US20110220094A1

US20110220094A1 - Secondary reflector for linear fresnel reflector system

Info

Publication number: US20110220094A1
Application number: US12/723,332
Authority: US
Inventors: David R. Mills; Matthew J. Lawrence
Original assignee: Ausra Inc
Current assignee: Areva Solar Inc
Priority date: 2010-03-12
Filing date: 2010-03-12
Publication date: 2011-09-15
Also published as: WO2011112632A2; WO2011112632A3; AU2011224454A1

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.

Description

1. FIELD

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

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.
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.
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

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.
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.
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.
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.
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.
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.
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.
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

FIG. 1 illustrates an exemplary solar collection system including a secondary reflector.

FIG. 2A illustrates a perspective view of an exemplary solar collection system including a secondary reflector.

FIG. 2B illustrates a zoomed-in perspective view of an exemplary solar collection system including a secondary reflector.

FIG. 3 illustrates an exemplary secondary reflector.

FIG. 4A illustrates the operation of an exemplary solar collection system including a secondary reflector.

FIG. 4B illustrates the operation of an exemplary solar collection system including a secondary reflector.

FIG. 5 illustrates an exemplary tertiary reflector.

FIG. 6A illustrates the operation of an exemplary solar collection system including a secondary reflector and tertiary reflector.

FIG. 6B illustrates the operation of an exemplary solar collection system including a secondary reflector and tertiary reflector.

FIG. 7 illustrates an exemplary secondary reflector.

FIG. 8 illustrates an exemplary secondary reflector.

DETAILED DESCRIPTION

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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 102 d. 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 102 r. For example, if secondary reflectors 107 were removed from solar collection system 100, the solar radiation represented by the bottom two ray lines 102 r reflected from the outer primary reflectors 101 would pass by solar receiver 103 and be lost.
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 102 r 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.
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.
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.
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.
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.
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×, the number of reflections in the secondary will be approximately 1 and the combined reflectance loss will be 0.940×0.940=0.884.
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.
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.
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.
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.
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.
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.
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.
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.
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 502 r may be reflected towards tertiary reflectors 515 where they are reflected at least one additional time towards solar collector 503. Additionally, rays 502 d 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 102 d.
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.
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.
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.
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.
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.
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.
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.

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.