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 
       [0001]    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 
       [0002]    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&#39;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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    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. 
         [0006]    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 
       [0007]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a cross-sectional view of a dual-optic solar concentrator unit; 
           [0009]      FIG. 2  provides a perspective view of an exemplary solar concentrator unit according to the present invention; 
           [0010]      FIG. 3  is a perspective view of an exemplary square secondary mirror; 
           [0011]      FIG. 4A  is a top view of the square secondary mirror of  FIG. 3 ; 
           [0012]      FIG. 4B  is a side view of the square secondary mirror of  FIG. 3 ; 
           [0013]      FIG. 4C  is a view of section A-A from the square secondary mirror of  FIG. 4A ; 
           [0014]      FIG. 5  depicts another embodiment of a square secondary mirror according to the present invention; 
           [0015]      FIG. 6  illustrates an alternative solar concentrator unit including a heat shield; 
           [0016]      FIG. 7  provides a perspective view of an exemplary heat shield; and 
           [0017]      FIG. 8  depicts an alternative solar concentrator unit incorporating a square secondary mirror and a heat shield. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0018]    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. 
         [0019]      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. 
         [0020]    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 . 
         [0021]    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. 
         [0022]    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 . 
         [0023]    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. 
         [0024]    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 ,  4 B, and  4 C 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 . 
         [0025]    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. 
         [0026]      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. 
         [0027]    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 . 
         [0028]    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. 
         [0029]    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 . 
         [0030]    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. 
         [0031]    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.