Abstract:
A concentrated photovoltaic (CPV) module includes a CPV cell mounted on a support surface. A primary optical element (POE) is a square Fresnel lens fixed in position over the CPV cell. A secondary optical element (SOE) is optically coupled to the cell for providing uniform light over the surface of the cell. The SOE has a square, sloping top portion and a truncated pyramid shaped bottom portion that tapers toward the cell. The top portion is formed of angled flat square rings that refract light entering the top portion from the POE. The bottom portion mixes the light using TIR. The SOE has an angled middle portion, and an integral support structure having one end attached to the middle portion surrounds the bottom portion and is attached to the cell support surface so there is little mechanical stress on the cell.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to an optical system for concentrating sunlight onto a small solar cell and, in particular, to an optical system that employs refraction, rather than mirrors, to concentrate sunlight onto the solar cell. 
       BACKGROUND 
       [0002]    A concentrated photovoltaic (CPV) system comprises an array of small solar cells (e.g., 1 cm 2  or less), where each cell receives light directed to it by an optical system that tracks the sun. The optical system for each cell typically has a light receiving area that is hundreds of times the area of the cell, so that the cell effectively receives energy from hundreds of suns. The alignment of the optical system to the sun (typically within one degree) is very important to maximize the energy impinging on the cell surface and maximize the uniformity of the light distribution over the cell surface. 
         [0003]    Such a system differs from less expensive solar cell systems where large panels of single-junction silicon cells are mounted in a fixed position on a rooftop. Such a system is generally referred to as a photovoltaic (PV) system, rather than a CPV system. A PV system has an overall efficiency of about 18%. 
         [0004]    A CPV cell typically has a stack of three semiconductor junctions, having the electrical characteristics of three diodes in series. Each junction is formed of a different set of semiconductor materials so as to be sensitive to a different range of wavelengths. The three groups of wavelengths are typically UV, visible, and infrared. Therefore, CPV systems use more of the sun&#39;s energy, and fewer CPV cells are needed to achieve the same power output as a PV system. Such CPV systems typically have an overall efficiency of about 28%. 
         [0005]    A common optical system for a CPV system comprises a large area Fresnel lens, called a primary optical element (POE), that ideally focuses all of the impinging sunlight onto a receiving surface of a much smaller secondary optical element (SOE). The SOE is directly optically coupled to the cell, such as by a transparent adhesive. The SOE mixes the light from the POE and has the goal of providing uniform illumination of the cell. 
         [0006]    The prior art optical systems, especially the SOE portions, suffer from drawbacks such as not achieving uniform light distribution, being very sun-alignment sensitive, being difficult to fabricate due to the robust materials needed to not degrade when subjected to the UV energy of hundreds of suns, and providing stress on the cell since the SOE is typically directly attached to the cell&#39;s top surface. 
         [0007]    What is needed is a CPV optical system where the SOE provides more uniform distribution of brightness and wavelengths over the cell surface, has a wider light acceptance angle from a rectangular Fresnel lens (the POE), is easy to fabricate, and does not exert significant mechanical stress on the cell. 
       SUMMARY 
       [0008]    An optically system for a standard CPV cell, having a top surface area of about 1 cm 2  or less, is described. A large rectangular (includes square) Fresnel lens is spaced from an SOE, such as by 10 cm. The Fresnel lens, in one embodiment, is a square having an area of about 625 cm 2  (e.g., 25 cm per side). 
         [0009]    The SOE has a bottom portion that resembles a truncated inverted pyramid shape. The bottom surface of the SOE has an area that matches the area of the cell and is optically coupled to the top surface of the cell with silicone. The top portion of the SOE comprises a light receiving surface that is made up of a small center square surrounded by rectangular concentric rings, where the inner rings are slightly angled up and the outer rings are at increasingly downward angles as the rings extend to the outer edge of the SOE, so that the top surface generally falls away toward the edges. In one embodiment of an SOE designed for a cell of about 3×3 mm, the size of the inlet top surface is about 9×9 mm and the SOE has a total height of about 14 mm. 
         [0010]    Since the Fresnel lens is rectangular, the light impinging on the SOE is generally rectangular and impinges on the top surface of the SOE at various angles. Since the SOE has a rectangular light receiving surface and concentric angled rectangular rings, the SOE top surface efficiently receives the light from the Fresnel lens at the various angles and improves light acceptance uniformity over its surface. The light entering through the rings is refracted downward by the SOE, and the light is totally internally reflected (TIR) by the bottom portion of the SOE so as to mix the incoming light to provide a uniform brightness and wavelength distribution over the cell surface. 
         [0011]    Between the sloping top portion and the truncated pyramid shaped bottom portion is a middle portion that has an inward angle connecting the outer square perimeter of the top portion to the narrower bottom portion. The angle of the middle portion is such that light refracted from the outer areas of the top portion does not significantly exit through the angled sides of the middle portion. The middle portion allows the edges of the top portion to overhang the outer edges of the bottom portion to accept more light at wider angles from the POE. This also allows the taper of the bottom portion to be at a steep angle to provide TIR of the light refracted by the top portion. 
         [0012]    The designs of the POE and SOE are such that light is not focused at a point or in a plane within the SOE. Rather, the Fresnel rings are designed to distribute their focal areas in a relatively large three-dimensional volume within the SOE to reduce the UV concentration and help enable better mixing of the light. In one embodiment, the Fresnel rings have a cloverleaf shape to distribute the focal areas inside of the SOE, where each Fresnel ring arc has a different focal point. In one embodiment, the focal areas extend along a 6 mm path within the SOE (about half the height of the SOE). Since the UV from many suns is not focused within a small point or area, the SOE material may be silicone instead of glass. Silicone is easy to mold, so the SOE can be made inexpensively with a stringent tolerance. 
         [0013]    Since the SOE is silicone, it can be optically coupled to the top surface of the cell using a silicone adhesive. To avoid the cell fully supporting the SOE, the SOE is molded to have an integral support structure connected to the middle portion of the SOE. The support structure may consist of four flat walls that are spaced from the bottom portion of the SOE by an air gap so as not to affect the TIR of the bottom portion. The middle portion is angled such that an insubstantial amount of light is tapped off by the support structure. The bottom surface of the support structure is adhesively affixed to the circuit board (or other substrate) supporting the cell. 
         [0014]    In another embodiment of the SOE  18 , the top down shape is not square but may be any shape, depending on the shape of the cell and POE. Additionally, the concentric rings on the SOE  18  need not be flat but may be rounded and form a smooth sloping surface. 
         [0015]    Other features are also described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1A  is a simplified and condensed cross-sectional view of the optical system providing light to a CPV cell, where the POE may be spaced from the SOE by over ten times the height of the SOE. 
           [0017]      FIG. 1B  is a close up of the top of the SOE. 
           [0018]      FIG. 1C  is a side view of the SOE supported over a cell, where  FIG. 1A  is a cross-section along the line  1 A- 1 A in  FIG. 1C . 
           [0019]      FIG. 1D  is a top down view of the SOE, illustrating the rectangular concentric facets of the top surface and the surrounding integral support structure of the SOE, where  FIG. 1A  is a cross-section along the line  1 A- 1 A in  FIG. 1D   
           [0020]      FIG. 2A  illustrates results from a computer simulation of light rays from the POE, when the POE is aligned with the sun, showing that a focal volume extends from about the top of the SOE to about half way into the SOE. 
           [0021]      FIG. 2B  is a close up of results from a computer simulation of light rays from the POE, when the POE is one degree off-axis with the sun, illustrating how most of the rays enter the SOE through the SOE&#39;s outer rings. 
           [0022]      FIG. 3A  is a simplified cross-sectional view of the SOE showing an approximate volume within which the various Fresnel prism arcs focus light. 
           [0023]      FIG. 3B  is a simplified top down view of the SOE showing an approximate volume within which the various Fresnel prism arcs focus light. 
           [0024]      FIG. 4  illustrates the general shape of the focal pattern formed by selected rings in the Fresnel lens at different depths into the SOE. 
           [0025]      FIG. 5  is a perspective view of one of many possible examples of a cell module containing the SOE of  FIG. 1A . 
           [0026]      FIG. 6  is a top down view of the Fresnel lens used as the POE, where there are 33 clover shaped rings in one embodiment, each ring quadrant being formed by an arc of a circle with its center point within the quadrant rather than at the center point of the lens. 
       
    
    
       [0027]    Elements labeled with the same numeral are the same or equivalent. 
       DETAILED DESCRIPTION 
       [0028]      FIG. 1A  is a cross-sectional view of a CPV module comprising an optical system and a CPV cell  10  mounted on a circuit board  12  or other substrate. The board  12  may be any electrically insulating substrate (e.g., ceramic) with electrical connectors for the cell  10 . The cell  10  may be mounted on the board  12  via a submount, which is then connected to the board  12 . The board  12  typically contains electrical connections for interconnecting identical cells in parallel to create a large current. In one embodiment, the cell  10  is a commercially available cell that generates about 6.8 amps at 2.68 volts using the optical system described herein. The cell  10  is preferably a triple junction cell, where each junction comprises different materials for the wavelength ranges of UV, visible, and infrared. The board  12  generally has a top metal layer  13  for connection to electrodes on each cell  10 , a connector for interconnection with other cells, and a heat sink layer for removing the high heat applied to each cell  10 . The cell  10  and board  12  may be conventional, and the board  12  is intended to represent any support surface for the cell  10 . 
         [0029]    The combination of the cell  10 , board  12 , and optical system is a module that is connected together by a housing, frame, or other structure to maintain the proper spacing and alignment. 
         [0030]      FIG. 1A  illustrates the module aligned with the sun so that the rays  14  are substantially normal to the primary optical element (POE)  16  (sun rays actually have a divergence of about 0.26 degrees). The POE  16  is a Fresnel lens having rings of differently angled prisms, described later in more detail with respect to  FIG. 4 . The POE  16  has an area that may be over 1000 times that of the cell  10 . In one embodiment, the cell  10  is a square having sides between 2.8 mm and 10 mm. For a 3.3×3.3 mm cell, the POE  16  is 94×94 mm for 1000 times the area and, for a 10×10 mm cell, the POE is 316×316 mm for 1000 times the area. 
         [0031]    The angled prisms of the POE  16  direct the sunlight toward the top surface of a secondary optical element (SOE)  18 , as shown by rays  20 . In one embodiment, the POE  16  is spaced about 10 cm from the top of the SOE  18 , and the height of the SOE  18  is about 14 mm. 
         [0032]      FIG. 1B  is a close-up view of the top portion of the SOE  18 . 
         [0033]      FIG. 1C  is a side view of the SOE  18  supported over a cell, where  FIG. 1A  is a cross-section along the line  1 A- 1 A in  FIG. 1C . 
         [0034]      FIG. 1D  is a top down view of the SOE  18 , where the width of the square top portion  23  (not including the support structure  22  in  FIG. 1A ) is about 8.8 mm. The SOE  18  tapers to the size of the cell  10 , such as 2.8×2.8 mm. 
         [0035]    The top portion  23  of the SOE  18  comprises a flat center area  24  and four square concentric rings  25 - 28 . The inner ring  25  slightly angles upward and rings  26 - 28  progressively increase in angle downward toward the edge so that each of the rings is generally normal to the light rays impinging on it from the POE  16 . There may be more inner or outer rings in an actual embodiment. For example, as shown in  FIG. 1A , light rays from the POE  16  near the edge of the POE  16  impinge upon the SOE  18  at a greater angle and are primarily received by the outer rings  26 - 28 , especially if the optical system is not aligned with the sun. Conversely, the light from near the center of the POE  16  impinges upon the center area  24  and inner ring  25 . Therefore, the POE  16  ring angles and SOE  18  shape are designed so that the light refracted by certain rings of the POE  16  impinge upon certain rings of the SOE  18  to provide a large focal area and uniformity of light. In one embodiment, the angle of the most outer ring  28  of the SOE  18  is about 45 degrees relative to the flat center area  24 . 
         [0036]    The generally downward slope of the SOE  18  top surface, in conjunction with its relatively wide width, provides a wide acceptance angle of light so the module does not need to be perfectly aligned with the sun. As the module becomes more out of alignment with the sun, more light will impinge upon the outer rings of the SOE  18 . Since each quadrant of the SOE  18  is identical, and each quadrant of the POE  16  is identical, there is very uniform light mixing inside the SOE  18 . 
         [0037]      FIG. 2A  illustrates a computer simulation of rays  20  from the POE  16  when the POE  16  is aligned with the sun. The rays  20  are “softly” focused within the SOE  18  so that there is a large three-dimensional volume within the SOE  18  where the light is focused from the different prism rings in the POE  16 . In one embodiment, the focal points begin near the top of the SOE  18  and extend about 6 mm into the SOE  18 , which is about half the height of the SOE  18 . The prism rings in the POE  16  are made non-circular so that the focal points do not just extend along a line down the center of the SOE  18 . By making the prism rings non-circular, the UV entering the SOE  18  is diffused, enabling the SOE  18  to be formed of silicone rather than a more UV tolerant glass or other transparent material. Silicone is inexpensive and easily moldable. 
         [0038]    The light rays  20  are mixed in the bottom portion  34  of the SOE  18 , where the light rays  20  reflect off the flat side walls by TIR. The bottom portion  34  has a truncated pyramid shape that extends from the cell surface to the middle portion  36  of the SOE  18 . 
         [0039]      FIG. 2B  is a close-up of a computer simulation of rays  20  from the POE  16  when the POE  16  is one degree off-axis with the sun. As seen, more of the rays  20  enter the SOE  18  from the outer rings of the top portion  23 , and more of the light is tapped off by the support structure  22 . Thus, the system is less efficient when not aligned with the sun; however, the shape of the top portion  23  causes it to accept more light from the POE without reflection, compared to prior art SOE&#39;s. 
         [0040]      FIG. 3A  illustrates the general focal volume  32  inside of the SOE  18  and slightly above the SOE  18  when the POE  16  is aligned with the sun and focuses the light rays  20  from the various rings of the POE  16 .  FIG. 3B  is a top down view of the focal volume  32 . 
         [0041]    The focal volume  32  has different cross-sectional shapes along its length since the different rings of the POE  16  create different patterns. For example, as shown in  FIG. 4 , the unbroken rings of the POE  16  create a generally circular focal pattern, and the broken outer rings of the POE  16  create an X shape focal pattern. The different top down views of the SOE  18  show the focal patterns at the depth at which the rays are best focused by the associated POE ring. The distribution of the focal patterns within the SOE  18  avoids any high UV concentrations, enabling the SOE  18  to be formed of silicone. In one embodiment, the depth of the focal volume  32  is about 6 mm, or about one-half the height of the SOE  18 . 
         [0042]    The SOE  18  can be formed of a thermoset silicone. Thermoset silicone can be easily molded using injection molding or compression molding. In one embodiment, the SOE  18  is molded directly over the board  12  (or other support surface) on which many cells  10  are mounted, so that many SOE&#39;s are formed simultaneously. The molding and curing process causes the SOE  18  to be adhered to the cell  10  and board  12  (including adhered to metal pads or conductors, etc.) without any special adhesive step. 
         [0043]    The SOE  18  can be easily molded to include the integral support structure  22 , shown in  FIG. 1A . The support structure  22  is generally a square skirt spaced from and surrounding the SOE  18  except where it is attached at the angled middle portion  36 . The angle of the middle portion  36  causes the outer surface of the middle portion  36  to not intersect any light rays refracted by the top portion  23  (when the system is aligned to the sun) so an insubstantial amount of light is tapped off by the silicone support structure  22 . The angled middle portion  36  allows the outer refracting ring  28  to extend over the edge of the bottom portion  34  to increase the acceptance angle of light from the POE  16 , while allowing the angled walls of the bottom portion  34  to have a relatively small angle to ensure TIR. 
         [0044]      FIG. 1A  shows the support structure  22  affixed to the metal layer  13  surface of the board  12  by a small integral silicone tab  37 . The tab  37  just allows the SOE  18  to be supported with a smaller footprint. The tab  37  can be located anywhere on the bottom of the support structure  22  to provide the least interference on the board  12 . The tab  37  is optional. In one embodiment, where the support structure  22  is affixed to the board  12  by a silicone adhesive, the tab  37  may represent the silicone adhesive. The support structure  22  may be affixed to the insulating surface of the board  12  if the metal layer  13  does not extend around the cell  10 . 
         [0045]    In one embodiment, the flat bottom surface of the bottom portion  34  is affixed to the top of the cell  10  by a silicone adhesive for good optical coupling. Since the SOE  18  is primarily supported by the support structure  22  affixed to the circuit board  12  (or other support structure) the SOE  18  does not mechanically stress the cell  10 . 
         [0046]    In another embodiment, the support structure  22  is not angled outward but is still separated from the bottom portion  34  by an air gap so as not to affect the TIR of the bottom portion  34 . In another embodiment, the support structure  22  is an extension of the top portion  23  rather than connected to the middle portion  36 . The support structure need not completely surround the bottom portion  34 . Many configurations of the support structure  22  are possible. 
         [0047]      FIG. 5  is a perspective view of the SOE  18  supported on a board  12 , where electrodes  39  on the board  12  lead to the cell. The board  12  (having a metal body for heat conduction) is intended to be bolted to a heat sink (not shown). 
         [0048]    In another embodiment of the SOE  18 , the top down shape is not square but may be any shape, depending on the optimal shape required for the system, such as depending on the shape of the cell and POE. The shape may also be a non-square rectangular, round, polygonal, or other shape. Additionally, the concentric rings on the SOE  18  need not be flat but may be rounded and form a smooth sloping surface (no separate rings). 
         [0049]      FIG. 6  is a top down view of the POE  16 , showing the non-symmetrical prism rings  40  to spread out the focal area in the SOE  18 . Each ring  40  may have a height of about 1 mm.  FIG. 6  illustrates a clover leaf pattern of the rings  40  formed by quadrants of circle portions, where the center point of each circle is within the quadrant rather than at the center point of the POE  16 , creating a non-symmetrical ring pattern. Other patterns may be used, such as rings having eight or more lobes. In one embodiment, there are 33 rings  40  on the POE  16 . 
         [0050]    The combination of the POE  16  and SOE  18  provides substantially uniform light over the top surface of the cell, where both brightness and wavelengths are uniformly distributed so that the three diode junctions in the cell are fully exposed to the concentrated sunlight for maximum current output. 
         [0051]    Having described the invention in detail, those skilled in the art will appreciate that given the present disclosure, modifications may be made to the invention without departing from the spirit and inventive concepts described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.