type fashion).
[0097] Although the above description largely presumes that slab
waveguides are employed and that PV cells need to be positioned
along edges of slab waveguides, as illustrated in FIGS. 27-28B,
this need not be the case in some embodiments. Indeed, the present
invention is intended to encompass a variety of embodiments having
a variety of different types and shapes of waveguides. For example,
not only six-sided slab waveguides (or slab waveguides with six
edges) but also slab waveguides with more than six sides/edges can
be employed in some embodiments. Also, in some embodiments, the
waveguides need not have sides/edges that are all flat, but instead
can include one or more sides that are curved. Further, given
appropriate prism facet configuration, light direction can be
controlled to such an extent that light can be effectively coupled
to PV cells even though the PV cells merely occupy minor regions
along one or more of the non-edge surfaces (e.g., the surfaces 22,
24 of the waveguide 18 of FIG. 1) of the waveguide.
[0098] Particularly as illustrated in FIG. 27, it is even possible
to provide a circularly-shaped waveguide 440 having an outer
cylindrical edge 442 that is mirrored/reflective that effectively
directs light to an off-edge PV cell 444 by appropriately
configuring prism facets 446 along that waveguide so as to direct
light in the directions shown by arrows emanating from those prism
facets, that is, in directions toward the location of the PV cell.
Due to the orientation of the prism facets 446, light is strongly
directed toward the PV cell at the center of the waveguide. Further
due to the mirrored edge 442, light is also reflected inward away
from the outer cylindrical circumference as indicated by an arrow
448. As the light reflects back and forth between the various
surfaces of the waveguide 440 it eventually proceeds to the
location of the PV cells 444. The particular location of the PV
cells 44 in terms of whether it is located on any particular one of
the two non-edged surfaces (that is, the surfaces corresponding to
the surfaces 22, 24 of FIG. 1) is not critical due to the number of
reflections that the light within the waveguide 440 undergoes. In
particular, there is no need for the PV cell to extend into the
waveguide in order for the PV cell to satisfactorily receive
light.
[0099] Given the ability to direct light within a given slab
waveguide by way of the prism facets (and also complementary
mirrored surfaces), not only can a radial concentrator be realized
having a single PV cell located at the center of the disk, but also
in some embodiments the disk can be replaced with hexagonal
sections to achieve higher fill factors between concentrator
elements. As shown in FIGS. 28A-28B, a plurality of hexagonal slab
waveguide portions 450 in particular can be assembled to form an
overall waveguide assembly 452, where each of the waveguide
portions 450 has a single associated PV cell 454 at its center
toward which all light within that waveguide portion tends to be
directed due to the orientation of the prism facets. Exemplary
orientation of prism facets within a section of an exemplary
hexagonal waveguide portion 450 is shown in FIG. 28A (particularly
in a section of one such waveguide portion), with the particular
configured orientations of the prism facets 456 being indicated by
arrows emanating from those prism facets. It can be further noted
that, in the rotationally-symmetric embodiments of FIGS. 27-28B,
the design of the coupling light extractor (e.g., in terms of prism
facet orientation) remains the same when rotated about a central
axis.
[0100] Because directionality of light flow within waveguides can
be achieved at least in part by appropriate configuration of the
prism facets, light can be further directed/coupled to PV cells
with fewer passes along the slab waveguide and therefore achieve
even greater efficiency. In at least some embodiments of the
present invention, it is envisioned that the use of prism facets to
achieve directionality and greater concentration can be combined
with the use of any one or more of the other above-described
techniques (e.g., those involving lenses, mirrors, reflectors,
light component separation, etc.) to achieve desired direction of
light within a slab waveguide toward PV cells and desired
concentration of that light. That is, the above-described methods
involving light control using prism facets are independent of, but
also combinable with, the other light-concentrating/extracting
designs also described above.
[0101] In view of the above description, it should be apparent that
the present invention is intended to encompass numerous embodiments
having a variety of different features, and the present invention
encompasses numerous variations on the particular embodiments
discussed above as well. In at least some additional embodiments of
the present invention, a solar energy system can employ one or more
of the features shown above in relation to one of the
above-described systems with other features shown above in relation
to other(s) of the above-described systems. Also, one or more of
the features can be modified in many different manners. For
example, in some alternate embodiments, it is possible to arrange
prism facets (or other injection features) along a surface of a
waveguide that is adjacent to a lens array rather than along the
opposite side of the waveguide. As already noted, a variety of
different types of injection features can be implemented depending
upon the application and embodiment.
[0102] From the above description, it should be apparent that, in
at least some embodiments, the present invention involves new types
of solar concentrators that allow for efficient and inexpensive
conversion of sunlight to electric power. In at least some such
solar concentrators, the concentrators collect sunlight from a
large upward facing surface having prism facets/injection features
and channel the rays via total internal reflection (TIR) within an
internal (slab waveguide) region, where they are directed towards
the edges of the structure. One or more PV cells are placed at
locations where light is allowed to leak for collection and energy
conversion. As described above, this can be at one or more ends of
the slab region, where the slab terminates and light can be
efficiently extracted. Yet in alternate embodiments, PV cells can
be placed periodically along the length of the slab waveguide by
providing a structure/device that allows for guiding of the light
out of the slab waveguide and into the PV cells. In some such
embodiments, this involves creation of a sharply curved region of
the slab waveguide proximate to the PV cell. One or more simple
bends in the slab waveguide/core will break the TIR conditions and
can thus simply allow for the extraction of light at several points
along a concentrator.
[0103] Solar PV systems typically are placed in the outside
environment to work, and are in general subject to degradation due
to prolonged exposure to weather. In concentrated PV systems, the
optical concentrator is exposed to weather, while the PV cell is
typically better protected. Recognizing that the PV cell is often
the largest single cost element in the system, it is desirable to
design a system so that a functional PV cell and associated
electronics can be `recycled` if the optical concentrator is
damaged.
In this regard, in at least some of the embodiments of the present
invention, the concentrator can be made as a continuous sheet which
is cut to the desired length, then attached to a linear PV cell.
The nature of a slab waveguide permits the guided light to be
efficiently stripped from the guided mode and directed into the PV
cell in several ways, for example: (1) by cutting the end surface
at an angle, (2) by removing the cladding or providing an
index-matching layer between the waveguide and the PV cell, or (3)
by introducing a sharp physical curvature or bend into the
waveguide, so that the light is incident at less than the critical
angle for total internal reflection. These features can be
pre-formed into the waveguide sheet, but they can also be
incorporated into the mounting for the PV cell, so as to be readily
implemented in relation to (for action upon) any region of a
waveguide to which they are attached.
[0104] Therefore, it is possible to design a linear photovoltaic
cell with a mounting that clamps onto the waveguiding concentrator
sheet, creating the feature that will strip the guided light and
direct it into the photovoltaic cell without the need for accurate
alignment. Assuming such a design, it is possible to have a modular
concentrated photovoltaic system where one or more photovoltaic
cells (and associated electrical connections) can be attached to
and/or removed from an optical concentrator in the field, both for
the initial installation, and subsequently for maintenance (e.g.,
if the optical concentrator needs to be replaced due to
environmental damage). Further, recognizing that the overall
collection efficiency of a waveguide-based concentrator depends on
the distance to the PV cell, it is possible to install a large-area
concentrator with a single PV cell and subsequently upgrade the
overall power output performance of the system by subsequently
adding more PV cells.
[0105] A slab waveguide typically is a multiple optical mode
structure which can guide light without loss as it propagates
through the slab. A slab waveguide typically consists of a high
index core surrounded by a lower index cladding on the top and/or
bottom. Converting light from normal incidence on the face of the
slab into light which is propagating within the slab requires some
kind of structure for deflecting the light, which will not then act
to eject (or allow excessive escaping of) light already trapped
within the slab. One way to achieve proper guiding is to provide a
localized coupling region with an index comparable to the core. As
described, in at least some embodiments, this can involve using a
colloidal suspension of high-index, sub-wavelength-sized, particles
within a lower index liquid. Bright incident light causes optical
trapping, increasing the density of the high index particles and so
increasing the overall refractive index where the incoming light
accumulates the particles yielding an increase in the average index
of refraction. A localized increase in the refractive index allows
light that scatters from the suspended particles, or reflects from
a nearby optical structure (which is otherwise not interacting with
the guided light within the slab), to be trapped within the slab
region and guided to the PV cell.
[0106] At least some embodiments of the above-described solar
energy systems/solar concentrators are suited for the roll-to-roll
processing method of manufacture. A roll process produces the
lenslets by embossing them onto a layer of low index plastic which
covers the higher index slab region. The back surface can be made
using a similar process, for the actively aligned version, or using
a sandwich of materials, such as a perforated mesh separating a
liquid-filled layer from a patterned rear surface. In all cases,
the multiple layers of the concentrator can be laid onto one
continuous substrate creating a long, flexible product at a very
low cost. Alternately the concentrator can be formed onto rigid
panels using a more conventional, if more expensive, manufacturing
process.
[0107] Thus, in at least some embodiments, the present invention
involves an overall slab waveguide concentrator geometry using
focusing lenses and localized injection features, where either the
localized injection features are permanent or alternatively the
injection features are reactive (formed in response to incident
light), where multiple specific materials and structures can be
used for formation of the injection features. The slab waveguide
format is extremely compact in comparison to many conventional
active or passive concentrator optics. Since material costs are a
significant part of the overall system cost, this entails a
potential cost savings.
[0108] Also, as noted, in the embodiments employing reactive solar
concentrators, the reactive nature of the concentrators eliminates
the need for the active tracking usually associated with solar
concentration. Indeed, such embodiments are distinctive in that no
absorption is required. The reacting material can potentially react
losslessly, as for example through a change in index. Even if the
reacting material does require some absorption, the light which is
guided within the slab does not (on average) encounter the reacting
material again, and so would experience only a single loss point.
The geometries of at least some of these embodiments are attractive
in that very high input to output area ratios can be achieved.
Although some embodiments will incorporate a lens array and
therefore only work with specular light, there is the potential for
significantly less loss by avoiding the absorption and remission of
photons. The overall geometry maintains the advantage of a high
collection area and incorporates a reactive, index-changing
material to avoid active tracking.
[0109] The highest conversion efficiency photovoltaic cells require
concentration of incident sunlight to work with maximum efficiency
(typically 100-1000.times. concentration). However, concentrator
optics are fundamentally incapable of efficiently collecting
diffuse sunlight onto a small area photovoltaic cell. Therefore,
the efficiency of a highly concentrated photovoltaic system drops
to nearly zero on cloudy days, whereas non-concentrated
photovoltaic systems (such as an amorphous silicon solar panel)
substantially maintain their performance. Given these
considerations, and further in view of the fact that many solar
installations (such as for residential and commercial rooftops)
involve limited areas upon which the solar collectors can be
implemented, at least some embodiments of the present invention are
intended facilitate achieving the benefits associated with both
concentrated PV systems as well as non-concentrated photovoltaic
systems by both collecting direct sunlight into a concentrated
high-efficiency photovoltaic cell, and also (possibly
simultaneously) directing diffuse sunlight into a less efficient
photovoltaic cell.
[0110] In this regard, at least some embodiments of the present
invention involve extracting and concentrating the direct sunlight
to the edge of the illuminated area for reception by one or more PV
cell(s), while allowing diffuse light to pass through the waveguide
for collection by one or more other PV cell(s). Referring again to
FIG. 1, light which enters the lens array 8 normally is focused
onto the replicated prism facets 26 and coupled into the slab
waveguide 18. In some embodiments (e.g., that of FIG. 10), light
which enters the lens array 8 at any other angle is focused by the
lenslets onto a transparent region of the rear surface of the
waveguide, missing the prism facets 26, and is transmitted through
the rear surface of the waveguide 24 substantially without
attenuation. Therefore, an area-efficient hybrid photovoltaic
system can be made by placing a conventional solar panel directly
beneath the micro-optic slab concentrator. On cloudless days, most
of the energy would be generated by the efficient photovoltaic cell
via the concentrator. By comparison, on cloudy days, a smaller
total amount of energy, bypassing the slab concentrator, would be
generated mostly by the photovoltaic panel.
[0111] Each of the above-described embodiments of solar energy
systems/solar concentrators are potentially manufacturable at
extremely low cost, as compared with the cost of manufacturing
conventional PV cell material from amorphous or crystalline
Silicon. Due to the compliance with roll-to-roll processing, it is
likely that this concentrator design will exist as flexible sheets
several meters in length. They could be fitted onto roofs or act as
tents to provide local power generation for homes or temporary
installations. Smaller units can be applicable for the powering of
laptop computers or other small electronics. At least some of the
above-described embodiments can be made from flexible materials, as
each local region is automatically aligned with the incident light.
This supports low cost deployment and unconventional uses: for
example as tent material or ground cover over non-flat terrain.
Although the above description describes physical orientations of
various components of solar energy systems relative to one another
(e.g., where one component is "above" or "below" another
component), these terms are only provided to facilitate description
of these embodiments but are not intended to limit the present
invention to embodiments satisfying these particular
characteristics.
[0112] It is specifically intended that the present invention not
be limited to the embodiments and illustrations contained herein,
but include modified forms of those embodiments including portions
of the embodiments and combinations of elements of different
embodiments as come within the scope of the following claims.
* * * * *