Abstract:
A system includes a solid light-transmissive element comprising a first surface and a second surface, first reflective material disposed on the second surface of the light-transmissive element, and a solar cell to convert light received at the first surface to electrical current. The light received at the first surface may pass through the light-transmissive element, reflect off the first reflective material and intercept an area of an interface between the first surface and an adjacent environment at an angle of incidence greater than arc sin(n x /n y ), where n x =an index of refraction of the adjacent environment and n y =an index of refraction of the light-transmissive element at the first surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/017,432, filed on Dec. 28, 2007 and entitled “Solid Concentrator With Total Internal Secondary Reflection”, the contents of which are incorporated herein by reference for all purposes. 
     
    
     BACKGROUND 
       [0002]    A solar radiation concentrator may convert received solar radiation (i.e., sunlight) into a concentrated beam and direct the concentrated beam onto a photovoltaic (or, solar) cell. The cell, in turn, may generate electrical current based on photons of the concentrated beam. A concentrator thereby enables a small solar cell to generate electrical current based on photons received over a larger area. 
         [0003]    U.S. Patent Application Publication No. 2006/0231133 describes several types of concentrating solar collectors. As generally described therein, solar radiation enters a solid transparent element and strikes reflective material disposed on a convex surface (i.e., a primary mirror) of the element. The radiation is reflected toward reflective material disposed on a smaller and opposite concave surface (i.e., a secondary mirror), and is reflected thereby toward an even smaller area from which a solar cell may receive the radiation. Such operation may allow the concentrator to convert the received solar radiation to electricity using smaller solar cells than would otherwise be required. 
         [0004]    The reflective material disposed on the secondary mirror prevents some solar radiation from reaching the primary mirror. The secondary mirror is located near the focus of the primary mirror in order to minimize this shading. However, this location requires the secondary mirror to exhibit a steeply curved aspheric surface and to satisfy precise geometric tolerances with respect to the primary mirror. Formation of such a primary mirror and a secondary mirror on opposite sides of an optically-transparent element (e.g., glass) is difficult and expensive. 
         [0005]    Improved solar concentrator designs are desired. Such designs may provide increased power generation per unit area, improved manufacturability, decreased cost, and/or other benefits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a cutaway side view of a solid concentrator according to some embodiments. 
           [0007]      FIG. 2  is a perspective top view of the  FIG. 1  solid concentrator according to some embodiments. 
           [0008]      FIG. 3  a perspective exploded view of a solid concentrator according to some embodiments. 
           [0009]      FIG. 4  is a perspective view of an array of solid concentrators according to some embodiments. 
           [0010]      FIG. 5  is a cutaway side view of a solid concentrator and lens according to some embodiments. 
           [0011]      FIG. 6  is a perspective top view of the  FIG. 5  solid concentrator and lens according to some embodiments. 
           [0012]      FIG. 7  is a perspective view of a solid concentrator and lens according to some embodiments. 
           [0013]      FIG. 8  is a perspective view of an array of solid concentrators and lenses according to some embodiments. 
       
    
    
     DESCRIPTION 
       [0014]    The following description is provided to enable any person in the art to make and use the described embodiments and sets forth the best mode contemplated for carrying out some embodiments. Various modifications, however, will remain readily apparent to those in the art. 
         [0015]      FIG. 1  is a cutaway side view of apparatus  100  according to some embodiments. Apparatus  100  includes substantially light-transparent core  105  and solar cell  110 . Core  105  may be composed of any suitable material or combination of materials. According to some embodiments, core  105  is configured to manipulate and/or pass desired wavelengths of light. Core  105  may be molded from low-iron glass, formed from a single piece of clear plastic, or formed from separate pieces which are glued or otherwise coupled together to form core  105 . 
         [0016]    Solar cell  110  may comprise a III-V solar cell, a II-VI solar cell, a silicon solar cell, or any other type of solar cell that is or becomes known. Solar cell  110  may comprise any number of active, dielectric and metallization layers, and may be fabricated using any suitable methods that are or become known. Solar cell  110  is capable of generating charge carriers (i.e., holes and electrons) in response to received photons. Although solar cell  110  is shown recessed into core  105 , solar cell  110  may be disposed at any suitable position with respect to core  105 . 
         [0017]    Primary mirror  120  is disposed on convex surface  125  of core  105  and reflective material  130  is disposed on flat surface  140  of core  105  as shown.  FIG. 2 , which is a top view of the  FIG. 1  apparatus, shows reflective material  130  disposed in a ring-like shape. Primary mirror  120  and reflective material  130  may comprise any suitable reflective material, including but not limited to silver or aluminum. Primary mirror  120  and reflective material  130  may be fabricated by sputtering or otherwise depositing a reflective material directly onto the larger convex surface of core  105  and the illustrated ring-shaped area of surface  140 . A reflective side of the deposited material faces the surface on which the material is deposited. 
         [0018]    Refractive lens  150  is disposed opposite from primary mirror  120 . Core  105  and lens  150  may comprise a single molded piece, or lens  150  may be fabricated separately and attached to core  105 . Accordingly, lens  150  may comprise a material different from core  105  in some embodiments. 
         [0019]    In operation, incoming on-axis (e.g., normal to surface  140 ) light  160  passes through ambient air and is received at surface  140  and lens  150  of apparatus  100 . For clarity,  FIG. 1  shows only incoming light  160  received on one half of apparatus  100 . Some of incoming light  160  is received at area A of surface  140  and is represented by dashed lines in  FIG. 1 . This light  160  received at area A passes through core  105  and reflects off of primary mirror  120 . The reflected light returns to an area at the interface of surface  140  and ambient air, where the reflected light experiences total internal reflection. 
         [0020]    More specifically, and with respect to the  FIG. 1  embodiment, the angle at which the reflected light  160  meets the area at the interface is greater than arc sin (n air /n core ), where n x  represents a refractive index of medium x. The reflective properties (efficiency, chromatic aberration, etc.) of a total internal reflection are superior to that of a reflective material coating. The reflected light proceeds from the interface toward an active area of solar cell  110  as shown. 
         [0021]    Dotted lines represent the incoming light  160  received at area B of surface  140 . This light  160  passes through core  105  and reflects off of primary mirror  120  as described above. This reflected light also returns to an area at the interface of surface  140  and ambient air, however, the angle at which the light meets the area is less than or equal to arc sin(n air /n core ). Since this light would not experience total internal reflection, reflective material  130  serves to reflect the light toward the active area of solar cell  110 . 
         [0022]    The reflectivity of a non-total internal reflection (angle of incidence≦arc sin (n air /n core ) may in some instances be greater than that provided by a reflective coating such as material  130 . Therefore, the exterior diameter of material  130  may be reduced so that the light received at some small annular zone immediately interior to area A reflects off of the air/surface  140  interface via a non-total internal reflection. 
         [0023]    As also shown in  FIG. 1 , incoming light  160  may reach reflective coating  130 . This light  160  is stopped at  130  and is not directed into core  105  and toward solar cell  110 . Incoming light  160  is also received by lens  150 . Lens  150  is shaped to refract the received light and to direct the light to the active area of solar cell  110 . Lens  150  may comprise a Fresnel lens, a continuous lens, a gradient index lens or some combination thereof. Refracted light may introduce chromatic dispersion, therefore some embodiments are designed to reduce a size and refractive angle of lens  150 . In some embodiments, the shape of lens  150  is less difficult to manufacture than the secondary mirror surfaces of prior designs. 
         [0024]    The dimensions of area A, area B, reflective material  130 , and lens  150  are subject to the geometry of primary mirror  120  and the refractive index of core  105 . In some embodiments, primary mirror  120  is paraboloidial-shaped and the refractive index of core  105  is ˜1.5. Any suitable mirror geometry and core material having any suitable refractive index may be used in some embodiments. 
         [0025]      FIG. 3  is an exploded view of apparatus  200  according to some embodiments. Apparatus  200  includes core  205 , primary mirror  220 , reflective material  230 , surface  240 , and lens  250 . Apparatus  200  may operate similarly to apparatus  100  described above. 
         [0026]    An upper periphery of core  205  of  FIG. 3  includes six contiguous facets. This six-sided arrangement may facilitate the formation of large arrays of apparatus  200  in a space-efficient manner.  FIG. 4  provides a perspective view of array  300  of apparatuses  200  according to some embodiments. Embodiments are not limited to the illustrated arrangement. For example, some embodiments may include four contiguous facets or no facets (e.g., apparatus  100 ). Irregular or semi-regular tessellations (e.g., a combination of octagons and squares) may also be employed. 
         [0027]    Primary mirror  220  includes conductive portion  222  and conductive portion  224 . Conductive portion  222  defines opening  226  through which concentrated light may exit apparatus  200  and be received by a solar cell. Primary mirror  120  of apparatus  100  may be substituted with primary mirror  220  and/or any other primary mirror illustrated and/or described herein. Alternatively, primary mirror  220  of apparatus  200  may be substituted with primary mirror  120  and/or any other primary mirror illustrated and/or described herein. 
         [0028]    Gap  227  is defined between conductive portions  222  and  224  to facilitate electrical isolation thereof. Accordingly, conductive portions  222  and  224  of primary mirror  220  may create a conductive path for electrical current generated by the solar cell. Conductive portions  222  and  224  may also, as described in above-mentioned Application Publication No. 2006/0231133, electrically link photovoltaic cells of adjacent collectors in a concentrating solar collector array. 
         [0029]      FIG. 5  is a cutaway side view and  FIG. 6  is a perspective top view of apparatus  400  according to some embodiments. Apparatus  400  includes substantially light-transparent core  405 , solar cell  410 , and primary mirror  420 , which may be implemented as described with respect to core  105 , cell  110  and mirror  120  of apparatus  100 . 
         [0030]    Apparatus  400  also includes lens  450  disposed at a distance d from surface  440  of core  405 . Lens  450  may comprise a material different from core  450  according to some embodiments. Lens  450  may reduce a need for reflective material disposed on surface  440 . As will be described below, some embodiments of apparatus  400  include reflective material on surface  440 . 
         [0031]    According to some embodiments, molding tolerances associated with lens  450  and core  405  provide improved manufacturability and decreased cost. 
         [0032]    In operation, incoming light  460  passes through ambient air and is received at surface  440  of apparatus  400 .  FIG. 5  shows only incoming light  460  received on one half of surface  440  for clarity. Light  460  received at area C passes through core  405  and reflects off of primary mirror  420 . The reflected light returns to the interface of surface  440  and ambient air where it experiences total internal reflection as described above. The reflected light proceeds from the interface toward an active area of solar cell  410  as shown. 
         [0033]    For some combinations of primary mirror geometries and core indices of refraction, some or all of the incoming on-axis light may be reflected using total internal reflection. For example, primary mirror  420  is not present along a periphery of surface  425  of core  405 . Light passing through core  405  and received at this periphery may intercept surface  425  at an angle sufficient to cause total internal reflection of the light toward surface  440 . Even if primary mirror  420  was present along the periphery of surface  425 , the light incident thereto (if received at a sufficient angle) may be reflected via total internal reflection rather than by primary mirror  420 . As total internal reflection exhibits substantially higher reflectivity than alternate reflective materials, the foregoing feature may improve system efficiency. 
         [0034]    Lens  450  receives incoming light  465 . Lens  450  is shaped to refract light  465  and to direct the light toward surface  440 . As shown in  FIG. 5 , light  465  is refracted three times prior to reaching solar cell  410 . Distance d, a shape of lens  450 , and a refractive index of lens  450  are therefore selected such that these refractions result in the delivery of light  465  to solar cell  410 . In addition, any suitable geometry of mirror  420  and refractive index of core  405  may be used in some embodiments. 
         [0035]    In some embodiments, some incoming normal light may miss lens  465  and intercept surface  440  at an area other than area C. Reflective material may be deposited on appropriate locations of surface  440  to reflect this light toward solar cell  410 . This reflective material may be disposed between lens  450  and surface  440  in some embodiments. 
         [0036]      FIG. 7  is a perspective view of apparatus  500  according to some embodiments. Apparatus  500  includes core  505 , primary mirror  520 , surface  540 , and lens  550 . Apparatus  500  may operate similarly to apparatus  400  described above. 
         [0037]    An upper periphery of core  505  includes six contiguous facets, but embodiments are not limited thereto. Primary mirror  520  may comprise a contiguous material, may be separated as described with respect to mirror  220 , and/or may comprise any suitable configuration. 
         [0038]      FIG. 4  provides a perspective view of array  600  of apparatuses  500  according to some embodiments. Each lens  550  is coupled to cover glass  650 , which provides environmental protection as well as a mounting surface for lenses  550 . Each lens may be coupled to glass  650  using an epoxy or other optically-transparent material. Selection of such a material may take into account a refractive index of glass  650 , a refractive index of lenses  550 , and/or thermal expansion properties to glass  650  and lenses  550 . 
         [0039]    A position of cover glass  650  may determine a distance d between lenses  550  and cores  505  of array  600 . In some embodiments, lenses  550  are mounted such that glass  650  is located between lenses  550  and cores  505 .