US20090114213A1

US20090114213A1 - Solar concentrator with square mirrors

Info

Publication number: US20090114213A1
Application number: US12/021,238
Authority: US
Inventors: Mark McDonald; Gill Shook; Michael Milbourne
Original assignee: Solfocus Inc
Current assignee: Solfocus Inc
Priority date: 2007-11-03
Filing date: 2008-01-28
Publication date: 2009-05-07
Also published as: WO2009058603A1

Abstract

The present invention is a solar concentrator system incorporating a square primary mirror, a square secondary mirror, and an optical receiver. The square secondary mirror provides highly efficient throughput of light in combination with the square primary mirror, with minimal shading. Manufacturing features may be incorporated into the square secondary mirror to assist in simplifying fabrication issues and assembly steps related to its non-circular shape. An optional heat shield around the optical receiver may be included, further enhancing performance of the solar concentrator system.

Description

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Ser. No. 60/985,200 filed on Nov. 3, 2007 entitled “Square Optical Mirror,” and also claims priority to U.S. Provisional Patent Application Ser. No. 61/016,314 filed on Dec. 21, 2007 entitled “Leadframe Receiver Package for Solar Concentrator Receiver,” both of which are hereby incorporated by reference as if set forth in full in this application for all purposes.

BACKGROUND OF THE INVENTION

Solar concentrators are solar energy generators which increase the efficiency of converting solar energy into DC electricity. Solar concentrators known in the art utilize, for example, parabolic mirrors and Fresnel lenses for focusing incoming solar energy, and heliostats for tracking the sun's movements in order to maximize light exposure. Another type of solar concentrator, disclosed in U.S. Patent Publication No. 2006/0266408, entitled “Concentrator Solar Photovoltaic Array with Compact Tailored Imaging Power Units,” utilizes a front panel for allowing solar energy to enter the assembly, with a primary mirror and a secondary mirror to reflect and focus solar energy through an optical receiver onto a solar cell. The surface area of the solar cell in such a concentrator system is much smaller than what is required for non-concentrating systems, for example less than 1% of the entry window surface area. Such a system has a high efficiency in converting solar energy to electricity due to the focused intensity of sunlight, and also reduces cost due to the decreased surface area of costly photovoltaic cells.
A similar type of solar concentrator is disclosed in U.S. Patent Publication No. 2006/0207650, entitled “Multi-Junction Solar Cells with an Aplanatic Imaging System and Coupled Non-Imaging Light Concentrator.” The solar concentrator design disclosed in this application uses a solid optic, out of which a primary mirror is formed on its bottom surface and a secondary mirror is formed in its upper surface. Solar radiation enters the upper surface of the solid optic, reflects from the primary mirror surface to the secondary mirror surface, and then enters a non-imaging concentrator which outputs the light onto a photovoltaic solar cell.
In these dual-optic type of solar concentrators, primary mirrors of various shapes have been described such as circular, hexagonal, and square, each shape having different impacts on design factors such as optical performance, array packaging, and manufacturability. Yet, secondary mirrors have remained circular in shape due to increased cost and complexity associated with non-circular parts. Typically, small mirrors such as a secondary mirror are fabricated using conventional lens grinding and polishing techniques, which are conducive to producing circular shapes. Various shapes may be produced with the process of glass molding. However, glass molding is challenged by substantially higher tooling costs, mitigation of which sometimes necessitates design changes of the desired part.
In addition to the cost and design obstacles from molding, manufacturing assembly issues arise from the use of a non-circular secondary mirror. For instance, a polygonal secondary mirror not only carries the same requirement as a circular secondary mirror of needing to be centered in the solar concentrator, but also should be oriented rotationally with respect to a corresponding polygonal primary mirror. That is, the edges of a polygonal secondary mirror should be aligned with the edges of a corresponding polygonal primary mirror in order for optimal light transmission to occur. This alignment condition adds complexity and cost to the manufacturing process of a solar concentrator unit.
Thus, improvements in a secondary mirror which may enhance optical performance for a non-circular primary mirror shape, while limiting impact on fabrication and manufacturing issues, can increase the success of a solar energy generator.

SUMMARY OF THE INVENTION

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a dual-optic solar concentrator unit;

FIG. 2 provides a perspective view of an exemplary solar concentrator unit according to the present invention;

FIG. 3 is a perspective view of an exemplary square secondary mirror;

FIG. 4A is a top view of the square secondary mirror of FIG. 3;

FIG. 4B is a side view of the square secondary mirror of FIG. 3;

FIG. 4C is a view of section A-A from the square secondary mirror of FIG. 4A;

FIG. 5 depicts another embodiment of a square secondary mirror according to the present invention;

FIG. 6 illustrates an alternative solar concentrator unit including a heat shield;

FIG. 7 provides a perspective view of an exemplary heat shield; and

FIG. 8 depicts an alternative solar concentrator unit incorporating a square secondary mirror and a heat shield.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference now will be made in detail to embodiments of the disclosed invention, one or more examples of which are illustrated in the accompanying drawings.
FIG. 1 depicts a cross-sectional view of an exemplary solar concentrator unit 100 as disclosed in U.S. Patent Publication No. 2006/0266408, entitled “Concentrator Solar Photovoltaic Array with Compact Tailored Imaging Power Units.” Solar radiation 160, represented by dashed lines, enters solar concentrator unit 100 through a front panel 110, reflects off a primary mirror 120, reflects again off a secondary mirror 130, and then enters an optical receiver 140. Optical receiver 140 includes a photovoltaic solar cell 145 where solar radiation 160 is converted to electricity, and optionally may also include a non-imaging concentrator 147. Non-imaging concentrator 147, if present, serves as a conduit to deliver solar radiation 160 to solar cell 145. Non-imaging concentrator 147 provides the potential to increase the acceptance angle of solar concentrator unit 100, and allows solar cell 145 to be located behind primary mirror 120 where heat sinking of solar cell 145 may be added.
U.S. Patent Publication No. 2006/0266408 describes this type of solar concentrator 100 with secondary mirror 130 being circular in shape and primary mirror 120 taking various shapes such as circular, hexagonal, and square. Non-circular shapes for primary mirror 120, such as hexagonal and square, allow for different configurations in packing solar concentrator units 100 into a solar panel array. For optimal light transmission, however, the design of secondary mirror 130 should be tailored for the distribution of light impinging from a particular shape of primary mirror 120. A circular secondary mirror 130 does not optimize light throughput for a non-circular primary mirror 120, largely due to excess shading. Excess shading reduces the amount of incoming light due to the presence of surface area on secondary mirror 300 which does not contribute to solar concentration. Thus, a circular secondary mirror 130 paired with a non-circular primary mirror 120 results in sub-optimal efficiency of solar concentrator unit 100.
In FIG. 2, a perspective view of a solar concentrator unit 200 according to the present invention is shown. Solar concentrator unit 200 includes a front panel 210, a primary mirror 220, a secondary mirror 230, and an optical receiver 240. In FIG. 2, both primary mirror 220 and secondary mirror 230 have square perimeters, formed by edges 225 of primary mirror 220 and edges 235 of secondary mirror 230. To achieve optimal light transmission from the paired square shapes of primary mirror 220 and secondary mirror 230 in FIG. 2, the edges 225 and edges 235 should be aligned substantially parallel to each other. Note that optical receiver 240 may represent an individual solar cell, or may represent a non-imaging concentrator combined with a solar cell as described previously in relation to FIG. 1. Optical receiver 240 is shown in FIG. 3 with a square cross-section, but may possess other cross-sectional shapes, such as a cylindrical shape, while still remaining within the scope of this invention.
Further details of an exemplary square secondary mirror 300 are shown in the perspective view of FIG. 3. In FIG. 3, the reflecting surface 310 of secondary mirror 300 is facing upward for clarity of the part. The exact curvature of reflecting surface 310 is designed to achieve the desired optical parameters within the overall dimensional constraints of the solar concentrator unit 200. Side faces 320 of secondary mirror 300 may incorporate features to simplify manufacturing of secondary mirror 300 and assembly of secondary mirror 300 into a solar concentrator unit. For instance, datum features 330 may be formed, such as by molding, into side faces 320 to orient secondary mirror 300 with respect to edges of a square primary mirror. In the exemplary embodiment of FIG. 3, datum features 330 are depicted as two recessed, rectangular notches which may used to register onto a tooling fixture (not shown) during assembly of a solar concentrator unit. However, datum features 330 may take other forms including protrusions, guide holes, and target markings for electronic positioning methods, separately or in combination, and may appear in other shapes including linear, circular, and triangular. Additionally, any number of datum features 330 may be incorporated onto a side face 320, and datum features 330 may be incorporated onto only one side face 320 or as many as all side faces 320.
Another manufacturing feature depicted in FIG. 3 for secondary mirror 300 are shelves 340 on side faces 320. Shelves 340 may be located on all side faces 320 for secure retention of secondary mirror 300 during the process of depositing mirror coating layers onto reflecting surface 310, a process which may involve rotation of the part in multiple orientations. Current forms of tooling used during deposition, such as locating pins, often contact the coated surface of a part which can cause shadowing of the deposited coatings. Such shadowing may lead to spatial non-uniformities in reflectivity as well as potential corrosion of secondary mirror 300 due to inadequate coating protection. Shelves 340 provide points for tooling to contact and secure secondary mirror 300 away from the reflecting surface 310, in a manner which allows for quick drop-in mounting of multiple parts into a deposition chamber.
Secondary mirror 300 may be fabricated from, for example, soda-lime glass using a molding process. In molding, a draft angle is required for releasing a part from its mold. FIGS. 4A, 4B, and 4C depict the effect of a draft angle around side faces 320 of secondary mirror 300. Draft angle 350 in the side view of FIG. 4B may be on the order of, for example, 5°. Because of the convex shape of reflecting surface 310, the draft angle 350 results in the removal of more material at midpoints 360 of side faces 320 as seen in FIG. 4A. That is, because side faces 320 are higher at their midpoints 360 than at their ends, the draft angle 350 cuts deeper into the convex reflecting surface 310 of secondary mirror 300 at midpoints 360. The resulting optical aperture contour of reflecting surface 310, which curves inward at midpoints 360, coincides with areas of the least light flux irradiance resulting from a square primary mirror. Thus, the amount of draft angle 350 may be adjusted to facilitate mold release as well as to contour the optical aperture of convex reflecting surface 310 to minimize excess shading. Section A-A, taken at the approximately the midline of secondary mirror 300 of FIG. 4A, is shown in FIG. 4C and provides a cross-sectional view of shelves 340 and of the narrowing of convex reflecting surface 310 at midpoint 360.
An alternative embodiment of the present invention is shown in FIG. 5, in which a secondary mirror 400 may have datum features 410 located on its mounting surface 420 instead of on its side faces as described in FIG. 3. Datum features 410 may be physical features such as the depicted circular recesses which align with matching features on a surface (such as front panel 110 of FIG. 1) to which secondary mirror 300 is to be mounted. In another embodiment, not shown, datum features 410 may be visual markings molded into mounting surface 420 to be sighted through transparent front panel 110 for alignment. Datum features 410 may be located near the center of mounting surface 420 as depicted in FIG. 5 or may be located around the perimeter of mounting surface 420, for example at opposing corners.
FIG. 6 illustrates a yet further embodiment of the present invention. A solar concentrator unit 500 includes a primary mirror 520 and a secondary mirror 530, with the addition of a heat shield 510 placed around an optical receiver 540. As shown in FIG. 6, an off-axis solar ray 560, which may result from tracking error of solar concentrator unit 500, does not focus at optical receiver 540. Instead, off-axis solar ray 560, which becomes highly concentrated after being reflected by primary mirror 520 and secondary mirror 530, can impinge upon and cause damage to primary mirror 520. Heat shield 510, which surrounds optical receiver 540, assists in preventing such off-axis solar rays 560 from straying outside the desired focal area. So as not to adversely affect useful light transmission within solar concentrator unit 500, the bounds of heat shield 510 ideally lie within an optically dead zone determined by the intersection of zones 570 and 575, shown by dotted lines. Zone 570 is the projection from opening 550, in which heat shield 510 and optical receiver 540 are inserted, to the focal point of primary mirror 520. Zone 575 is the region shaded by secondary mirror 530. Both zones 570 and 575 represent a family of surfaces calculated for the desired optical characteristics for solar concentrator unit 500, such as a specific target acceptance angle.
For a circular opening 550 and a square secondary mirror 530, zone 570 is conical and zone 575 is a pyramidal prism, the intersection of which creates a heat shield 600 shaped with four undulations around its upper edge 610 as shown in FIG. 7. For ideal light transmission, heat shield 600 should have its corners 620 oriented with the corners of square primary mirror 520 and square secondary mirror 530 of solar concentrator 500 of FIG. 6. Orientation of heat shield 600 may be achieved by a registration feature such as notch 630 corresponding to a mating feature in opening 550 of primary mirror 520, or by datum features similar to those described with respect to FIG. 3.
Note that opening 550 of primary mirror 520 may alternatively be non-circular in shape, which would modify the resulting contour of upper edge 610 and outer surface 640 of heat shield 600. Moreover, the outer surface 640 of heat shield 600 need not be limited by the exact regions delineated by zones 570 and 575. For example, heat shield 600 may be larger than the calculated zones 570 and 575, which sacrifices some light transmission to allow for greater manufacturing tolerances of solar concentrator unit 500. Also, inner surface 650 of heat shield 600 may be reflective and may be tailored with a profile to capture a desired range of off-axis angles.
In an alternative type of solar concentrator system 700 shown in FIG. 8, a dielectric 710 fills the space between a primary mirror 720 and a secondary mirror 730. Dielectric 710 is chosen with a suitable index of refraction “n,” such as a value of “n” being, for example, 1.4 to 1.5. In a situation where dielectric 710 is a solid material such as glass, a square primary mirror 720 and a square secondary mirror 730 may be formed and aligned directly into the lower and upper surfaces, respectively, of dielectric 710. A heat shield 750 may be utilized around optical receiver 740 of solar concentrator system 700 similarly as described with respect to FIGS. 6 and 7.
Although embodiments of the invention have been discussed primarily with respect to specific embodiments thereof, other variations are possible. For instance, while the invention utilizes a square, the design principles disclosed herein may apply to other polygonal components such as hexagonal mirrors. Furthermore, although datum features in this invention have been described to orient square mirror substantially parallel with each other, circumstances may arise in which other non-parallel orientations may be desired. Lenses or other optical devices might be used in place of, or in addition to, the primary and secondary mirrors or other components presented herein. For example, a Fresnel lens could be used to focus light onto the solar concentrator system, or to focus light at an intermediary phase of the solar concentrator. Other embodiments can use optical or other components for focusing any type of electromagnetic energy such as infrared, ultraviolet, or radio-frequency. There may be other applications for the fabrication method and apparatus disclosed herein, such as in the fields of light emission or sourcing technology (e.g., fluorescent lighting using a trough design, incandescent, halogen, spotlight, etc.) where a light source is put in the position of the photovoltaic cell. Other types of energy conversion, such as thermal transfer to a fluid system, may be used instead of conversion to electricity by a photovoltaic cell may be used.
While the specification has been described in detail with respect to specific embodiments of the invention, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. These and other modifications and variations to the present invention may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present invention, which is more particularly set forth in the appended claims. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention. Thus, it is intended that the present subject matter covers such modifications and variations as come within the scope of the appended claims and their equivalents.

Claims

1. A solar concentrator system, comprising:

a substantially planar surface;

a curved primary mirror for reflecting solar radiation, said curved primary mirror having a first perimeter which is substantially square, and wherein at least a portion of said first perimeter is in contact with said substantially planar surface;

a curved secondary mirror positioned to reflect said solar radiation from said primary mirror, said curved secondary mirror having a second perimeter which is substantially square, and wherein at least a portion of said second perimeter is in contact with said substantially planar surface; and

an optical receiver positioned to receive said solar radiation from said curved secondary mirror.

2. The solar concentrator system of claim 1, wherein said curved secondary mirror comprises molded glass.

3. The solar concentrator system of claim 1, wherein said curved secondary mirror is convex.

4. The solar concentrator system of claim 1, wherein said curved secondary mirror further comprises a datum feature for orienting said second perimeter of said curved secondary mirror with respect to said first perimeter of said curved primary mirror.

5. The solar concentrator system of claim 4, wherein said curved secondary mirror further comprises a side face, wherein said datum feature is located on said side face.

6. The solar energy system of claim 1, further comprising a heat shield around said optical receiver.

7. The solar energy system of claim 6, wherein said a heat shield has an upper edge with a contour, wherein said contour of said upper edge is derived from a zone shaded by said curved secondary mirror.

8. The solar concentrator system of claim 1, wherein said optical receiver comprises a photovoltaic cell.

9. The solar concentrator system of claim 8, wherein said optical receiver further comprises a non-imaging concentrator.

10. The solar concentrator system of claim 9, wherein said non-imaging concentrator is a truncated pyramid.

11. The solar concentrator system of claim 1, further comprising a space between said curved primary mirror and said curved secondary mirror, wherein said space is filled with a dielectric material.

12. The solar concentrator system of claim 11, further comprising a heat shield around said optical receiver.

13. A method of concentrating solar radiation with a curved primary mirror having a first perimeter which is substantially square, a curved secondary mirror having a second perimeter which is substantially square and an optical receiver, comprising the steps of:

attaching at least a portion of said first substantially square perimeter of said curved primary mirror to a substantially planar surface;

attaching at least a portion of said second substantially square perimeter of said curved secondary mirror to said substantially planar surface;

positioning said optical receiver to receive solar radiation that passes through said substantially planar surface, is reflected from said curved primary mirror, and is reflected from curved secondary mirror.

14. The method of concentrating solar radiation of claim 13, wherein said optical receiver comprises a photovoltaic cell.

15. The method of concentrating solar radiation of claim 14, wherein said optical receiver further comprises a non-imaging concentrator.

16. The method of concentrating solar radiation of claim 15, wherein said non-imaging concentrator is a truncated pyramid.

17. The method of concentrating solar radiation of claim 13, further comprising the step of positioning a heat shield around said optical receiver.

18. The method of concentrating solar radiation of claim 17, wherein a portion of said solar radiation is reflected off of said heat shield.

19. The method of concentrating solar radiation of claim 15, wherein said curved secondary mirror comprises molded glass.

20. The method of concentrating solar radiation of claim 15, wherein a dielectric material is located between said curved primary mirror and said curved secondary mirror.