Abstract:
An apparatus is disclosed for generation of electricity using sunlight focused onto multi junction photovoltaic cells having high conversion efficiency. The apparatus includes a large paraboloidal mirror of back-silvered glass, turned to the sun throughout the day, so as to provide an intense focus. Multiple photovoltaic cells are provided at the focus. The optics are configured to distribute sunlight without significant loss into separate regions matched to the photovoltaic cell size. A secondary optical system takes strongly focused sunlight near the focus of a single paraboloidal mirror and distributes it equally between the cells, and regions of equally concentrated sunlight are matched to cell size and are substantially co-planar, so that the cells may be grouped on flat circuit cards.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    In the past, solar generators aimed at exploiting the high efficiency of multi-junction solar cells to generate electricity typically employed many small solar focusing optical systems for each individual photovoltaic cell. Such generators were deficient in that the packaged assemblies of numerous optical systems and cells were both large and complex, and consequently suffered from a relatively high cost that made such solar generators uncompetitive with alternative methods of generating electricity. Such generators also required large, unique facilities for their manufacture, and were expensive to transport from the factory to an installation site. 
         [0002]    Some previous designs of solar generators have been disclosed that use single large reflectors to power arrays of multi-junction photovoltaic cells. U.S. Patent Application Publication No. 2011/0168234, by John Lasich, titled “Photovoltaic Device for a Closely Packed Array,” describes a solar generator with a densely-packed array of solar cells near the focus of a large paraboloidal reflector dish. Planar mirrors are arranged around the perimeter of a densely packed array. One drawback of the proposed configuration by Lasich is that no provision is made to direct light away from the light-insensitive electrical connections on the front surface of the arrayed cells, causing losses and reduced efficiency. Another drawback is that the illumination is not uniformly distributed across the array, causing loss of power when individual cells are connected in series. Yet another drawback is that small mispointing of the optical axis away from the sun would cause the illumination to become more uneven, further reducing power output. Lasich proposes the use of stiff, heavy trackers to mitigate this problem by maintaining accurate pointing, but such trackers drive up cost. 
         [0003]    U.S. Pat. No. 8,350,145, by Roger P. Angel, titled “Photovoltaic Generator with a Spherical Imaging Lens for Use with a Paraboloidal Solar Reflector,” uses a spherical ball lens at the focus of a paraboloidal dish reflector. The lens stabilizes the light against mispointing at the image of the dish reflector, formed on a concave surface, and tiled with tapered optical funnels. At each funnel output, the light is distributed into discrete square regions, with a photovoltaic cell located at each region. 
         [0004]    However, because the apparatus disclosed in U.S. Pat. No. 8,350,145 relies on spherical symmetry to realize equal apportionment of sunlight to a plurality of photovoltaic cells arranged in a concave array, the cells and optical funnels are configured in a concave array. In practice, the manufacturing costs involved in making curved reflecting surfaces and supporting structures for the concave array of photovoltaic cells have been relatively high. In addition, the lens itself is preferably made as a full sphere (ball lens), and both the optical funnels and the photovoltaic cells are deployed on concentric concave spherical surfaces. Some embodiments use photovoltaic cells of many different shapes and sizes to tile the spherical surface, and in practice, this added complexity has increased costs. Some embodiments use identical square cells, but complex funnel shapes are configured to fit together seamlessly to tile a spherical surface at their input, and to match the square cell dimension at their output. In practice, such embodiments have been relatively expensive to manufacture, because the funnels are manufactured with many different odd shapes to fit together, and the individual reflective surfaces of a funnel, instead of being flat, are twisted to bring the light from an odd entrance shape to a square output to match the square photovoltaic cell. In addition, providing the funnel surfaces with high specular reflectance, and subsequent coating for very high reflectivity, tend to be more expensive to manufacture. 
         [0005]    Mounting photovoltaic cells to conform to a spherical surface may be problematic. If individual flat photovoltaic cells are to be mounted individually on electrically insulating but thermally conductive substrates, and such substrates to be attached to a concave, faceted surface, with the facets tangent to a sphere, the mounting process is further complicated by the additional requirement for transfer of high flows of both heat and electricity from the substrates. 
         [0006]    In some prior designs, compensation for shadowing of a primary mirror by a central assembly of secondary optics and any supporting structure is achievable only by eliminating partly blocked cells from a series-connected chain. This may waste light, and consequently lead to reduced efficiency and power output. 
         [0007]    It follows that many prior designs have suffered from relatively high manufacturing costs. In addition, some prior designs may have inevitable light blockages that break the continuous sunlight beam from a primary reflector, and as a result, may cause current imbalances and reduced power output. There is therefore room for improvement. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is an apparatus for generation of solar electricity by focusing sunlight onto small multi-junction photovoltaic cells having exceptionally high conversion efficiency. The invention addresses the key requirement for solar generation, namely low manufacturing cost and high overall efficiency. To this end, the apparatus includes a large paraboloidal mirror of back-silvered glass, turned to the sun throughout the day, so as to provide an intense focus. Solar reflectors of large back-silvered glass segments already used extensively to concentrate sunlight for solar thermal generation have been proven to have long life in field operation and are relatively inexpensive per unit of solar power brought to a focus. However, to exploit such large collectors in an economical system, it is necessary to inexpensively convert into electricity the powerful sunlight provided at the focus. The conversion cannot be accomplished by a single high-efficiency multi-junction cell placed at the focus, as is common for cells used with small lens collectors, because the electrical current would be so large as to cause a single cell to fail. 
         [0009]    This invention provides for efficient operation of multi-junction photovoltaic cells at the powerful focus, by dividing the light to illuminate multiple small cells, each operating at a safe, reduced current. The secondary optics of the invention located near the powerful focus are configured so as to distribute the light without loss into individual separate regions each matched to the size of a single cell, or small group of parallel-connected adjacent cells acting as a single large cell. These regions are set slightly apart, so as to provide room for electrical connections between the cells or groups. The secondary optics of this invention provide for equal division of light between all the series-connected cells or groups. This equality is required for efficient power generation by simple series connection of the cells, because in such connection, power is lost unless the photovoltaic current, and therefore the amounts of light received by all of the individual cells, is very nearly equal. 
         [0010]    A particular feature of the optical design of this invention is its matching of the secondary optical system specifically to the particular pattern of illumination of the primary reflector. This matching is required in a practical system to ensure equal division of light between the cells or groups despite the uneven illumination of the primary collector. Such unevenness is inevitable in practice because of local shadowing of a large axisymmetric reflector by system elements blocking the sun ahead of it. 
         [0011]    A second feature is to maintain balance despite slight mispointing of the apparatus away from the sun. Maintaining such balance is needed to avoid the cost for heavy solar trackers needed to point accurately in the wind. 
         [0012]    A third important feature of this invention is that the regions of equal concentrated sunlight output by the secondary optics are matched to cell size are arranged to be co-planar (not on a spherical surface), so that the cells may be grouped on flat circuit cards. This is done to reduce manufacturing cost, because flat circuit cards are simple to assemble with photovoltaic cells and the other circuit elements, by methods well developed in the electronics industry. Flat cards are also conveniently adapted for active cooling with thermal transfer liquid, needed to keep the cells cool despite high thermal loads. 
         [0013]    A fourth important feature of this invention is the design of the secondary optics, in which a lens is combined with secondary reflecting elements in the form of sharp-edged wedges to cleanly separate the light directed toward different cells. The wedges are readily made from sheets of inexpensive flat material that has been pre-polished to a high specular finish and silvered and overcoated for both very high solar reflectivity and long-term resistance to tarnishing. In one aspect assemblies of multiple wedges are made as “origami optics”. The flat retlecting material is deeply grooved along fold lines such that when folded it forms multiple, sharply defined reflector wedges in the correct geometrical configuration to illuminate multiple cells. This method is inexpensive and yields wedge arrays of very high optical throughput. 
         [0014]    Two embodiments are shown which differ in their configurations of cells and secondary optics within the power conversion units. 
         [0015]    In the first, the cells are configured in a single planar array. The secondary optics to distribute the focused light evenly across the flat surface include a telecentric entrance lens with two elements, one having an aspheric surface. 
         [0016]    In the second embodiment, the cells are configured in four planar quadrants, tilted with respect to each other. In this case, the secondary optics include a single element entrance lens. 
         [0017]    In both embodiments, the lens serves both as an entrance window to a sealed chamber containing the wedge reflectors and photovoltaic cells and preventing contamination. The lens reformats the light into an image of the primary reflector which is fixed in position within the receiver package and stabilized against pointing error. This image has a sharply defined edge and includes the detail of any obscuring elements. The stability and detail of this image make possible the subsequent division of the light so that each cell group receives the same amount despite blocked areas and mispointing. In both embodiments, the lens is constructed so as to direct rays to arrive substantially perpendicular to the planar arrays of cells. This allows the wedge reflectors to be optimized so that the light reflected down to the cells (or cell groups) remains evenly divided even when the apparatus is slightly mispointed from the sun. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a schematic diagram of a solar concentrating apparatus according to the present invention, supported on a two-axis tracking mount. 
           [0019]      FIG. 2   a  is a cross-sectional view of an optical window, a lens design, and a photovoltaic cell array according to a first embodiment of the present invention. 
           [0020]      FIG. 2   b  is a perspective view of the optical window, lens design, and photovoltaic cell array shown in  FIG. 2   a.    
           [0021]      FIG. 3   a  is a linear cross-sectional view of a lens design and a photovoltaic cell array according to a first embodiment of the present invention showing on-axis rays of light. 
           [0022]      FIG. 3   b  is a linear cross-sectional view of a lens design and a photovoltaic cell array according to a first embodiment of the present invention showing off-axis rays of light. 
           [0023]      FIG. 4  is a perspective view of an individual photovoltaic cell. 
           [0024]      FIG. 5  is a schematic diagram illustrating a cross-sectional view of a reflective wedge over wiring between adjacent photovoltaic cells. 
           [0025]      FIG. 6   a  is a schematic diagram illustrating on-axis rays impinging upon an array of wedges and photovoltaic cells according to  FIG. 5 . 
           [0026]      FIG. 6   b  is a schematic diagram illustrating off-axis rays impinging upon an array of wedges and photovoltaic cells according to  FIG. 5 . 
           [0027]      FIG. 7  is a diagram illustrating a symmetric four-fold division of a focal surface. 
           [0028]      FIG. 8   a  is a perspective view of three adjacent photovoltaic cells. 
           [0029]      FIG. 8   b  is a perspective view of three groups of three adjacent photovoltaic cells. 
           [0030]      FIG. 9   a  is a perspective view of a photovoltaic cell circuit card. 
           [0031]      FIG. 9   b  is a plan view of the photovoltaic cell circuit card shown in  FIG. 9   a  with a wedge assembly framing the photovoltaic cells. 
           [0032]      FIG. 9   c  is a perspective view of the photovoltaic cell circuit card and wedge assembly shown in  FIG. 9   b.    
           [0033]      FIG. 10   a  is a plan view of a dish reflector and cantilever arm supporting a Power Conversion Unit in accordance with the present invention. 
           [0034]      FIG. 10   b  is a plan view of the layout of a corresponding photovoltaic cell circuit card and wedge assembly depicting the area affected by the shadow of the corresponding cantilever arm and Power Conversion Unit shown in  FIG. 10   a.    
           [0035]      FIG. 11  is a perspective view of a photovoltaic cell circuit card and wedge assembly in accordance with a first embodiment showing a central secondary reflector. 
           [0036]      FIG. 12   a  is a diagram of cell illumination distribution for a first embodiment according to the present invention showing on-axis illumination. 
           [0037]      FIG. 12   b  is a diagram of cell illumination distribution for a first embodiment according to the present invention showing off-axis illumination. 
           [0038]      FIG. 13   a  illustrates irradiation patterns for photovoltaic cell groups corresponding to a first embodiment during on-axis pointing. 
           [0039]      FIG. 13   b  illustrates irradiation patterns for photovoltaic cell groups corresponding to a first embodiment during off-axis pointing. 
           [0040]      FIG. 14   a  depicts graphs of a histogram of cell illumination distribution and a current-voltage curve corresponding to on-axis pointing of a first embodiment according to the present invention. 
           [0041]      FIG. 14   b  depicts graphs of a histogram of cell illumination distribution and a current-voltage curve corresponding to off-axis pointing of 0.25 degree for a first embodiment according to the present invention. 
           [0042]      FIG. 14   c  depicts graphs of a histogram of cell illumination distribution and a current-voltage curve corresponding to off-axis pointing of 0.50 degree for a first embodiment according to the present invention. 
           [0043]      FIG. 14   d  depicts graphs of a histogram of cell illumination distribution and a current-voltage curve corresponding to off-axis pointing of 0.75 degree for a first embodiment according to the present invention. 
           [0044]      FIG. 15  is a contour diagram of output power as a function of system pointing error for a first embodiment according to the present invention. 
           [0045]      FIG. 16  is a graph depicting output power as a function of azimuth and elevation pointing errors for a first embodiment according to the present invention. 
           [0046]      FIG. 17   a  is a cross-sectional view of a lens design and a plurality of planar photovoltaic cell arrays according to a second embodiment of the present invention. 
           [0047]      FIG. 17   b  is a perspective view of the lens design and plurality of planar photovoltaic cell arrays shown in  FIG. 17   a.    
           [0048]      FIG. 18   a  is a cross-sectional diagram of a second embodiment of the present invention depicting rays of sunlight reflected from a dish reflector during on-axis pointing of the dish reflector. 
           [0049]      FIG. 18   b  is a cross-sectional diagram of a second embodiment of the present invention depicting rays of sunlight reflected from a dish reflector during off-axis pointing of the dish reflector. 
           [0050]      FIG. 19   a  is a cross-sectional view of a wedge assembly and photovoltaic cells according to a second embodiment of the present invention showing rays of sunlight during on-axis pointing of a dish reflector. 
           [0051]      FIG. 19   b  is a cross-sectional view of a wedge assembly and photovoltaic cells according to a second embodiment of the present invention showing rays of sunlight during off-axis pointing of a dish reflector. 
           [0052]      FIG. 20   a  is a perspective diagram of a plurality of photovoltaic cell circuit cards. 
           [0053]      FIG. 20   b  is a perspective diagram of a photovoltaic cell circuit card and wedge assembly. 
           [0054]      FIG. 20   c  is a perspective view of a plurality of photovoltaic cell circuit cards and corresponding wedge assemblies according to a second embodiment of the present invention. 
           [0055]      FIG. 21   a  is a plan view of a dish reflector and cantilever arm supporting a Power Conversion Unit in accordance with a second embodiment of the present invention. 
           [0056]      FIG. 21   b  is a plan view of the layout of a corresponding photovoltaic cell circuit card and wedge assembly depicting the area affected by the shadow of the corresponding cantilever arm and Power Conversion Unit shown in  FIG. 21   a.    
           [0057]      FIG. 22  is a perspective view of a photovoltaic cell circuit card and wedge assembly in accordance with a second embodiment showing a central secondary reflector. 
           [0058]      FIG. 23   a  is a diagram of cell illumination distribution for a second embodiment of the present invention showing on-axis illumination. 
           [0059]      FIG. 23   b  is a diagram of cell illumination distribution for a second embodiment of the present invention showing off-axis illumination. 
           [0060]      FIG. 24   a  illustrates irradiation patterns for photovoltaic cell groups corresponding to a second embodiment during on-axis pointing. 
           [0061]      FIG. 24   b  illustrates irradiation patterns for photovoltaic cell groups corresponding to a second embodiment during off-axis pointing. 
           [0062]      FIG. 25   a  depicts graphs of a histogram of cell illumination distribution and a current-voltage curve corresponding to on-axis pointing of a second embodiment of the present invention. 
           [0063]      FIG. 25   b  depicts graphs of a histogram of cell illumination distribution and a current-voltage curve corresponding to off-axis pointing of 0.25 degree for a second embodiment of the present invention. 
           [0064]      FIG. 25   c  depicts graphs of a histogram of cell illumination distribution and a current-voltage curve corresponding to off-axis pointing of 0.50 degree for a second embodiment of the present invention. 
           [0065]      FIG. 25   d  depicts graphs of a histogram of cell illumination distribution and a current-voltage curve corresponding to off-axis pointing of 0.75 degree for a second embodiment of the present invention. 
           [0066]      FIG. 26  is a contour diagram of output power as a function of system pointing error for a second embodiment according to the present invention. 
           [0067]      FIG. 27  is a graph depicting output power as a function of azimuth and elevation pointing errors for a second embodiment of the present invention. 
           [0068]      FIG. 28   a  is a diagram depicting a quadrant of a wedge reflector assembly. 
           [0069]      FIG. 28   b  is a diagram depicting a step in a method of manufacturing a quadrant of a wedge reflector assembly. 
           [0070]      FIG. 28   c  is a diagram depicting a step in a method of manufacturing a quadrant&#39;s interior wedge reflector. 
           [0071]      FIG. 28   d  is a diagram depicting a step in a method of manufacturing a quadrant&#39;s interior wedge reflector. 
           [0072]      FIG. 28   e  is a diagram depicting a step in a method of manufacturing a quadrant&#39;s interior wedge reflector. 
           [0073]      FIG. 28   f  is a diagram depicting a step in a method of manufacturing a quadrant&#39;s perimeter wedge reflector. 
           [0074]      FIG. 28   g  is a diagram depicting a step in a method of manufacturing a quadrant&#39;s perimeter wedge reflector. 
           [0075]      FIG. 28   h  is a diagram depicting a step in a method of manufacturing a quadrant&#39;s perimeter wedge reflector. 
           [0076]      FIG. 28   i  is a diagram depicting a step in a method of manufacturing a quadrant&#39;s perimeter wedge reflector. 
           [0077]      FIG. 28   j  is a diagram depicting a step in a method of manufacturing a quadrant of a wedge reflector assembly. 
           [0078]      FIG. 29   a  is a diagram showing the configuration of four quadrants of a wedge reflector assembly according to the first embodiment of the present invention. 
           [0079]      FIG. 29   b  is a diagram showing the configuration of four quadrants of a wedge reflector assembly according to the second embodiment of the present invention. 
           [0080]      FIG. 30  is a perspective view of photovoltaic cell groups of three cells, where the groups are connected in series on a circuit card. 
           [0081]      FIG. 31  is a schematic diagram of the electrical connections for a plurality of photovoltaic cells connected in parallel groups of three cells each with a bypass diode, where the groups of cells are connected in series. 
           [0082]      FIG. 32  is a plan view of the details of a circuit board for photovoltaic cells in accordance with the first embodiment. 
           [0083]      FIG. 33  is a plan view of the details of a circuit board for photovoltaic cells in accordance with the second embodiment. 
           [0084]      FIG. 34  is a partially cut-away perspective view of a circuit board for photovoltaic cells showing a thermal pathway between wedge support structure and the circuit board. 
           [0085]      FIG. 35  is a perspective view of a solar concentrating apparatus according to the present invention showing an array of a plurality of dish reflectors and corresponding Power Conversion Units. 
           [0086]      FIG. 36  is a schematic diagram of a solar concentrating apparatus according to the present invention showing coolant flow for the array of Power Conversion Units shown in  FIG. 35 . 
           [0087]      FIG. 37  is a cross-sectional schematic diagram of an example of a Power Conversion Unit. 
           [0088]      FIG. 38  is a diagram of a flat sheet of pre-coated reflective material used to make wedge reflectors  16 . 
           [0089]      FIG. 39  is a diagram of the reflective material shown in  FIG. 38  after an undercut is made in the sheet. 
           [0090]      FIG. 40  is a diagram of the reflective material shown in  FIG. 39  after the material is folded to form a wedge reflector. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0091]      FIG. 1  is a schematic diagram of a solar concentrating apparatus  1  according to the present invention, supported on a two-axis tracking mount  3 . A substantially square paraboloidal dish reflector  2  has an axis  19 , and incoming sunlight  22  striking the dish reflector  2  is reflected in the direction indicated by rays  122  shown in  FIG. 1  to a focus  7 . During operation, the axis  19  of the reflector  2  is aligned to the direction of the sun by a dual-axis mount  3 , so that sunlight reaching the dish reflector  2  is concentrated at the focus  7 . 
         [0092]    The converging rays  122  of sunlight enter a Power Conversion Unit  20 , or PCU  20 , positioned near the focus  7 . The focused sunlight entering the PCU  20  is converted into electricity by the PCU  20 . The PCU  20  is supported by an arm  25  which is attached to a cantilevered post  26 . The cantilevered post  26  is rigidly attached to a support structure  27 . The reflector  2  is also disposed upon or attached to the support structure  27 . In this way, the PCU  20  is constrained to remain aligned with the focus  7  of the reflector  2 . 
         [0093]    The PCU  20  comprises a plurality of photovoltaic cells  30  configured in one or more planar arrays. The PCU  20  includes secondary optics comprising a single or compound lens  29  which is positioned near the focus  7 , together with sharp wedge reflectors  16 . The secondary optics are configured so that the lens  29  and the wedge reflectors  16  cooperate to apportion the sunlight from the dish reflector  2  onto the plurality of photovoltaic cells  30  in substantially equal amounts so that the photovoltaic cells  30  generate substantially equal electrical current when illuminated with sunlight. In a first embodiment, the lens  29  comprises a compound telecentric lens  29  as shown in  FIG. 2   a ,  FIG. 2   b ,  FIG. 3   a , and  FIG. 3   b . In a second embodiment, the lens may be a single lens such as the double convex lens  70  shown in  FIG. 17   a ,  FIG. 17   b ,  FIG. 18   a , and  FIG. 18   b.    
         [0094]    The example shown in  FIG. 1  depicts a single dish reflector  2  and PCU  20  carried by a single dual-axis mount  3 . However, it should be understood that a plurality of dish reflectors  2 , each having a corresponding PCU  20 , may be mounted upon a single rigid support structure  27  on a two-axis mount  3  as shown, for example, in  FIG. 35 . 
       First Embodiment 
       [0095]    As shown in  FIG. 1 , solar rays  22  parallel to the axis  19  are reflected by the large reflector  2 , and the reflected solar rays  122  converge upon the focus  7 . Turning now to  FIG. 2   a , the reflected rays  122  pass through the focus  7  and impinge upon the lens  29 . In accordance with a first embodiment of the present invention, the lens  29  shown in  FIG. 2   a  is a telecentric lens  29 . 
         [0096]      FIG. 2   a  shows details of components comprising the PCU  20  according to a first embodiment of the present invention. In the illustrated example, a flat window  6  forms the entrance to the PCU  20 . Within the PCU  20  and behind the window  6 , is a two-element telecentric lens  29 , a plurality of wedge reflectors  16 , and a single planar array  18  of photovoltaic cells  30 , which generate electricity. In this example, the telecentric lens  29  comprises a first piano-convex element  8 , having a flat entrance surface  9  and a convex aspheric back surface  10 . The telecentric lens  29  further comprises a second lens element  11  which is a double convex lens element  11 , having a spherical entrance surface  12  and an exit surface  13 . Although  FIG. 2   a  shows a two element compound lens as the illustrated example of a telecentric lens  29 , it should be understood that the telecentric lens  29  may comprise other telecentric multi-element lens designs in which the chief rays are collimated and parallel to the optical axis in image space and provide uniform image plane illumination. 
         [0097]    The reflector  2  (not shown) would be located off to the right of  FIG. 2   a , and the reflected rays  122  converge to the PCU  20 . These converging rays  122  pass through the flat window  6 , and generally converge to the focus  7 . The sun is not a point source of light. Instead, the sun is a disc as seen from planet Earth.  FIG. 2   a  shows additional converging rays  4  which originate from the top edge of the sun&#39;s disc, and converge to a corresponding focal point  701 .  FIG. 2   a  shows additional converging rays  5  which originate from the bottom edge of the sun&#39;s disc, and converge to a corresponding focal point  702 . 
         [0098]    The telecentric lens  29  forms a flat, square image of the primary reflector  2  as shown by the dashed line  23  in  FIG. 2   a . Each of the sharp wedge reflectors  16  is located with its apex  50  in the image plane  23 . The sharp wedge reflectors  16  function to direct rays  4  and  5  away from gaps between the photovoltaic cells  30  in the array  18 . Perimeter reflectors  15  surround the wedge array perimeter, to bring edge rays onto the photovoltaic cells  30  around the perimeter of the array  18 . 
         [0099]      FIG. 2   b  shows the main elements of the PCU  20  in perspective: the entrance window  6 , the first lens element  8  and the second lens element  11  of the compound telecentric lens  29 , and a secondary array assembly  17  which includes the wedge reflectors  16 , the perimeter reflectors  15 , and the planar cell array  18 . 
         [0100]    The telecentric lens  29  used in the first embodiment is designed to have three characteristics that are important for the efficient operation of the apparatus  1 . First, the lens  29  reformats the concentrated light at the focus  7  of the primary reflector  2  into a sharply defined image  23 , which is stabilized against mispointing and is also flat, and thus matched to the flat cell array  18 . This allows high efficiency coupling of the concentrated sunlight to a flat array of photovoltaic cells  18 . 
         [0101]    Second, the lens  29  is free of distortion. As a result, solar rays  22  which are evenly spaced on entering the apparatus  1  are also evenly spaced as they form the image  23 . Freedom from distortion is highly desirable, since it results in the concentrated sunlight having substantially uniform brightness at the image plane  23  near to where the cells  30  and the wedge reflectors  16  are located. A second valuable attribute of the distortion free telecentric lens  29  is that the image  23  has the same shape as the primary reflector  2 , namely square, so it can efficiently be coupled to the array  18  of square or rectangular photovoltaic cells  30 . 
         [0102]    The third important characteristic of the telecentric lens  29  is to redistribute the concentrated sunlight as a collimated beam  14  substantially perpendicular to the image plane  23 , as shown in  FIG. 3   a .  FIG. 3   a  shows rays originating from a distant point aligned with the optical axis  19  of the apparatus  1 , for example, the center of the sun&#39;s disc. Sun rays  22  enter the apparatus parallel to each other and to the axis  19 , as shown in  FIG. 1 . After reflection by the primary reflector  2 , these rays  122  are brought to a point focus  7 . Referring now to  FIG. 3   a , after passage through the telecentric lens  29 , the rays  14  have been refracted to be parallel to the axis  19  of the reflector  2 . This is a result of the telecentricity of the lens  29 . The rays  14  thus strike the planar cell array  18  at normal incidence, which is perpendicular to the photovoltaic cells  30 . 
         [0103]      FIG. 3   b  illustrates an example of rays  124  from a distant point not aligned with the optical axis  19 . This may be the result of the reflector  2  not being pointed directly at the sun. In this case, the rays  124  converge on a point  703  which is displaced away from the optical axis  19 . But after refraction by the telecentric lens  29 , the rays  24  strike the wedge reflectors  16  and the cell arrays  18  in nearly the same square image area  23  as the example shown in  FIG. 3   a . The telecentric lens  29  compensates for the mispointing of the reflector  2 . The rays  24  also remain parallel to each other and uniformly spaced, although tilted to the axis  19 . Uniform distribution of sunlight across the photovoltaic cells  30  and wedge array  18  is thus maintained even when the reflector  2  is slightly mispointed away from the sun. 
         [0104]    It will be understood by those with skilled in the art that the two element lens  29  illustrated in this example is simply one example of a telecentric lens  29  with flat field and freedom from distortion. Other telecentric lens configurations using two or more elements to achieve these properties may be employed without departing from the scope of the present invention. Similarly, lenses with different prescription may be designed to accommodate dish reflectors with different focal ratio and dimensions may be used. 
         [0105]    The telecentric lens elements  8  and  11  are preferably of fused quartz, to minimize light loss and heating by absorption of the highly concentrated sunlight. Preferably to avoid contamination of the front lens surface  9 , the PCU  20  is provided with an entrance window  6 , where the flux levels are reduced and less likely to result in burned-on contamination. In a preferred first embodiment using these materials, antireflection coatings may be applied to the four lens surfaces  9 ,  10 ,  12  and  13  and to both sides of the window  6 . 
         [0106]    An important feature of the present invention is that the secondary optics accommodate mispointing errors. The telecentric lens  29  used in the secondary optics functions so that the rays of sunlight reaching the photovoltaic cells  30  are either perpendicular to the flat cell array  18 , or have only a limited range of ray angles away from perpendicular. The wedge reflectors  16  and the perimeter reflectors  15  used in the secondary optics, positioned just above the photovoltaic cells  30 , function to direct light away from gaps between the photovoltaic cells  30  and light insensitive areas on the photovoltaic cells  30 , and direct that light onto the light sensitive areas of the photovoltaic cells  30  that are operative to convert the light into electricity. 
         [0107]      FIG. 4  shows a perspective view of a single multi-junction photovoltaic cell  30  used in the array of cells  18 . Each photovoltaic cell  30  is made on a square or rectangular substrate  34 , and has a photovoltaically active front area  32 . The electrical current created at the cell&#39;s front surface  32  flows from the metallization on the back  35  of the cell, the positive electrode  35 , through the cell  30  to the active area  32  where it is transmitted via thin surface conductors  33  to metallic edge busbars  31 , the negative electrodes  31 . The very high efficiency of the photovoltaic cells  30  is in part a result of their use of current collecting busbars  31  on both sides of the cell  30 , to split the current and reduce ohmic losses in the thin surface conductors  33 . However, the two metallic busbars  31  are opaque and insensitive to sunlight. In accordance with the present invention, wedge reflectors  16  are used to steer incoming sunlight away from the busbars  31  onto the cell active area  32 , and thereby avoid wasted sunlight. 
         [0108]      FIG. 5  shows a cross-section view of a wedge reflector  16  located above the busbars  31  of two adjacent photovoltaic cells  30  of the array  18 . The wedge reflector  16  has a first planar reflective side surface  51  and a second planar reflective side surface  52  that meet at a sharp wedge knife-edge apex  50 . In  FIG. 5 , the illustrated rays  14  of sunlight have been reflected by the reflector  2  and passed through the telecentric lens  29  in the case of on-axis illumination. The parallel, on-axis rays of light  14  from the telecentric lens  29  that are incident on the wedge reflector  15  are re-directed to photovoltaically-active areas  32  of the photovoltaic cells  30 . Consequently, the re-directed light contributes to the electricity generated by the photovoltaic cells  30 . If instead, the wedge reflector  16  was not used, and rays of light were to be allowed to impinge upon the busbars  31 , no electricity would be generated from such light striking the busbars  31 . 
         [0109]    As shown in  FIG. 5 , the photovoltaic cells  30  are mounted via their back metallization  35  to a first electrically conductive land  203  and a second electrically conductive land  202 , which are both attached to an electrically insulating planar substrate  83 . The cell circuit card  44  comprises the substrate  83  and a plurality of conductive lands  201 ,  202 ,  203  and  204 . In the gap between the photovoltaic cells  30 , there is a third strip of a land  201  and a fourth strip of a land  202 . Electrical connection between the busbars  31  is made to the third strip of land  201  by first wirebonds  40 , and electrical connection is made to the fourth strip of land  202  by second wirebonds  41 . 
         [0110]    To provide a thermal pathway for the surfaces of the thin wedge reflector sides  51  and  52  to transmit their absorbed heat, the inside of the wedge reflector  16  is bonded via thermal adhesive  47  to a thermally conductive wedge support structure  53 . This wedge support structure  53  not only provides a thermal pathway for a plurality of wedge reflectors  16  on a photovoltaic cell array  18 , but also acts as a mechanical skeleton support to locate the wedge reflectors  16  accurately above and between the photovoltaic cells  30 . In order to prevent electrical contact of the first and second wirebonds  40  and  41  to the wedge reflector  16 , the wedge reflector sides  51  and  52  have undercuts  54  provided, and the underside of the wedge support structure  53  is raised to clear the first and second wire bonds  40  and  41 . 
         [0111]      FIG. 6   a  a schematic diagram illustrating on-axis rays impinging upon an array of wedge reflectors  16  and photovoltaic cells  30 .  FIG. 6   a  illustrates the action of wedge reflectors  16  under illumination by parallel light rays from the exit surface  13  of the telecentric lens  29  for the case of light entering the reflector  2  on-axis relative to axis  19 . In  FIG. 6   a  the rays  14  striking the first and second side surfaces  51  and  52  of the wedge reflectors  16  are reflected to the active areas  32  of the photovoltaic cells  30 , brightening the illumination equally along both sides of these active areas  32 . 
         [0112]      FIG. 6   b  illustrates the action of wedge reflectors  16  under illumination by parallel light rays from the exit surface  13  of the telecentric lens  29  for the case of light entering the reflector  2  off-axis. In  FIG. 6   b , the tilted rays  24  strike only the first wedge surfaces  51  on one side of each wedge reflector  16  and are reflected further across the active areas  32  of the photovoltaic cells  30 , brightening the illumination across most of the active cell areas  32 .  FIG. 6   b  graphically shows the importance of the telecentric lens  29  in controlling the range of angles of the rays  24  so that the light rays  24  are close to normal to the planar cell array  18 . Without the telecentric lens  29 , light rays  24  at too large of an angle to normal, after reflection by the wedge reflectors  16 , would be reflected away from the active cell areas  32  and thus not generate electricity. Thus, the telecentric lens  29  and the wedge reflectors  16  together comprise secondary optics that maintain high and substantially uniformly divided illumination of the photovoltaic cells  30  even when the reflector  2  is not accurately pointed at the sun. 
         [0113]    As shown in  FIG. 2   a , the knife edges  50  of the reflective wedges  16  are located in a plane that is essentially coincident with the plane of the flat image  23  of the dish reflector  2  formed by the telecentric lens  29 . In this way, essentially all of the light rays from the primary dish reflector  2  passing through the telecentric lens  29  are directed to photovoltaic active areas  32  of the photovoltaic cells  30 , even for misalignment as in the example shown in  FIG. 6   b  for off-axis rays  24 . 
         [0114]    A preferred example of the full optical system of the first embodiment that optimizes performance of the system, comprising a dish reflector  2 , telecentric lens  29 , wedge reflectors  16 , and a planar array  18  of photovoltaic cells  16 , is described below. In this preferred example,  FIG. 7  shows how the cells in a planar array  18  may be laid out as four substantially identical and symmetrically placed rectangles  89  having a length “a” and a width “b”, which are arranged specifically to match both the image  23  of the dish reflector  2 , and to leave a square hole  88 , having a length indicated by reference numeral  91 , at the center. This hole  88  may for example correspond to the central shadow cast on the primary dish reflector  2  by the PCU  20 . This layout is configured to avoid uneven illumination of the photovoltaic cells  30  that would arise if shadowing by the PCU  20  was not taken into account. This central area  88  may be tailored to different sizes by changing the length “a” and width “b” of the rectangle  89  designated for the parallel cell groups  36  in each quadrant, for example by adjusting the geometry and gap width of the cells in the groups  36 . 
         [0115]      FIG. 8   a  and  FIG. 8   b  illustrate a configuration of photovoltaic cells  30  in one of the rectangles  89  for this preferred example of the first embodiment. As  FIG. 8   a  illustrates, groups  36  of photovoltaic cells  30  are configured with three individual cells  30  connected in parallel so their light sensitive areas form a rectangular area, and the group  36  is electrically connected to essentially perform like a single rectangular cell. The individual photovoltaic cells  30  are oriented with their busbars  31  running along the long edges of the rectangular array  36  so as to facilitate electrical connection in parallel to form the group  36 . It will be understood by those skilled in the art that the function of the cell group  36  could alternatively be accomplished with a single long rectangular cell, or with two rectangular cells placed end to end.  FIG. 8   b  illustrates three parallel groups  36  of cells  30  placed next to each other to form a first group  37 , a second group  38 , and a third group  39 , that will be connected electrically in series to form cell configuration in on rectangle  89 . 
         [0116]      FIG. 9   a  shows the location of twelve cell groups  36  of cells on a flat circuit card  44  conforming to the layout of the four rectangles  89  shown in  FIG. 7 . Also shown in  FIG. 9   a  are bypass diodes  45  included on the flat circuit card  44 . Each rectangle area  89  forms a quadrant of nine photovoltaic cells  30  arranged in three cell groups  36 , where each group  36  has three photovoltaic cells  30 . The three cell groups  36  comprise a first outer group  37 , a second middle group  38 , and a third inner group  39 , as illustrated in  FIG. 8   a . The twelve cell groups  36  are all connected in series by the circuit on the flat circuit card  44 . In this symmetric arrangement of electrically connected photovoltaic cells  30 , balanced photocurrent in the series chain is achieved by dividing the light evenly between the first outer group  37 , the second middle group  38 , and the third inner group  39 . 
         [0117]      FIG. 9   b  and  FIG. 9   c  show an array of wedge reflectors  16  located above the light-insensitive areas between the photovoltaic cells  30  in cell groups  36 . Each wedge reflector  16  is constructed and installed in accordance with the detailed illustration provided in  FIG. 5 . Together with the taller inward sloping reflectors  15  around the perimeter of the cell array  18 , the wedge reflectors  16  direct the incoming sunlight to the photovoltaically active areas  32  of the photovoltaic cells  30  in this preferred example of the first embodiment. 
         [0118]    The optical design of this preferred example of the first embodiment is made by adjustments to the optical parameters, which includes the power and figure of the lens surfaces  9 ,  10 ,  12 , and  13 , and the positions, placement and angling of the wedge reflectors  16  and perimeter edge reflectors  15 . In the design process, the telecentric lens design is first optimized as an independent unit for flat field and telecentricity—such that all rays arrive parallel to each other and normal to the image surface—to give a square image  23  of the primary dish reflector  2  that is free from distortion. The design process then proceeds with changes made in the parameters of the aspheric lens  8  and double-convex lens  11  as well as the wedge reflector parameters, in order to obtain uniform power division between parallel cell groups  37 ,  38  and  39 , for both on-axis and off-axis illumination. 
         [0119]    As a practical matter, the PCU  20  must be supported above the center of the reflector  2 . Entering sunlight will thus be blocked to some degree by the PCU  20  and its support structure  25 . In the case of a support arm  25  in the example shown in  FIG. 1 , the loss of light will be localized below the support arm  25 , and will lead to asymmetrical light distribution unless compensated in some way. 
         [0120]      FIG. 10   a  is a plan view looking down the system axis  19  of a primary square reflector  2  obscured in part by a PCU  20  of square cross-section and a support arm  25 . In this example, the PCU  20  outline is square, and will therefore cast a square shadow on the primary reflector  2 .  FIG. 10   b  is a plan view of the corresponding cell array  18  that employs the configuration illustrated in  FIG. 9   b , and shows the image formed by the lens  29  of the primary reflector  2  and the shadow of the PCU  20 , and the support arm  25 , in relation to the wedge reflector knife edges  50  which define the areas of light within the image plane  23  that are reflected to the different groups  36  of photovoltaic cells  30 .  FIG. 10   b  shows the region of obscuration—the image of the support arm  25  appears as a dark line  96 , causing a reduction in the illumination of the cell group  97 . The image of the PCU  20  shadow falls on the central region  88 , but there are no photovoltaic cells  30  in this region. However, the reduction in the illumination of cell group  97  needs to be addressed. 
         [0121]      FIG. 11  shows structure provided in accordance with a preferred example of the first embodiment in accordance with the example discussed above, for example, in connection with  FIG. 10   b , using the layout depicted in  FIG. 7 . Referring to  FIG. 11 , a central reflector  98  is positioned in the central region  88  to compensate for the reduction in the illumination of cell group  97  and the associated shadowing loss. Central reflector  98  directs light rays  99  from a central unshadowed area  95  onto cell group  97  that, in the illustrated example, is obscured by the image of the shadow  96  of the support arm  25 . The size  91  of the central square  88  between the cell groups  36  in this preferred example of the first embodiment is chosen such that even after the central obscuration caused by the square outline of the PCU  20 , there is still enough light reflected by the central reflector  98  to compensate for the shadowing by the support arm  25 . In this preferred example of the first embodiment, the area of 95, which is the part of the central area  88  not blocked by the shadow  94  of the PCU  20 , is substantially the same as the area of the shadow  96  of the cantilever arm  25 . 
         [0122]    Table 1 gives specific dimensions and design details for an especially preferred optimized example of the first embodiment. In this example, the primary dish reflector  2  is a paraboloid with 1.5 m focal length and a 1.6 m square perimeter  92  (projected along the optical axis of the paraboloid). The central obscuration caused by the PCU  20  is a sixteen centimeter diameter square, while the oversized central length  91  is 25.6 cm as projected onto the dish reflector  2 . 
         [0123]    In the example provided in Table 1, the optical system is designed to illuminate thirty six 10 mm×10 mm square photovoltaic cells  30  configured as twelve groups  36 , each group  36  having three cells  30  in parallel, and each group  36  having a total photovoltaic active area  32  of 30 mm×10 mm. The twelve groups are configured as shown in  FIG. 9   b , with 5 mm wide gaps between the photovoltaic active areas  32  of adjacent cell groups  36 . Above each such gap is a 5 mm wide and 10 mm high wedge reflector  16 , meaning the first wedge surface  51  and the second wedge surface  52  are disposed at an angle from normal equal to 14°. The wedge knife edges  50  are made coincident with the flat image plane  23  of the primary reflector  2 . This image is created by a two-element telecentric lens  29  that resides behind the parabolic focus  7  of the dish reflector  2 . 
         [0124]    In Table 1, the F/# is defined as the ratio of focal length to diagonal of the square dish  2  and the geometric concentration factor of 710 X is taken as the ratio of dish collector area to cell active area. 
         [0125]    Using on the parameters of Table 1,  FIG. 12   a  and  FIG. 13   a  give the results of illumination performance calculations for this especially preferred optimized example of the first embodiment, based upon on-axis solar illumination, and  FIG. 12   b  and  FIG. 13   b  give the results for 0.5° off-axis solar illumination.  FIG. 12   a  and  FIG. 12   b  show the relative strength of the total optical power received by the different parallel cells groups  36 .  FIG. 13   a  and  FIG. 13   b  show the irradiance pattern on the active cell area  32  of each parallel cell group  36 .  FIG. 14   a  provides the calculated electrical performance of this especially preferred optimized example in the case of on-axis pointing. The right hand graph shows the computed output curve of current against voltage (IV curve) with the maximum power point indicated, and normalized to 100%. The power contributed to the maximum by each of the 12 cell groups if shown by the histogram on the left.  FIG. 14   b ,  FIG. 14   c , and  FIG. 14   d  further detail the calculated electrical performance of this especially preferred optimized example as it undergoes mispointing from the sun of 0.25°, 0.5°, and 0.75°, respectively.  FIG. 15  shows a contour plot of the maximum power points calculated for system mispointing from the optical axis  19  out to 1.20 in all directions.  FIG. 16  deconstructs the contour plot of  FIG. 15  into a more detailed view of the maximum power as a function of pointing error in the azimuth and elevation directions. The modeled system is based on the parameters of Table 1 and includes compensation for central obscuration by a support arm  25  of width 25 mm. The solar illumination is modeled as coming from a disc of uniform brightness and subtending 0.5 degrees diameter. 
         [0126]    In this especially preferred optimized example, for on-axis pointing 98.2% of the sunlight rays incident across the full aperture of the reflector  2  reach the photovoltaic cells  30 , i.e., 8.2% of the rays are received by each parallel cell group  36 . The ray-blocking contributions are 1% by the shadow  94  of the PCU  20  and 0.8% by the shadow  96  of the support arm  25 . Additional loss of sunlight power entering the full aperture will arise on passage to the photovoltaically active areas  32  of the cells because of less than perfect reflection by the primary reflector dish  2 , and dielectric reflection losses at the six surfaces of the window  6  and two lens elements  9  and  11 . Further loss from the slight rounding of the tips  50  of the wedge reflectors  15  of origami optics made by the method described herein is estimated to be 2%. Absorption losses are negligible for fused silica. For a somewhat soiled dish having reflectivity of 90%, and dielectric losses of 1% for each antireflection coated surface, these total additional losses amounts to 18%, and thus the total system loss is 20% for on-axis illumination. If used with cells of 43% conversion efficiency, the system is thus estimated to have end-to-end conversion efficiency of 34%. From  FIG. 16 , the additional ray loss from mispointing reaches 10% only for mispointing angles of 0.7 degrees, thus total system efficiency will remain above 30% even at 0.7 degrees of mispointing. 
       Second Embodiment 
       [0127]    A second embodiment of the present invention is described below which provides a different implementation of the power conversion unit or PCU  20  having a single lens element  70 , and having photovoltaic cells  30  configured in four planar arrays  18 . 
         [0128]    Turning now to  FIG. 17   a , a second embodiment according to the present invention is shown comprising a PCU  20  having a single lens element  70 . Solar rays  22  parallel to the axis  19 , after reflection by the dish reflector  2 , then converge as rays  122  in the PCU  20 . The incoming light rays  122  converge to a focus  7 . Additional converging rays  4  and  5  are shown which originate from opposite points on the edge of the sun&#39;s disc, and converge to the two corresponding focal points  701  and  702 . The foci  7 ,  701  and  702  are formed within the single lens  70 , which also forms the entrance window to the PCU  20  (as shown in  FIG. 37 ). 
         [0129]    The lens  70  shown in  FIG. 17   a  comprises a single biconvex element with entrance surface  12  and exit surface  13 . Rays exiting the surface  13  form a curved image  28  of the primary reflector  2 . The image  28  has a substantially square boundary corresponding to the square boundary of the primary reflector  2 . Behind the lens  70  is a contiguous arrangement of four cell arrays  18  of photovoltaic cells  30 . The four cell arrays  18  are tilted with respect to each other, so as to approximate the concave curved shape of the image  28 . Located between the lens  70  and the planar arrays  103  is an array of wedge reflectors  102  composed of interior reflectors  16  and perimeter reflectors  15 . The function of the arrays of wedge reflectors  102  is to direct the uniformly distributed rays  4  and  5  emerging from the lens  70  to the photovoltaic cells  30  in the planar arrays  103 , and away from gaps between the photovoltaic cells  30 . The entire segmented assembly  101  is comprised of the array of wedge reflectors  102  and all four planar arrays  103  of cells  30 . 
         [0130]    As shown in  FIG. 17   a , the knife edges  50  of the interior wedges  16  are located closely coincident with the plane of the curved image  28  of the dish reflector  2  formed by the lens  70 . In this way, essentially all the light rays  4  and  5  from the primary dish reflector  2  passing through the lens  70  are directed to photovoltaic active areas  32  of the cells  30 . 
         [0131]      FIG. 17   b  shows the main elements of the PCU  20  in perspective: the lens element  70 , the wedge reflector assembly  102  composed of interior reflectors  16  and perimeter reflectors  15 , the planar cell arrays  103 , and the extent of the whole segmented assembly  101 . 
         [0132]    The lens  70  has two characteristics that may be important for the efficient operation of an apparatus according to this second embodiment. First, the boundary of the image formed by the lens  70  is preferably sharp and preferably has approximately the same shape as the primary reflector  2 , namely square. This allows high efficiency coupling of the concentrated sunlight to the four square, flat arrays of photovoltaic cells  103 . 
         [0133]    A second characteristic of the lens  70  that may be important is to deliver light in a direction that is locally approximately perpendicular to the curved image surface  28  and thus approximately perpendicular to the planar cell assemblies  103 . This is believed to be an important factor for effective use of wedge reflectors  16 . 
         [0134]    This is further illustrated in  FIG. 18   a  and  FIG. 18   b .  FIG. 18   a  shows the incoming on-axis rays  122  after reflection by the primary reflector  2 . These rays  122  are brought to a point focus  7 . After passage through the lens  70 , these refracted rays  14  are locally perpendicular to the image surface  28 .  FIG. 18   b  shows rays from a distant point source not aligned with the optical axis  19 . In the example shown in  FIG. 18   b , the off-axis converging rays  124  shown in the drawing are now brought to a focus  703  which is displaced away from the optical axis  19 . But after continued refraction through the lens  70 , the rays  24  strike the wedge reflector assemblies  102  substantially close to the same region as before (this is a property of an image formed by the lens  70 ). The angle at which these rays  24  locally strike the curved image surface  28  is displaced away from normal incidence, by an amount that depends on the degree of mispointing of the reflector  2  and associated PCU  20  from the distant source. The rays  24  are thus either substantially perpendicular locally to the image surface  28  or with only a limited range of ray angles away from perpendicular, determined by the degree of mispointing. In this second embodiment, the four planar arrays of cells  103  are configured with their centers substantially parallel to the local image surface  28 , and perpendicular to incoming on-axis refracted rays  14 . Using this configuration, it is possible to use quadrants  102  of interior wedge reflectors  16  and perimeter reflectors  15  to direct light away from the gaps and the light insensitive areas on the photovoltaic cells  30  and onto the light sensitive areas  32  of the photovoltaic cells  30 . 
         [0135]      FIG. 19   a  illustrates the action of interior wedge reflectors  16  and perimeter wedge reflectors  15  under the illumination from the lens  70  for the case of light entering the apparatus on axis.  FIG. 19   b  illustrates the action of interior wedge reflectors  16  and perimeter wedge reflectors  15  under the illumination from the lens  70  for the case of light entering the apparatus off-axis. In  FIG. 19   a  the on-axis rays  14  exiting the rear surface  13  of the lens  70  and striking the first planar reflective side surfaces  51  and the second planar reflective side surfaces  52  of the interior wedge reflectors  16  and perimeter wedge reflectors  15  are reflected to the active areas  32  of the solar cells  30 , brightening the illumination along the sides of these areas  32 . In  FIG. 19   b , the off-axis rays  24  exiting the rear surface  13  of the lens  70  are tilted off-perpendicular and generally strike the wedge reflectors  16  primarily on the first planar reflective side surfaces  51 , and are reflected further across the active areas  32  of the photovoltaic cells  30 , brightening the illumination across most or all of the cell area  32 .  FIG. 19   b  illustrates the value of the lens  70  in controlling the range of angles of the rays  24  to be approximately perpendicular to the cell array quadrants  103 . Rays far from normal to the array  103 , after reflection by the interior wedge reflectors  16  and perimeter wedge reflectors  15 , may not reach the active cell area  32  and thus would not generate electricity. 
         [0136]    It will be understood by those with common knowledge of optics that the single element lens  70  illustrated is simply one example illustrative of a singlet lens yielding rays near-normal to the local curved image surface  28 . Those skilled in the art, after having the benefit of this disclosure, will appreciate that other lens configurations with these properties are possible without departing from the spirit or scope of the present invention. Similarly, those skilled in the art, after having the benefit of this disclosure, will appreciate that lenses with different focal length designed to accommodate dish reflectors  2  with different focal ratios and dimensions are possible without departing from the spirit or scope of the present invention. 
         [0137]    The lens  70  is made preferably of fused quartz, to minimize light loss and heating by absorption of the highly concentrated sunlight. Antireflection coatings are preferably applied to the entrance surface  12  and to the exit surface  13  of the lens  70 . 
         [0138]    In order to maintain uniform division of concentrated sunlight across a plurality of photovoltaic cells  30 , groups  36  of cells  30  may be electrically connected in parallel as discussed in connection with  FIG. 8   a.    
         [0139]      FIG. 20   a  shows the location of a plurality of cell groups  36  on four flat circuit cards  103 . The cards  103  are substantially identical, and correspond to the four identical rectangles  89  of the type illustrated in  FIG. 7 . Also shown in  FIG. 20   a  are three bypass diodes  45  included on each card  103 . Each card  103  with nine photovoltaic cells  30  comprises three cell groups  36 , specifically an outer group  37 , a middle group  38 , and an inner group  39 , arranged next to each other as illustrated in  FIG. 20   a  and in  FIG. 8   b . The three cell groups  36  are connected in series by a printed circuit on each cell card  103 . In a PCU  20 , the four cards  103  are themselves connected electrically in series. In this highly symmetric arrangement of connecting a total of thirty-six individual photovoltaic cells  30 , the objective of achieving balanced photocurrent in a series chain of photovoltaic cells  30  comes down to ensuring that the light is divided evenly between the outer groups  37 , the middle groups  38 , and the inner groups  39 . 
         [0140]      FIG. 20   b  shows a flat cell card  103  configured with interior wedge reflectors  16  located as in  FIG. 5 , above the light-insensitive areas  31  between the outer cell groups  37 , the middle cell groups  38 , and the inner cell groups  39 . Together with the perimeter sloping reflectors  15  around the perimeter of the cell array  102 , the interior wedge reflectors  16  direct the incoming light to the photovoltaically active areas  32  of the three parallel groups  36  of cells  30  on each card  103 , specifically the outer cell groups  37 , the middle cell groups  38 , and the inner cell groups  39 . 
         [0141]      FIG. 20   c  shows an assembly of three such cell cards  103  with reflector quadrants  102 , with the fourth card  103  and reflector quadrant  102  being set in place to complete the full segmented assembly  101  of a PCU  20  according to the second embodiment of the invention. 
         [0142]    The final optical design of this second embodiment is preferably made by adjustments to the optical parameters, which include the radii of the lens surfaces  12  and  13 , their spacing, and positions and the placement and angling of the interior wedge reflectors  16  and the exterior wedge reflectors  15  that comprise each quadrant of wedge arrays  102 . In the design process, the lens design is first optimized as an independent unit so as to give a curved, square image  28  of the primary dish reflector  2 . In the subsequent system optimization, the merit criterion is changed to be uniform power division between parallel cell groups  36 , including the effects of edge reflection to redirect rays onto the active cell areas  32 , the uniformity to be maintained for both on-axis and off axis illumination. 
         [0143]    As a practical matter, the PCU  20  must be supported above the center of the reflector  2 . Entering sunlight rays  22  will thus be blocked to some degree by the support structure for the PCU  20 . In the case of a cantilever arm  25 , the loss of light will be localized below the arm, and lead to asymmetrical light distribution. 
         [0144]    In accordance with a second embodiment of the present invention,  FIG. 21   a  is a view down the system axis  19  of a primary square reflector  2  obscured in part by a PCU  20  and PCU support arm  25 . In this embodiment, the PCU  20  outline is circular, as shown in  FIG. 21   a .  FIG. 21   b  is a view down the system axis of the wedge/cell assembly  101  within the PCU  20 , showing the image formed by the lens  70  of the PCU  20  and PCU support arm  25  in relation to the wedge knife edges  50  which define the areas of light within the image plane  28  that are reflected to the different cell groups  36 . This shows the region of obscuration—the image of the support arm  25  appears as a dark bar  96 , causing a reduction in the illumination of the cell group  36  located in area  97 . 
         [0145]      FIG. 22  shows a preferred method to compensate for this shadowing loss. A central reflector  98  is positioned to direct rays  99  from central un-shadowed area  95  onto cell group  97  that is obscured by the image of the shadow of the cantilever arm  96 . The size of the central square  91  between the wedge reflectors  16  in this particular embodiment is chosen such that even after the central obscuration  94  caused by the PCU  20 , there is still enough light  99  reflected by the central reflector  98  to compensate for the shadowing of cell group  97  by the support arm  25 . The area of 95, the part of the central area not blocked by the PCU&#39;s shadow, is substantially the same as the area  96  of the image of the shadow of the cantilever arm. 
         [0146]    Table 2 gives the prescription of a preferred example of the second embodiment providing an optimized lens prescription and placement, as shown in  FIG. 17  and  FIG. 18 . It has the appropriate optical power, size, and location so as to bring focused light to a curved, suitably sized image  28  of the primary reflector  2  that matches the area of the chosen cell groups  36  and wedge array  102  dimensions. It will be understood that this design is simply an illustrative example, and that other designs with different dimensions, numbers of cells and cell groupings will fall within the scope of this invention. 
         [0147]    In this preferred example of the second embodiment, the primary dish reflector  2  is a paraboloid with 1.5 m focal length and a 1.6 m square perimeter  92  (projected along the optical axis of the paraboloid). The central obscuration caused by the PCU  20  is a 15.2 cm diameter circle, while the oversized central length  91  being 19.6 cm as projected onto the dish reflector  2 . 
         [0148]    The optical system is designed to illuminate a total of thirty-six 8.8 mm×8.8 mm square photovoltaic cells  30  configured as three groups  36  on each of four planar cards  103 . Each group  36  having three cells  30  in parallel, and each group  36  having a total photovoltaic active area  32  of approximately 26.4×8.8 mm. The three groups on each planar card  103  are configured as shown in  FIG. 20 , with 3 mm wide gaps between the photovoltaic active areas  32  of adjacent cell groups  36 . 
         [0149]    Above each gap is a 4.2 mm wide and 9 mm high wedge reflector  16 , with the wedge surfaces  51  and  52  having an angle from normal averaging approximately 13°. The wedge knife edges  50  are made substantially coincident with the curved image plane  28  of the primary dish reflector  2 . This image  28  is created by a lens  70 . The parabolic focus  7  of the dish reflector  2  falls within the lens  70 . 
         [0150]    In Table 2, the F/# is defined as the ratio of focal length to diagonal of the square dish  2  and the geometric concentration factor of 918 X is taken as the ratio of total dish collector area to total cell active area. 
         [0151]    Using on the parameters of Table 2,  FIG. 23   a  and  FIG. 24   a  give the results of illumination performance calculations for this especially preferred optimized example of the second embodiment, based upon on-axis solar illumination, and  FIG. 23   b  and  FIG. 24   b  give the results for 0.5° off-axis solar illumination.  FIG. 23   a  and  FIG. 23   b  show the relative strength of the total optical power received by the different parallel cells groups  36 .  FIG. 24   a  and  FIG. 24   b  show the irradiance pattern on the active cell area  32  of each parallel cell group  36 .  FIG. 25   a  provides the calculated electrical performance of this especially preferred optimized example in the case of on-axis pointing. The right hand graph shows the computed output curve of current against voltage (IV curve) with the maximum power point indicated, and normalized to 100%. The power contributed to the maximum by each of the twelve cell groups is shown by the histogram on the left.  FIG. 25   b ,  FIG. 25   c , and  FIG. 25   d  further detail the calculated electrical performance of this especially preferred optimized example as it undergoes mispointing from the sun of 0.25°, 0.5°, and 0.75°, respectively.  FIG. 26  shows a contour plot of the maximum power points calculated for system mispointing from the optical axis  19  out to 1.2° in all directions. 
         [0152]      FIG. 27  deconstructs the contour plot of  FIG. 26  into a more detailed view of the maximum power as a function of pointing error in the azimuth and elevation directions. The modeled system is based on the parameters of Table 2 and includes compensation for central obscuration by a support arm  25  of width 25 mm. The solar illumination is modeled as coming from a disc of uniform brightness and subtending 0.5 degrees diameter. 
         [0153]    In this especially preferred optimized example, for on-axis pointing 98.2% of the sunlight rays incident across the full aperture of the reflector  2  reach the photovoltaic cells  30 , i.e., 8.2% of the rays are received by each parallel cell group  36 . The ray-blocking contributions are 1% by the shadow  94  of the PCU  20  and 0.8% by the shadow  96  of the support arm  25 . Additional loss of sunlight power entering the full aperture will arise on passage to the photovoltaically active areas  32  of the cells because of less than perfect reflection by the primary reflector dish  2 , and dielectric reflection losses at the two surfaces lens  70 . Further loss from the slight rounding of the tips  50  of the wedge reflectors  15  for origami optics made by the method described herein is estimated to be 2%. Absorption losses are negligible for fused silica. For a somewhat soiled dish having reflectivity of 90%, and dielectric losses of 1% for each antireflection coated surface, these total additional losses amounts to 14%, and thus the total system loss is 16% for on-axis illumination. If used with cells of 43% conversion efficiency, the system is thus estimated to have end-to-end conversion efficiency of 36%. From  FIG. 16 , the additional ray loss from mispointing reaches 12% only for mispointing angles of 0.7 degrees, thus total system efficiency will remain above 31% even at 0.7 degrees of mispointing. 
       Method of Manufacturing 
       [0154]      FIG. 38 ,  FIG. 39 , and  FIG. 40  illustrate various steps of a method of manufacturing origami optics having a wedge reflector  16  with sharp edges from flat sheets of reflective material  55 . A preferred reflective material  55  is thin aluminum, polished to high specularity and coated with silver  151 . A protective layer over the reflective coating  151  may also be provided. Such material is commercially manufactured in large areas with very high and stable reflectivity. Multiple dielectric layers may be used to enhance reflectivity and stability without significantly increasing manufacturing costs, because manufacturing methods for coating very large flat sheets at high speed are well developed. 
         [0155]      FIG. 38  shows in a detail a cross-section of a sheet of material  55 , such as pre-coated aluminum, having a reflective surface  151 .  FIG. 39  shows the sheet  55  after undercutting a groove  54  in the bottom side  152  of the sheet  55 . The groove  54  has a first groove side  153  that forms a shallow angle to the reflective surface  151 , and a second groove side  154  that similarly forms a shallow angle to the reflective surface  151 . The groove  54  has a sharpness and depth so as to nearly part the sheet  55 , leaving a very narrow, thin joining region  150 . 
         [0156]      FIG. 40  shows the sharp edge  50  formed in the wedge reflector  16  by folding the sheet  55  by bringing the first groove side  153  into close proximity with the second groove side  154 . Referring to  FIG. 39 , the sheet  55  shown in  FIG. 39  is folded down along the thin joining region  150  in order to arrive at the wedge reflector  16  shown in  FIG. 40 . 
         [0157]    While  FIG. 38 ,  FIG. 39 , and  FIG. 40  show detailed steps of how a sheet  55  of pre-coated aluminum is folded to make a single wedge reflector with a sharp knife edge  50 , the present invention includes a method of conveniently manufacturing an assembly of interior wedge reflectors  16  and perimeter inward sloping reflectors  15  from reflector sheets  55  by cutting the sheet  55  with multiple grooves  54 , so that each grove  54  may be folded to form a plurality of both perimeter reflectors  15  and wedge reflectors  16  from one sheet  55 , with the correct geometry incorporated into the pattern of grooves that are cut into the sheet  55 . For purposes of the present invention, “origami optics” is defined as a configuration of a plurality of wedge reflectors  16  and perimeter reflectors  15  made from a sheet  55  of reflective material by cutting a pattern of grooves in the sheet  55  and folding the sheet in accordance with the pattern of grooves to form the plurality of wedge reflectors  16  and perimeter reflectors  15 . 
         [0158]      FIGS. 28   a  through  28   j  illustrate steps in the manufacture of an assembly designed for use with cells in identical quadrants, each quadrant having three elongated cell groups. It will be apparent to those familiar with the art that the method could be applied to configurations with differently shaped groups and different numbers of groups. 
         [0159]      FIG. 28   a  shows one of four identical folded reflectors which, when fitted together, will form a complete reflector wedge assembly  102 . The assembly in  FIG. 28   a  incorporates seven of the eight planar surfaces that reflect light to one quadrant, and one of the eight surfaces that reflect light to the next quadrant—reflector surface  59 . This configuration is chosen so that the knife edge  50  that splits light between adjacent quadrants is made by a fold in one piece of reflector material, and does not require the difficult butting of separate pieces to form a knife edge. The three cutouts that form assembly  102  are two interior reflector cutouts  71  and a perimeter reflector cutout  72 .  FIG. 28   b , shows the underside of a rectangle  55  of pre-coated reflective material with the outlines  71  and  72  of cut-outs which will be folded to become an interior reflector  16  and perimeter reflector  15 . Undercuts  54 , shown also in  FIG. 5 , and perimeter wedge notches  73 , are milled away from the sheet  55  prior to cutting out each reflector. 
         [0160]    To create an interior reflector  16 , the outline  71  shown is cut from the reflective sheet  55 , as shown in  FIG. 28   c , and is bent around the central undercut  54  edge, as shown in  FIG. 28   d . This creates a single interior wedge reflector  16 , as shown in  FIG. 28   e , with a knife-edge  50 , and a first planar reflective side  51  and a second planar reflective side  52 . The undercuts  54  are made so as to almost cut through the material, so that folding yields a sharp knife-edge  50  on the reflective side  151 . To create the perimeter reflector  15 , the cutout  72  from the sheet  55  shown in  FIG. 28   f  has a more complex shape, to yield four linked planar facets  56 ,  57 ,  58 , and  59  after folding. The perimeter reflector left side  56  is bent away from the undercut edge  54  dividing it from the perimeter reflector right side  57 , while the perimeter reflector roof outside  59  is bent around the undercut edge  54  dividing it from the perimeter reflector roof inside  58 , as shown in  FIG. 28   g . The perimeter reflector roof comprising  58  and  59  is then bent around the undercut edge  54  dividing it from the perimeter reflector right side  57 , as shown in  FIG. 28   h . When rotated around, this three-fold part creates the entire perimeter reflector  15 , as shown in  FIG. 28   i . Two interior wedge reflectors  16  and a perimeter wedge reflector  15  are then combined as in  FIG. 28   j  to create a complete wedge reflector quadrant  102 . 
         [0161]      FIG. 29   a  details how each wedge reflector quadrant  102  is inserted into the first embodiment, and  FIG. 29   b  details how each wedge reflector quadrant  102  is inserted into the second embodiment. In  FIG. 29   a , four of the wedge reflector quadrants  102  are brought together to create the secondary optics of the secondary assembly  17  of the first embodiment. In  FIG. 29   b , four of the wedge reflector quadrants  102  are brought together to create the secondary optics of the complete segmented wedge assembly  101  of the second embodiment. This four-part segmented construction is also shown in  FIG. 20   c.    
         [0162]    The electrical and thermal connection for the first embodiment and the second embodiment may be described as follows. Referring to  FIG. 5 , mechanical support, cooling and partial electrical connection of the complete flat cell array is made by soldering the cell groups  36  and the bypass diodes  45  to a ceramic circuit card  44 . The circuit is made through lands  201  and  202  formed by etching gaps in the copper directly bonded to the thermally conductive ceramic  83 . The circuit is completed by interconnections made between the cells  30  by wire or ribbon conductors  40  and  41  connected to the cell face negative electrodes  31  shown in  FIG. 4 . It will be understood by those skilled in the art of electrical circuitry that the bonds  40  and  41  could be wire or ribbon or welded foil. Within each parallel cell group  36 , the common connection of the three cell base positive electrodes  35  is made by their all being soldered to the same continuous copper land  203 , so they are all at the same electric potential. 
         [0163]    A method of linking the three common front electrodes and of making the series connection between the adjacent groups is shown in  FIG. 30 , for a series chain of a first parallel group of three photovoltaic cells  37 , a second parallel group of three photovoltaic cells  38 , and a third parallel group of three photovoltaic cells  39 . 
         [0164]    The circuit card is etched to form four discrete continuous lands  201 ,  202 ,  203  and  204 , that are interdigitated between each other in the plane of the card. Wirebonds  40  from the left hand top electrodes of the first parallel group of cells  37  link to a strip of land that is part of the U-shaped land  203 . Wirebonds  41  from the right hand top electrodes of the first parallel group of cells  37  link to a strip of land this is also part of the U-shaped land  203 . These strips of land  203  are then connected to the base electrodes  35  of the second parallel group of cells  38  completing the series connection of the first group  37  and the second group  38 . Similarly wirebonds  40  from the left hand top electrodes of the second parallel group of cells  38  link to a strip of land that is part of the U-shaped land  202  that underlies the third parallel group of cells  39 . Wirebonds  41  from the right hand top electrodes of the second parallel group of cells  38  link to a strip of land that is part of the U-shaped land  202 , completing the series connection of the second parallel group of cells  38  and the third parallel group of cells  39 . 
         [0165]    In a preferred implementation, the electrical circuit linking all twelve cell groups in a PCU  20  is shown schematically in  FIG. 31 . There are three cells  30  in each parallel group  36 . Each parallel group  36  includes a bypass diode  45 . Twelve parallel groups are connected electrically in series in the example shown in  FIG. 31 . The electrical potentials at each node are numbered as shown in  FIG. 31 , starting at  301 , the negative output terminal, and numbered sequentially to  313  for the positive output terminal. 
         [0166]      FIG. 32  shows a highly preferred layout that implements the wiring diagram of  FIG. 31  on a single planar circuit card  44 , as used in the first embodiment. In this example, each parallel group  36  is comprised of three photovoltaic cells  30 , and the fill circuit has twelve such parallel groups  36 , three in each quadrant, i.e., an outer group  37 , a central group  38 , and an inner group  39 . It will be clear that other configurations with a different number of cells in each group, and a different number of groups per quadrant, could also be used as desired to optimize for cell and concentrator size. A preferred circuit card  44  comprises copper direct bonded (DBC) onto a thermally conductive ceramic substrate such as aluminum nitride. It should be understood that other thermally conductive ceramics such as alumina or beryllia could also be used. 
         [0167]    The circuit card  44  as shown, together with the wirebonds  40  and  41 , provides all the parallel and series connections, for parallel cell groups  37 ,  38 , and  39 , and also for the bypass diodes  45 , which are connected electrically in parallel with each parallel cell group as in  FIG. 31 . For clarity, the wirebonds  40  and  41  are not shown in  FIG. 32 , but it should be understood that such wirebonds will be used as illustrated in  FIG. 5 . The circuit as illustrated in  FIG. 31  has thirteen regions of different electrical potential, from the positive output terminal  301  to the negative output terminal  313 . As in  FIG. 30 , most lands are substantially U-shaped, and receive current through wirebonds  40  and  41  from both side of electrodes in given parallel group  37 ,  38 , and  39 , and are separated electrically by etched outlines  43  on the cell card&#39;s face. 
         [0168]      FIG. 33  shows a highly preferred circuit layout that implements the wiring diagram of  FIG. 31  of one of four identical quadrant circuit cards, as used in the second embodiment. The four cards are used together as shown in  FIG. 20   a . Interconnections between the four cards to complete the circuit of  FIG. 31  are made between the positive electrical potential on land  301  which is wired to the negative potential  313  of the adjacent circuit card via connectors on each land. 
         [0169]      FIG. 37  is a cross-sectional schematic diagram showing how the wedge support structure  53  may be used to maintain optical and mechanical alignment of the components of the secondary assembly  17 , within the PCI  20 . Thermal adhesive  47  between the wedge support structure  53 , interior wedge reflectors  16 , perimeter wedge reflectors  15 , and the cell cards  44  provides mechanical support and alignment, as well as heat transfer from the wedge reflectors  16  to the cell cards  44 . The heat is removed from the back surface of the cell cards  49  by fluid coupling to a heat transfer system (not shown) which does not provide mechanical support. 
         [0170]    Within the PCU  20 , alignment of secondary assembly  17  to the PCU&#39;s lens  70  is provided by the PCU housing structure  68  with the lens O-ring  48 . This structure  68  is in turn held in position and attached to the PCU support arm  25  by the PCU attachment bracket  69 . 
         [0171]    In the interest of efficiency and scale, a preferred implementation has multiple PCUs  20  and reflectors  2  on a single two-axis tracking system  3 .  FIG. 35  details one such implementation where eight PCU&#39;s  20 , are supported above eight dish reflectors  2  held to face the sun by a single two-axis tracking system  3 . 
         [0172]    The wedge reflectors receive heat during operation, because in a practical system their reflectivity is not perfect. A preferred method to dissipate this heat is by thermal conduction to the cell card below. The conduction path for the folded wedge reflector assemblies of  FIG. 28   e  and  FIG. 28   i  is via a wedge support structure  53  using a thermal adhesive  47 , as shown in  FIG. 5 . The heat absorbed by the interior wedge reflectors  16  is preferably thermally conducted outward along the wedge support structure  53  to its perimeter lying under the perimeter wedges  15 . As shown in  FIG. 28   i , gaps are provided through the notches  73  located on faces  57  and  59  for the support structure  53 . As shown in  FIG. 34 , at the notch  73  location where interior wedge reflectors  16  are fitted into a perimeter edge reflector  15 , the wedge support structure  53  is then bonded to the cell card  44  along the perimeter using thermal adhesive  47  over the adhesive footprint region  46 . In this way, heat is carried by conduction down through the perimeter&#39;s thermal adhesive footprint  46  into the ceramic cell circuit card  44 . This thermal adhesive footprint  46  surrounding the parallel groups of cells  37 ,  38 , and  39  is also shown in  FIG. 32  for the first embodiment and  FIG. 33  for the second embodiment. The thickness and compliance of this thermal adhesive layer is preferably chosen so as to take the differential thermal expansion between the wedge support structure  53  and the circuit card  44  without excessive mechanical stress, and the thermal conductivity is chosen to be high enough to transmit the heat without excessive temperature gradient. 
         [0173]    Referring to  FIG. 36 , in a preferred implementation of the complete cooling system, the heat from each cell is not transferred by thermal conduction to the air locally, as in most prior art, but the heat from multiple cells on a circuit card is transferred by a heat transfer fluid running through microchannels or between pins attached the rear surface  49  of each cell card. The fluid passes through a plumbing manifold to a single fan/radiator unit  60 , rigidly attached to the elevation mirror support structure  27  of the concentrated photovoltaic generator  1 . This common fan/radiator unit  60  serves the multiple PCUs  20  and acts as a partial counterweight to the multiple dish reflectors  2  and PCUs  20  of a complete generator  1 . 
         [0174]      FIG. 36  illustrates the plumbing configuration for a cooling system. Upon leaving the common generator pump  61 , coolant enters the single fan/radiator unit  60  and passes into the main parallel inlet manifold  62  that runs along the length of the generator&#39;s elevation axis. The manifold is then split in parallel at each of the multiplicity of cantilevered pillars  26  that transfer the coolant up to the PCU support arm  25 . The manifold is then split further in parallel through each support arm  25  to a PCU  20 . 
         [0175]    At the end of the PCU support arm  25  the coolant leaves the inlet manifold  62  and enters the PCU  20  via a quick-disconnect inlet  64 , passes behind the cell circuit card  44  or cards  101  and out the outlet manifold  63  via a quick-disconnect outlet  65 . The quick-disconnect junctions  64  and  65  are used so that the small PCUs  20  can be easily removed and replaced. The coolant then flows through the outlet manifold  63 , mirroring the same parallel connection path of the inlet manifold  62  until it passes through the common generator pump  61  and back into the fan/radiator unit  60 . 
         [0176]    One goal of the present invention is to provide an inexpensive and efficient way to couple clustered, small photovoltaic cells to sunlight focused by a single large and inexpensive dish reflector. The present invention greatly reduces manufacturing cost by using secondary optics that provide for cells in flat arrays on small circuit cards and using secondary reflectors with flat, pre-manufactured, foldable surfaces. 
         [0177]    Those skilled in the art, after having the benefit of this disclosure, will appreciate that modifications and changes may be made to the embodiments described herein, different materials may be substituted, equivalent features may be used, changes may be made in the steps of manufacturing processes, and additional elements and steps may be added, all without departing from the scope and spirit of the invention. This disclosure has set forth certain presently preferred embodiments and examples only, and no attempt has been made to describe every variation and embodiment that is encompassed within the scope of the present invention. The scope of the invention is therefore defined by the claims appended hereto, and is not limited to the specific examples set forth in the above description. 
         [0000]    
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Primary reflector 
                 F/0.66 Square Paraboloidal Mirror with 2.56 m 2  area, f = 1.5 m 
               
               
                 Optical Window 
                 Flat, n = 1.53 @ λ = 500 nm 
               
               
                   
                 Dimensions: 16 cm square, at +85 mm from parabolic focus 
               
               
                   
                 Thickness: 4 mm 
               
               
                 Lens Element 1 
                 f = 106 mm, n = 1.46 @ λ = 500 nm 
               
               
                   
                 Surface 1: R 1  = 0 at −40 mm from parabolic focus 
               
               
                   
                 Thickness: 46 mm 
               
               
                   
                 Surface 2: R 2  = 53.56 mm with Conic = −0.9 
               
               
                   
                 Material: fused silica 
               
               
                 Lens Element 2 
                 f = 183 mm, n = 1.46 @ λ = 500 nm 
               
               
                   
                 Surface 1: R 1  = 160 mm at −87 mm from parabolic focus 
               
               
                   
                 Thickness: 41 mm 
               
               
                   
                 Surface 2: R 2  = −160 mm2 
               
               
                   
                 Material: fused silica 
               
               
                 Non-Imaging Optics 
                 Type: Flat silvered wedges 
               
               
                   
                 Angle: 14° 
               
               
                   
                 Location: 191 mm from parabolic focus 
               
               
                   
                 Thickness: 20 mm toward parabolic focus at edges, 10 mm around 
               
               
                   
                 cells 
               
               
                 Solar Cells 
                 Type: Triple Junction Solar Cells 
               
               
                   
                 Array Size: 36 × 10 mm square @ 191 mm from parabolic focus 
               
               
                   
                 Concentration Factor: 710x 
               
               
                 System Properties 
                 Silica Mass: 400 g/m 2   
               
               
                   
                 90% Power Point @ 0.7° in the Elevation Pointing direction 
               
               
                   
                 Geometric concentration 710 X 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
             
             
               
                 Collection Aperture 
                 F/0.66 Square Paraboloidal Mirror with 
               
               
                   
                 2.56 m 2  area, f = 1.5 m 
               
               
                 Lens Element 1 
                 f = 48.1 mm, n = 1.46 @ λ = 500 nm, vertex 
               
               
                   
                 located 60 mm in front of Parabolic Focus 
               
               
                   
                 Surface 1: R 1  = 60 mm 
               
               
                   
                 Thickness: 95 mm 
               
               
                   
                 Material: fused silica 
               
               
                   
                 Surface 2: R 2  = −35 mm 
               
               
                 Non-Imaging Optics 
                 Type: Flat silvered wedges 
               
               
                   
                 Angle: 14° 
               
               
                   
                 Location: Center of quadrants located at 83 mm 
               
               
                   
                 from parabolic focus 
               
               
                   
                 Thickness: 6 mm toward parabolic focus from 
               
               
                   
                 quadrants 
               
               
                 Solar Cells 
                 Type: Triple Junction Solar Cells 
               
               
                   
                 Array Size: 36 × 8.8 mm square on 4 separate 
               
               
                   
                 circuit cards 
               
               
                   
                 Concentration Factor: 918x 
               
               
                 System Properties 
                 Silica Mass: 450 g/m 2   
               
               
                   
                 90% Power Point @ 0.6° for both azimuth 
               
               
                   
                 and elevation mispointing 
               
               
                   
                 Geometric concentration 918 X