U.S. patent application number 12/723332 was filed with the patent office on 2011-09-15 for secondary reflector for linear fresnel reflector system.
This patent application is currently assigned to Ausra, Inc.. Invention is credited to Matthew J. Lawrence, David R. MILLS.
Application Number | 20110220094 12/723332 |
Document ID | / |
Family ID | 44558752 |
Filed Date | 2011-09-15 |
United States Patent
Application |
20110220094 |
Kind Code |
A1 |
MILLS; David R. ; et
al. |
September 15, 2011 |
SECONDARY REFLECTOR FOR LINEAR FRESNEL REFLECTOR SYSTEM
Abstract
A solar collection system having a secondary reflector is
provided. The secondary reflector may be located above a field of
primary reflectors and below a solar receiver. The secondary
reflector may form a portion of an ellipse or macrofocal ellipse
and be operable to reflect at least a portion of the solar
radiation reflected by the primary reflectors onto the solar
receiver. The secondary reflector may be disposed away from the
solar receiver and associated tertiary reflectors such that it does
not intercept reflected solar radiation from the outer primary
reflectors that would otherwise have struck the solar receiver and
tertiary reflectors had the secondary reflector not been
present.
Inventors: |
MILLS; David R.; (Palo Alto,
CA) ; Lawrence; Matthew J.; (San Francisco,
CA) |
Assignee: |
Ausra, Inc.
Mountain View
CA
|
Family ID: |
44558752 |
Appl. No.: |
12/723332 |
Filed: |
March 12, 2010 |
Current U.S.
Class: |
126/651 ;
126/685 |
Current CPC
Class: |
F24S 2023/83 20180501;
Y02E 10/47 20130101; F24S 2023/838 20180501; F24S 23/79 20180501;
F24S 30/40 20180501; F24S 23/77 20180501; F24S 2023/876
20180501 |
Class at
Publication: |
126/651 ;
126/685 |
International
Class: |
F24J 2/24 20060101
F24J002/24; F24J 2/18 20060101 F24J002/18 |
Claims
1. A collector system comprising: at least two separately pivotable
primary reflectors operable to reflect solar radiation; at least
one solar receiver operable to receive solar radiation, wherein the
at least one solar receiver is positioned on a level above the at
least two separately pivotable primary reflectors, wherein the
collection aperture of the at least one solar receiver is downward
facing; and at least one secondary reflector operable to reflect at
least a portion of a solar radiation reflected by the at least two
separately pivotable primary reflectors onto at least a portion of
the at least one solar receiver, wherein the at least one secondary
reflector is positioned on a level below the at least one solar
receiver, and wherein the secondary reflector comprises a first
reflective surface.
2. The collector system of claim 1 further comprising at least one
tertiary reflector associated with the at least one solar receiver,
the at least one tertiary reflector operable to enlarge the
collection aperture of the at least one solar receiver.
3. The collector system of claim 1, wherein the first reflective
surface forms at least a portion of an ellipse.
4. The collector system of claim 3, wherein the ellipse is defined
by foci located at an end of one of the at least one solar receiver
and a top edge of one of the at least two separately pivotable
primary reflectors at its highest position of use.
5. The collector system of claim 3, wherein the ellipse is defined
by foci located with a portion of the solar receiver and a portion
of one of the at least two separately pivotable primary
reflectors.
6. The collector system of claim 3 further comprising a tertiary
reflector associated with the at least one solar receiver, the
tertiary reflector operable to enlarge the collection aperture of
the at least one solar receiver, wherein the ellipse is defined by
foci located at a portion of the at least one solar receiver, a
portion of the tertiary reflector, or a portion of one of the at
least two separately pivotable primary reflectors.
7. The collector system of claim 1, wherein the at least one
secondary reflector further comprises a second reflective surface
opposite the first reflective surface.
8. The collector system of claim 1, wherein the at least one
secondary reflector comprises two secondary reflectors.
9. The collector system of claim 8, wherein a top portion of each
secondary reflector of the at least one secondary reflector is
separated by a space operable to allow heated air to pass
through.
10. The collector system of claim 1 further comprising at least one
light barrier operable to at least partially block the solar
radiation reflected by the at least one the primary reflector.
11. The collector system of claim 10, wherein the at least one
secondary reflector further comprises a non-reflective surface
opposite the first reflective surface.
12. The collector system of claim 10, wherein the at least one
secondary reflector further comprises a double-sided reflector.
13. The collector system of claim 10, wherein the light barrier is
a horizontal light barrier.
14. The collector system of claim 10, wherein the light barrier is
a vertical light barrier.
15. The collector system of claim 1, wherein the at least two
separately pivotable primary reflectors comprises at least two
primary reflectors located on opposite sides of the at least one
solar receiver.
16. The collector system of claim 1, wherein the at least one solar
receiver comprises at least one absorber tube for carrying a heat
transfer fluid.
17. The collector system of claim 1, wherein the at least two
separately pivotable primary reflectors are flat reflectors.
18. The collector system of claim 1, wherein the at least two
separately pivotable primary reflectors are parabolic
reflectors.
19. The collector system of claim 1, wherein the first reflective
surface forms at least a portion of macrofocal ellipse.
20. A secondary reflector for use with a Linear Fresnel Reflector
system, the secondary reflector comprising: a first reflective
surface having a curvature defined by an ellipse, the reflective
surface operable to reflect at least a portion of solar radiation
reflected by a primary reflector directly onto a solar receiver
while the secondary reflector is positioned on a level above the
primary reflector and below the solar receiver.
21. The secondary reflector of claim 20, wherein the ellipse is
defined by foci located with a portion of the solar receiver and a
portion of one of the at least one primary reflector.
22. The secondary reflector of claim 20 further comprising a second
reflective surface opposite the first reflective surface.
23. The secondary reflector of claim 20 further comprising a light
barrier configured to at least partially block a portion of the
solar radiation directed towards a non-reflective surface opposite
the first reflective surface.
24. The secondary reflector of claim 23, wherein the light barrier
is a horizontal light barrier.
25. The secondary reflector of claim 23, wherein the light barrier
is a vertical light barrier.
Description
1. FIELD
[0001] The present disclosure relates generally to solar collection
systems, and more particularly, to secondary reflectors, tertiary
reflectors used with said secondary reflectors, solar collection
systems having secondary reflectors, and solar collection systems
having secondary reflectors and tertiary reflectors.
2. RELATED ART
[0002] Current Linear Fresnel Reflector (LFR) systems generally
include an array of parallel reflector lines focusing sunlight to a
linear receiver above. The receiver may contain heat transfer
fluids or may be photovoltaic or thermoelectric absorbers. The
receiver may be mounted downward or to the side, but most current
systems are downward-facing.
[0003] One drawback of such downward-facing LFR systems is that the
performance of the outer mirrors or heliostats is relatively poor
compared to that of the inner minors. This is because conventional
downward-facing systems offer the largest apparent receiver
aperture to the heliostats which have the smallest images (minors
directly below the receiver) and the smallest apparent receiver
aperture to those with the largest images (mirrors farthest from
the receiver). Thus, while the heliostats directly below the
receiver perform with relatively high efficiency, the heliostats
farthest away from the receiver perform with relatively low
efficiency since a large portion of the reflected solar radiation
spills past the small apparent aperture of the receiver. In
addition, this also has the effect of limiting the number of
mirrors that can be practically or economically used, and
accordingly, the flux concentration that can be achieved.
[0004] One solution that has been used in an attempt to solve the
problem described above is to add a downward-facing secondary
mirror placed above the receiver. This secondary mirror reflects
solar radiation that spills past the receiver back onto the top of
the receiver. While the additional minor increases the amount of
solar radiation absorbed by the receiver, the shape and position of
the mirror results in hot air becoming trapped against the surface
of the mirror. The air causes the minor to increase in temperature,
thereby possibly causing a degradation in the reflectance of the
mirror and reducing the overall efficiency of the system.
Furthermore, such arrangements tend to increase the average number
of reflections required to strike the receiver, thereby diminishing
system collection efficiency.
BRIEF SUMMARY
[0005] In one example, a collector system is described comprising
at least two separately pivotable primary reflectors operable to
reflect solar radiation and at least one solar receiver operable to
receive solar radiation, wherein the at least one solar receiver is
positioned on a level above the at least two separately pivotable
primary reflectors, and the collection aperture of the at least one
solar receiver is downward facing. The system further includes at
least one secondary reflector operable to reflect at least a
portion of a solar radiation reflected by the at least two
separately pivotable primary reflectors onto at least a portion of
the at least one solar receiver, wherein the at least one secondary
reflector is positioned on a level below the at least one solar
receiver, and wherein the secondary reflector comprises a first
reflective surface.
[0006] In another example, the first reflective surface of the
secondary reflector may form at least a portion of an ellipse. The
ellipse may be defined by foci located at an end of one of the at
least one solar receiver and a top edge of one of the at least two
separately pivotable primary reflectors at its highest position of
use. Alternatively, the ellipse may be defined by foci located with
a portion of the solar receiver and a portion of one of the at
least two separately pivotable primary reflectors. In another
example, the first reflective surface of the secondary reflector
may form at least a portion of a macrofocal ellipse.
[0007] In another example, the collector system may further include
at least one tertiary reflector operable to enlarge the collection
aperture of the solar receiver. In yet another example, the first
reflective surface of the secondary reflector may form at least a
portion of an ellipse having foci located at a portion of the solar
receiver, a portion of the tertiary reflector, or a portion of one
of the at least two separately pivotable primary reflectors.
[0008] In another example, the collector system may include two or
more secondary reflectors. The secondary reflectors may be
separated by a space operable to allow heated air to pass
through.
[0009] In another example, the collector system may include at
least one light barrier operable to at least partially block the
solar radiation reflected by the at least one primary reflector.
The light barrier may be a horizontal light barrier or a vertical
light barrier.
[0010] In one example, a secondary reflector is described, the
secondary reflector comprising a first reflective surface having a
curvature defined by an ellipse, the reflective surface operable to
reflect at least a portion of solar radiation reflected by a
primary reflector directly onto a solar receiver while the
secondary reflector is positioned on a level above the primary
reflector and below the solar receiver.
[0011] In another example, the first reflective surface of the
secondary reflector may form at least a portion of an ellipse
defined by foci located with a portion of the solar receiver and a
portion of one of the at least one primary reflector.
[0012] In yet another example, the secondary reflector may further
include a light barrier configured to at least partially block a
portion of the solar radiation directed towards a non-reflective
surface opposite the first reflective surface. The light barrier
may be a horizontal light barrier or a vertical light barrier.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 illustrates an exemplary solar collection system
including a secondary reflector.
[0014] FIG. 2A illustrates a perspective view of an exemplary solar
collection system including a secondary reflector.
[0015] FIG. 2B illustrates a zoomed-in perspective view of an
exemplary solar collection system including a secondary
reflector.
[0016] FIG. 3 illustrates an exemplary secondary reflector.
[0017] FIG. 4A illustrates the operation of an exemplary solar
collection system including a secondary reflector.
[0018] FIG. 4B illustrates the operation of an exemplary solar
collection system including a secondary reflector.
[0019] FIG. 5 illustrates an exemplary tertiary reflector.
[0020] FIG. 6A illustrates the operation of an exemplary solar
collection system including a secondary reflector and tertiary
reflector.
[0021] FIG. 6B illustrates the operation of an exemplary solar
collection system including a secondary reflector and tertiary
reflector.
[0022] FIG. 7 illustrates an exemplary secondary reflector.
[0023] FIG. 8 illustrates an exemplary secondary reflector.
DETAILED DESCRIPTION
[0024] The following description is presented to enable a person of
ordinary skill in the art to make and use the various embodiments.
Descriptions of specific devices, techniques, and applications are
provided only as examples. Various modifications to the examples
described herein will be readily apparent to those of ordinary
skill in the art, and the general principles defined herein may be
applied to other examples and applications without departing from
the spirit and scope of the various embodiments. Thus, the various
embodiments are not intended to be limited to the examples
described herein and shown, but are to be accorded the scope
consistent with the claims.
[0025] Various embodiments are described below relating to solar
collection systems. In particular, a secondary reflector and solar
collection system with one or more secondary reflectors are
described below. The solar collection system may include primary
and secondary reflectors for directing solar radiation to a solar
receiver, e.g., a receiver having a plurality of absorber tubes.
The secondary reflector may be positioned below the absorber tubes
and configured to reflect solar radiation from the primary
reflectors onto the absorber tubes, e.g., solar radiation reflected
from the primary reflectors that might otherwise miss the absorber
tubes. The absorber tubes may carry a heat transfer fluid that is
heated during operation by solar radiation reflected by the primary
and secondary reflectors to the absorber tubes. Additionally, the
solar collection system may include one or more tertiary reflectors
to increase the apparent aperture of the solar collector.
[0026] FIG. 1 illustrates a side view of an exemplary solar
collection system 100 according to the present disclosure. Solar
collection system 100 may include a field of ground mounted primary
reflectors 101 for reflecting solar radiation onto solar receiver
103. Primary reflectors 101 may be made of any reflective material
or any material coated with a reflective substance. For example,
primary reflector 101 may be a polished reflective metal,
transparent plastic reflector with an embedded layer of reflective
material or optical layer which acts as a reflector, a glass mirror
with a reflective coating on the front or back surface, or the
like. Primary reflectors 101 may have any shape, for example, they
may be flat mirrors, parabolic mirrors, or the like. In one
example, primary reflectors 101 may be of the type described in
International Patent Applications numbered PCT/AU2004/000883 and
PCT/AU2004/000884, filed Jul. 1, 2004, both of which are
incorporated herein by reference.
[0027] In one example, a group of primary reflectors 101 may be
arranged in one or more rows and associated with a particular solar
receiver 103. In another example, each row of primary reflectors
101 and, hence, each solar receiver 103 may have an overall length
(into the drawing) of 400 meters. Solar receiver 103 may be
supported at a suitable height above primary reflectors 101, e.g.,
a height of approximately 10 to 20 meters, by stanchions which may
be stayed by ground-anchored guy wires. The width of primary
reflectors 101 may be approximately 0.5 to 5 meters and adjacent
primary reflectors 101 may be separated by 1 to 15 meters depending
on both the width of the reflector chosen and its position in the
field. One of ordinary skill in the art will appreciate that other
similar or different sized reflectors and configurations may be
employed.
[0028] As will be discussed in greater detail below, primary
reflectors 101 may be driven collectively or regionally, as rows or
individually, to track movement of the sun (relative to the earth)
and to reflect incident radiation to respective ones of solar
receiver 103.
[0029] Solar receiver 103 may include one or more absorber tubes
105 for absorbing solar radiation. Absorber tubes 105 may carry a
heat exchange fluid (e.g., water or, following heat absorption,
water-steam or steam, molten salt, air, or heat transfer oil).
Absorber tubes 105 may be made from any thermally conductive
material, such as aluminum, stainless steel, and the like.
[0030] Absorber tubes 105 may vary in diameter from about 25 mm to
500 mm depending on the number of tubes used and the overall size
and optical concentration of the system. Further, the length of
absorber tubes 105 may range from approximately 50 meters to 1,000
meters. Absorber tubes 105 may also be doubled around at one end to
allow free thermal expansion of the tubes. Further, each of the
absorber tubes 105 may be coated, along its length and around a
portion of its circumference that is exposed to incident solar
radiation, with a solar absorptive coating. The coating may
comprise a solar spectrally selective surface coating that remains
stable under high temperature conditions in ambient air or it may
comprise a black paint that is stable in air under high-temperature
conditions.
[0031] Solar receiver 103 may include any number of absorber tubes
105 to suit specific system requirements. In one example, solar
receiver 103 may include between two and thirty absorber tubes 105.
While solar receiver 103 has been described as comprising one or
more absorber tubes 105, it will be appreciated by one of ordinary
skill in the art that other types of solar receivers, such as
photovoltaic or thermoelectric absorbers may be used as solar
receivers 103.
[0032] In one example, as illustrated by FIG. 1, solar receiver 103
may be configured to receive reflected solar radiation from primary
reflectors 101 located on opposite sides of solar receiver 103. In
particular, FIG. 1 shows two rows of primary reflectors 101 on each
side of solar receiver 103. In another example, solar receiver 103
may receive reflected radiation from twelve rows of primary
reflectors 101; for example, solar receiver 103 may receive
reflected radiation from six rows of primary reflectors 101 located
on one side of the collector and from six rows located on the other
side of the collector. One of ordinary skill will appreciate that
in some examples the number of primary reflectors 101 on each side
of solar receiver 103 may not be equal. Further, in other examples,
solar receiver 103 may not receive reflected solar radiation from
more than one side.
[0033] Solar collection system 100 may further include one or more
secondary reflectors 107 for reflecting at least a portion of the
solar radiation reflected by primary reflectors 101. Secondary
reflectors 107 may be configured to reflect solar radiation from
primary reflectors 101 onto one or more absorber tubes 105 of solar
receiver 103. Secondary reflectors 107 may be made of any
reflective material or any material coated with a reflective
substance. For example, secondary reflectors 107 may be polished
reflective metals or coated glass minors identical, similar, or
different than primary reflectors 101.
[0034] In one example, as will be discussed in greater detail below
with respect to FIG. 3, the reflective surface of secondary
reflectors 107 may form an arc approximating a portion of an
ellipse. In another example, the reflective surface of secondary
reflectors 107 may form an arc approximating a portion of a
macrofocal ellipse, for example, as described in Rabl, Ari and
Winston, Roland, Ideal Concentrators for Finite Sources and
Restricted Exit Angles, Applied Optics, Vol. 15, Issue 11, pp.
2880-2883 and Chaves, Julio, Introduction to Nonimaging Optics,
Light Presciptions Innovators, page 3, which are incorporated
herein by reference. The size of the secondary reflector may be
determined by the distance to the farthest primary reflector row,
the distance to the innermost primary reflector row, and the shape
and dimensions of the solar receiver. For example, for a system
having a receiver comprised of many small tubes, the width of each
of the two secondary reflector components may be approximately
30-100% of the width of the receiver tube group. In one example,
the width of each of the two secondary reflector components may be
between 50% and 60% of the width of the receiver tube group.
[0035] Secondary reflectors 107 may be disposed within solar
collection system 100 using various techniques. For example,
secondary reflectors 107 may be attached to a common structure
supporting solar receiver 103 or one or more primary reflectors
101. Alternatively, secondary reflectors 107 may be supported by a
separate structure. Further, mechanisms to rotate or translate
secondary reflectors 107 may be included to adjust the position of
secondary reflectors 107, e.g., for different positions of the sun,
changes in relative positions of primary reflectors 101 and solar
receiver 103, and so on.
[0036] In one example, solar collection system 100 may include two
secondary reflectors 107 positioned below solar receiver 103 with
the reflective surfaces of secondary reflectors 107 facing outwards
in opposite directions. Secondary reflectors 107 may be further
spaced a vertical distance below solar receiver 103, which may
allow at least a portion of solar radiation reflected by primary
reflectors 101 to directly strike a surface of absorber tubes 105.
The reflection of solar radiation will be described in greater
detail below with respect to FIGS. 4A and 4B.
[0037] FIG. 2A illustrates a three-dimensional perspective view of
solar collection system 200. Solar collection system 200 includes
22 primary reflectors 101 directed towards solar receiver 103. In
this example, solar receiver 103 is placed at a height of 15
meters. In this illustrated example, primary reflectors 101 are
positioned at distances of 4.1747, 8.5809, 13.1290, 17.7485,
23.3570, 28.4186, 34.6933, 40.9410, 48.3207, 57.1195, and 66.3706
meters from the base of solar receiver 103. However, as explained
above, any number of solar collectors may be used and may be
positioned at any distance from solar receiver 103. Further, solar
receiver 103 may be placed at any height depending on the desired
system configuration. FIG. 2B illustrates a zoomed-in view of solar
collection system 200 of FIG. 2A.
[0038] In one example, multiple solar collection systems 100 may be
placed in a row. In such a configuration, the farthest primary
reflector 101 of one solar collection system 100 is placed adjacent
to the farthest primary reflector 101 of the neighboring solar
collection system 100. Further, primary reflectors 101 may be
driven collectively or regionally, as rows or individually, to
track movement of the sun (relative to the earth) and to reflect
incident radiation to respective ones of solar receiver 103.
[0039] In another example, primary reflectors 101 may be operable
to flip and reorientate to reflect incident radiation onto either
solar receiver 103 of two adjacent solar collection systems 100.
Primary reflectors 101 may be redirected depending on the sun's
position throughout the day. Mirror flipping provides improved
system efficiency by placing more reflectors in an approximately
sun-facing orientation that intercepts a greater amount of solar
energy and reflects said radiation to the solar receiver 103 that
will absorb the most solar radiation for that given position of the
sun. Mirror flipping is described in greater detail in U.S. Pat.
No. 5,899,199 and U.S. Pat. No. 6,131,565, which are incorporated
herein by reference.
[0040] Mirror flipping improves the efficiency of solar collection
system 100 by improving the efficiency of the primary reflectors
101 farthest from solar receiver 103. As will be discussed in
greater detail below, sideways-facing secondary reflectors provide
improved efficiency over downward-facing secondary reflectors for
the primary reflectors farthest away from the solar receiver by
providing the largest apparent aperture to the heliostats with the
largest image (minors farthest from the receiver). Thus, using
sideways-facing secondary reflectors increases the efficiency
gained by mirror flipping. In one example, the performance benefit
provided by minor flipping with sideways-facing secondary
reflectors may range from 0% (when the sun is directly overhead) to
approximately 4.5% in the late afternoon.
[0041] FIG. 3 illustrates one side of solar collection system 100
having an exemplary secondary reflector 107. As discussed above,
the reflective surface of secondary reflector 107 may form an arc
approximating a portion of an ellipse. In one example, the
reflective surface of secondary reflector 107 may form an arc
approximating a portion of ellipse 111 having focus 113 located at
or near the outer edge of solar receiver 103 (or outermost absorber
tube 105) and focus 114 located at or near the top edge of the
outermost primary reflector 101 when said reflector top edge is at
its highest position of usage. By positioning and shaping secondary
reflector 107 in the manner described above, solar radiation
reflected onto the surface of secondary reflector 107 by any of the
primary reflectors 101 facing secondary reflector 107 may be
reflected onto a portion of solar receiver 103 as illustrated in
FIGS. 4A and 4B.
[0042] In another example, where primary reflectors 101 are
operable to flip and reorientate to reflect incident radiation onto
different solar receivers, the reflective surface of secondary
reflector 107 may form an arc approximating a portion of an ellipse
having a first focus located at or near the outer edge of solar
receiver 103 (or outermost absorber tube 105) and a second focus
located at or near the top edge of the outermost primary reflector
101 operable to reorientate towards that solar receiver 103.
[0043] In another example, instead of reflecting to a point,
secondary reflector 107 may reflect an extreme ray from the top of
the outermost primary reflector 101 to be tangent to the outer
circumference of the outer absorber tube 105. In this example, the
reflective surface of secondary reflector 107 may form an arc
approximating a portion of a macrofocal ellipse.
[0044] In another example, the reflective surface of secondary
reflector 107 may form an arc approximating a portion of a
parabolic curvature with an optic axis parallel to the ray passing
between the top of the outermost primary reflector 101 and the
bottom of the secondary reflector 107.
[0045] In other examples, the reflective surface of secondary
reflector 107 may form an arc approximating a portion of an ellipse
having other foci than shown in FIG. 3, e.g., associated with a
portion of one of the primary reflectors and the solar absorber, or
other components of solar collection system 100.
[0046] The exemplary shapes of secondary reflector 107 described
above allow secondary reflector 107 to be placed in a substantially
vertical orientation and at least partially below solar receiver
103. Positioning secondary reflector 107 in such a manner allows a
sizeable fraction of rays coming in from the sides to directly
strike the receiver. Further, the vertical orientation may also
allow rays from reflectors close to the base supporting secondary
reflectors 107 to bypass secondary reflectors 107. This reduces the
average absorption loss in the reflectors. Additionally, the
vertical orientation allows secondary reflector 107 to cool more
rapidly than conventional horizontal secondary reflectors because
hot air is allowed to rise up and away from the vertical reflector
instead of being trapped against the underside of the horizontal
reflector.
[0047] In one example, another secondary reflector 107 on the
opposite side of solar collection system 100, as seen in FIG. 1,
may be configured to mirror that of secondary reflector 107
illustrated in FIG. 3. In other words, the secondary reflector 107
on the opposite side of solar collection system 100 may form a
portion of an ellipse having foci at the opposite end of solar
receiver 103 (or outermost absorber tube 105) and at the top edge
of the outermost primary reflector 101 at the opposite end of solar
collection system 100.
[0048] FIGS. 4A and 4B illustrate the operation of exemplary solar
collection system 100. For clarity, the reflections caused by the
outer primary reflectors 101 and the inner primary reflectors 101
have been broken up into FIGS. 4A and 4B, respectively. However, it
should be appreciated that during actual operation, reflections
from both the outer and inner primary reflectors 101 may be
generated simultaneously.
[0049] FIG. 4A illustrates reflections caused by the outer primary
reflectors 101 of exemplary solar collection system 100. In one
example, solar radiation may be reflected by the outer primary
reflectors 101 towards solar receiver 103. However, since the outer
primary reflectors 101 are the reflectors farthest away from solar
receiver 103, they generate the largest image while having the
smallest collector aperture. As a result, only a portion of the
solar radiation reflected by the outer primary reflectors 101
directly strikes absorber tubes 105 of solar receiver 103. The
portion of the reflected solar radiation that directly strikes
solar receiver 103 is represented by the top three ray lines
extending away from the outer primary reflectors 101, shown as rays
102d. The portion of the reflected solar radiation that does not
directly strike solar receiver 103 is represented by the bottom two
ray lines extending away from the outer primary reflectors 101,
shown here as rays 102r. For example, if secondary reflectors 107
were removed from solar collection system 100, the solar radiation
represented by the bottom two ray lines 102r reflected from the
outer primary reflectors 101 would pass by solar receiver 103 and
be lost.
[0050] Thus, in one example, one or more secondary reflectors 107
may be positioned below solar receiver 103 to redirect at least a
portion of the reflected solar radiation 102r from primary
reflectors 101 that would otherwise pass by solar receiver 103. In
this example, these portions of solar radiation may be reflected a
second time by secondary reflectors 107 onto one or more absorber
tubes 105 of solar receiver 103. Accordingly, more of the solar
radiation reflected by primary reflectors 101 may be directed to
solar receiver 103 either directly from primary reflectors 101 or
from primary reflectors 101 via secondary reflectors 107 than
without the secondary reflectors 107.
[0051] Secondary reflectors 107 may be positioned a vertical
distance below solar receiver 103 to allow the reflected solar
radiation aimed at solar receiver 103 to directly strike solar
receiver 103 while still being operable to reflect at least some of
the solar radiation that would otherwise pass by solar receiver
103. Since each reflection reduces the efficiency of energy
transfer, disposing secondary reflectors 107 vertically below solar
receiver 103 may improve the overall efficiency of solar collection
system 100 by preserving much of the single reflection solar
radiation (solar energy reflected by primary reflectors 101)
delivered to solar receiver 103, and reflecting otherwise lost
solar radiation to solar receiver 103 via secondary reflectors 107.
For example, secondary reflectors 107 are disposed so as to provide
secondary reflection for solar radiation that may pass by solar
receiver 103, but to provide little or no secondary reflection for
solar radiation that would directly strike solar receiver 103. In
one example, secondary reflectors 107 may be placed such that the
uppermost end of secondary reflectors 107 just intersect the most
horizontal rays reflected by the outermost primary reflectors 101
that would directly strike the outermost point on solar receiver
103 if secondary reflectors 107 were not present.
[0052] In one example, solar collection system 100 may be
configured such that approximately 50% of the solar radiation
reflected by the outer primary reflectors 101 is aimed directly
towards solar receiver 103. The remaining 50% of the solar
radiation reflected by the outer primary reflectors 101 may be
directed below solar receiver 103 and reflected by secondary
reflectors 107. It should be appreciated by one of ordinary skill
in the art that in other examples, different distributions of solar
radiation reflected by primary reflectors 101 may be applied
between solar receiver 103 and secondary reflectors 107.
[0053] FIG. 4B illustrates reflections caused by the inner primary
reflectors 101 of exemplary solar collection system 100. In one
example, solar collection system 100 may be configured such that
all, or almost all, of the solar radiation reflected by the inner
primary reflectors 101 directly strike solar receiver 103. In this
example, none, or almost none, of the reflections caused by the
inner primary reflectors 101 are reflected by secondary reflectors
107. In other examples, a portion of the solar radiation reflected
by the inner primary reflectors 101 may be reflected by secondary
reflector 107 prior to contacting solar receiver 103.
[0054] While FIGS. 4A and 4B show only two rows of primary
reflectors 101 on each side of solar collection system 100, it
should be appreciated that any number of primary reflectors 101 or
rows of primary reflectors 101 may be positioned on each side of
solar collection system 100. Further, it should be appreciated that
the amount of solar radiation reflected onto secondary reflectors
107 by the primary reflectors 101 positioned between the inner and
outer primary reflectors 101 may vary depending upon the distance
and angle between the primary reflector 107 and solar receiver 103.
This is due to the change in image size and apparent collector
aperture size as the distance and angle between the reflector and
collector change.
[0055] In one example, if 50% of the rays collected from the outer
reflectors use secondary reflectors 107 and none of the rays
collected from the innermost reflectors use secondary reflectors
107, on average, approximately 25% of the total rays collected will
use secondary reflectors 107. As a result, secondary reflectors 107
may add approximately 25% to the overall reflection loss of the
system. For a system with a minor having a reflectance of 0.940,
for example, the effective reflection loss from the primary would
be 0.940 and from the secondary would be approximately 0.985 for a
net of 0.926. This is better than conventional single pipe
downward-facing secondary reflection systems which typically have
between 50% and 100% of the total rays collected using the
secondary reflectors. For example, for a non-imaging secondary such
as a compound macrofocal elliptical concentrator (CMEC) (e.g., as
described in Rabl, An and Winston, Roland, Ideal Concentrators for
Finite Sources and Restricted Exit Angles, Applied Optics, Vol. 15,
Issue 11, pp. 2880-2883 and Chaves, Julio, Introduction to
Nonimaging Optics, Light Presciptions Innovators, page 3) having a
secondary geometrical concentration of about 1.times., the number
of reflections in the secondary will be approximately 1 and the
combined reflectance loss will be 0.940.times.0.940=0.884.
[0056] In another example, tertiary reflectors may be placed near
the ends of the solar receiver to enlarge the apparent aperture of
the receiver. This arrangement may allow a larger number of primary
reflectors to be placed in the reflector field. Further, as will be
discussed in greater detail below, this arrangement may also lessen
ray spillage and increase the overall solar concentration on the
solar receiver.
[0057] FIG. 5 illustrates one side of solar collection system 500
having exemplary secondary reflectors 507 and exemplary tertiary
reflectors 515. (Note that tertiary reflectors 515 are shown
schematically, and should not be considered as a limitation upon
the shape and/or relative size of tertiary reflectors 515 to other
elements of solar collection system 500.) Elements 501, 503, and
505 of FIG. 5 may be similar to elements 101, 103, and 105 of FIG.
1, respectively. Secondary reflectors 507, however, may differ from
secondary reflectors 107 in that the reflective surface of
secondary reflectors 507 may form an arc approximating an ellipse
having foci located at or near the top edge of the outermost
primary reflector 501 when said reflector top edge is at its
highest position of usage and at or near the bottom outer edge of
the tertiary reflector 515 on the same side of solar collection
system 500. However, it should be appreciated that the reflective
surface of secondary reflector 507 may form an arc approximating a
portion of an ellipse having other foci, e.g., associated with a
portion of one of the primary reflectors and the solar absorber,
tertiary reflectors, or other components of solar collection system
500.
[0058] In another example, instead of reflecting to a point,
secondary reflector 507 may reflect an extreme ray from the top of
the outermost primary reflector 501 to be tangent to the outer
circumference of the outer absorber tube 505. In this example, the
reflective surface of secondary reflector 507 may form an arc
approximating a portion of a macrofocal ellipse.
[0059] In the illustrated example, tertiary reflectors 515 are
shown extending outwards from the top of each outer absorber tube
505. The reflective surfaces of the top portions of tertiary
reflectors 515 may form an arc approximating an involute centered
around the outer absorber tubes 505 of solar receiver 503. In one
example, the top portion of tertiary reflector 515 may be the
portion of the reflector located above the intersection point 519
between ray 517 and tertiary reflector 515. Ray 517 represents the
ray reflected from the outer primary reflector 501 on the opposite
end of solar collection system 500 that passes just below solar
receiver 503. The reflective surfaces of the bottom portions of
tertiary reflectors 515 (the portions below intersection point 519)
may form an arc approximating a portion of ellipse 511 having focus
513 located at or near the outer edge of solar receiver 503 (or
outermost absorber tube 505) and focus 514 located at or near the
top edge of the outermost primary reflector 501 when said reflector
top edge is at its highest position of usage.
[0060] In another example, instead of reflecting to a point, the
bottom portion of tertiary reflector 515 may reflect an extreme ray
from the outermost primary reflector 501 to be tangent to the outer
circumference of the outer absorber tube 505. In this example, the
reflective surface of the bottom portion of tertiary reflector 515
may form an arc approximating a portion of a macrofocal
ellipse.
[0061] By positioning and shaping tertiary reflector 515 in the
manner described above, solar radiation reflected by primary
reflectors 501 and secondary reflectors 507 that would otherwise
miss solar receiver 503 may be reflected onto solar receiver 503.
It will be appreciated that other shapes and curves for tertiary
reflector 515 may be used.
[0062] In one example, secondary reflectors 507 may be positioned a
vertical distance below solar receiver 503 to allow the solar
radiation aimed at solar receiver 503 and tertiary reflectors 515
to directly strike the receiver and tertiary reflector. For
example, ray 521 (as well as rays aimed above ray 521) may be
allowed to pass above secondary reflectors 507. This configuration
increases the efficiency of solar collection system 500 by
maximizing direct ray hits on the receiver and thus reducing the
number of unnecessary reflections.
[0063] FIGS. 6A and 6B illustrate the operation of exemplary solar
collection system 500, which includes both secondary reflectors and
tertiary reflectors. Like FIGS. 4A-4B described above, the
reflections caused by the outer primary reflectors 501 and the
inner primary reflectors 501 have been broken up into FIGS. 6A and
6B, respectively. However, it should be appreciated that during
actual operation, reflections from both the outer and inner primary
reflectors 501 may be generated simultaneously.
[0064] FIG. 6A illustrates reflections caused by the outer primary
reflectors 501 of exemplary solar collection system 500. The
operation of solar collection system 500 is similar to that of
solar collection system 100 shown in FIG. 4A. However, since the
reflective surface of secondary reflectors 507 may form an arc
approximating an ellipse having foci different from the foci of the
ellipse approximated by the arc formed by the reflective surface of
secondary reflectors 107, rays 502r may be reflected towards
tertiary reflectors 515 where they are reflected at least one
additional time towards solar collector 503. Additionally, rays
502d may strike both solar receiver 503 and the tertiary reflector
515 on the opposite end of solar receiver 503, rather than only
solar receiver 103 as is the case with rays 102d.
[0065] FIG. 6B illustrates reflections caused by the inner primary
reflectors 501 of exemplary solar collection system 500. The
operation of solar collection system 500 is similar to that of
solar collection system 100 shown in FIG. 4B. However, rather than
having all, or almost all, of the solar radiation reflected by the
inner primary reflectors 101 directly strike solar receiver 103 as
shown in FIG. 4B, all, or almost all of the solar radiation
reflected by the inner primary reflectors 501 may directly strike
both solar receiver 503 and tertiary reflectors 515. In this
example, none, or almost none, of the reflections caused by the
inner primary reflectors 501 are reflected by secondary reflectors
507. In other examples, a portion of the solar radiation reflected
by the inner primary reflectors 501 may be reflected by secondary
reflector 507 prior to contacting solar receiver 503.
[0066] While FIGS. 6A and 6B show only two rows of primary
reflectors 501 on each side of solar collection system 500, it
should be appreciated that any number of primary reflectors 501 or
rows of primary reflectors 501 may be positioned on each side of
solar collection system 500.
[0067] FIG. 7 illustrates exemplary secondary reflectors 703 and
705 that may be used in any of the examples provided herein. In one
example, the top portion of secondary reflectors 703 and 705 may be
separated by a space 701. Space 701 may allow hot air between the
two secondary reflectors 703 and 705 to rise up and out from
between secondary reflectors 703 and 705, thereby creating a
"chimney effect" to cool the reflectors, or at least prevent the
hot air from becoming trapped there between.
[0068] Although solar collection system 100 may be configured so
that most of the solar radiation reflected by the primary
reflectors is not incident to the underside of secondary reflectors
703 and 705, imperfections in the primary reflectors and
imperfections in the arrangement of components of solar collection
system 100 may cause stray reflections to strike the underside of
secondary reflectors 703 and 705. Thus, in one example, the inner
surfaces of secondary reflectors 703 and 705 may be coated with
highly reflective and non-absorbing paint or material to reduce the
amount of heat absorbed by the inner surfaces of secondary
reflectors 703 and 705 due to reflections from the primary
reflectors. For example, the inner surfaces of secondary reflectors
703 and 705 may be coated with a highly reflective white or silver
coating such as paints containing Titanium Oxide. In other
examples, secondary reflectors 703 and 705 may be constructed such
that the silver coating is protected by glass on both sides to
create a double sided reflector having a high reflectance for the
inside surface of secondary reflectors 703 and 705.
[0069] In another example, secondary reflectors 703 and 705 may
include light barrier 707 for blocking at least a portion of the
stray reflections from the primary reflectors that may strike the
underside of the secondary reflectors. Light barrier 707 may be
configured to connect to the bottom portions of each reflector.
Light barrier 707 may be positioned horizontally, or close to
horizontally, relative to the ground. The surface of light barrier
707 may be highly reflective and non-absorbing. For example, the
surface of light barrier 707 may be coated with a highly reflective
white or silver coating. Additionally, when light barrier 707 is
used, the inner surfaces of secondary reflectors 703 and 705 may be
coated with a coating having a high emissivity to reduce heating.
For example, the inner surfaces of reflectors 703 and 705 may be
coated with a black paint or glass surface having a high infrared
emissivity. In one example, the horizontal light barrier 707 may
include holes or apertures designed to admit air for upward
circulation and cooling of the underside of the secondary
reflectors. In another example, light barrier 707 may also be
designed to minimize light intrusion.
[0070] In another example, illustrated by FIG. 8, secondary
reflectors 703 and 705 may include vertical light barriers 803 and
805 for blocking at least a portion of the solar radiation
reflected by the primary reflectors. Vertical light barriers 803
and 805 may extend vertically downwards a sufficient distance from
secondary reflectors 703 and 705 to block solar radiation reflected
by the closest primary reflectors that would otherwise strike the
underside of the secondary reflectors 703 and 705, thereby ensuring
that secondary reflectors 703 and 705 are protected from at least a
portion of the stray light produced by the reflector field. The
surfaces of vertical light barriers 803 and 805 may be highly
reflective and non-absorbing. For example, the surface of light
barriers 803 and 805 may be coated with a highly reflective white
or silver coating, or may be vertical extensions extending downward
from the secondary reflector made of glass or other specular
reflector material.
[0071] Although a feature may appear to be described in connection
with a particular embodiment, one skilled in the art would
recognize that various features of the described embodiments may be
combined. Moreover, aspects described in connection with an
embodiment may stand alone.
* * * * *