Patent Publication Number: US-2011061718-A1

Title: Passively Cooled Solar Concentrating Photovoltaic Device

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/381,999, entitled “Passively Cooled Solar Concentrating Photovoltaic Device” filed May 5, 2006. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to solar power generators, more particularly to managing the heat generated at and around the photovoltaic (PV) cell in solid dielectric solar concentrator photovoltaic (CPV) devices. 
     BACKGROUND OF THE INVENTION 
     Photovoltaic solar energy collection devices used to generate electric power generally include flat-panel collectors and concentrating solar collectors. Flat collectors generally include PV cell arrays and associated electronics formed on semiconductor (e.g., monocrystalline silicon or polycrystalline silicon) substrates, and the electrical energy output from flat collectors is a direct function of the area of the array, thereby requiring large, expensive semiconductor substrates. Concentrating solar collectors reduce the need for large semiconductor substrates by concentrating light beams (i.e., sun rays) using, e.g., a parabolic reflectors or lenses that focus the beams, creating a more intense beam of solar energy that is directed onto a small PV cell. Thus, concentrating solar collectors have an advantage over flat-panel collectors in that they utilize substantially smaller amounts of semiconductor. Another advantage that concentrating solar collectors have over flat-panel collectors is that they are more efficient at generating electrical energy. 
     A problem with conventional concentrating solar collectors is that they are expensive to operate and maintain. The reflectors and/or lenses used in conventional collectors to focus the light beams are produced separately, and must be painstakingly assembled to provide the proper alignment between the focused beam and the PV cell. Further, over time, the reflectors and/or lenses can become misaligned due to thermal cycling or vibration, and become dirty due to exposure to the environment. Maintenance in the form of cleaning and adjusting the reflectors/lenses can be significant, particularly when the reflectors/lenses are produced with uneven shapes that are difficult to clean. 
     Another problem associated with conventional concentrating solar collectors is damage to the PV cell and mirror structure due to the excessive temperatures generated by the focused light. For reliable operation it is essential to keep the PV cell and its surrounding packaging within safe limits, which is typically well under 100 degrees Celsius (100° C.). Because flat plate photovoltaic modules are exposed to direct (i.e., unfocused) solar light, the temperature rise of most flat plate photovoltaic modules under peak isolation is about 25° C. above ambient in zero wind, which produces a maximum PV cell temperature of about 70° C. (i.e., assuming an ambient temperature of 45° C.). In contrast, concentrating solar collectors produce flux densities of 300 to over 1000 suns at the PV cell, with typically less than half of the energy is converted into electricity and the remainder occurring as heat, producing PV cell temperatures that can reach well above 100° C. A conventional approach to reducing peak PV cell temperatures in concentrating solar collectors includes using a forced liquid cooling system to cool the PV cell, but such forced liquid cooling systems are expensive to produce and maintain, thus significantly increasing the overall production and operating costs of such concentrating solar collectors. 
     What is needed is a concentrator PV (CPV) device that avoids the expensive assembly and maintenance costs associated with conventional concentrator-type PV cells, and also maintains the CPV device within reliable operating temperatures in a cost effective and reliable manner. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a Cassegrain-type CPV device that induces the efficient radiation of heat out the front of the concentrator by utilizing a heat spreader to evenly distribute heat from the centrally located PV cell over the backside surface of a solid optical element, and by utilizing the solid optical element to transfer the heat from the heat spreader to a front aperture surface, from which the heat is radiated into space. This arrangement facilitates the radiation of more than 30% of the generated heat through the front aperture surface, thus improving passive cooling performance by approximately a factor of two over hollow concentrator systems that radiate heat out the back surface. In addition, the solid optical element facilitates the direct formation of primary and secondary mirrors thereon, thus automatically and permanently aligning the concentrator optics and maintaining optimal optical operation while minimizing maintenance costs. 
     In accordance with an aspect of the invention, a lateral thermal resistance of the heat spreader is less than a transverse thermal resistance of the solid optical element, thereby optimizing radiant heat transfer by maximizing the heat distribution to maintain the optical element and, hence, the aperture surface at a substantially uniform temperature. In one embodiment, the solid optical element includes a low-iron glass structure having a thickness in the range of 5 to 12 mm and a diameter of approximately 28 mm, and the heat spreader includes copper heat-distributing layer having a nominal thickness of approximately 70 microns. At this thickness, a lateral thermal resistance of thermal resistance of the copper heat-distributing layer is greater than the transverse thermal resistance of the optical element, thereby producing the desired uniform heating and radiation from the front aperture surface. 
     In accordance with an embodiment of the present invention, the heat spreader includes a thermal conductive layer (e.g., copper) formed on a flexible substrate (e.g., a polyimide film such as Kapton® produced by DuPont Electronics), and the PV cell is mounted on the heat spreader prior to assembly onto the solid optical element, thereby greatly simplifying the assembly process. In one embodiment the flexible substrate is cut or otherwise separated into a plurality of radial arms that extend from a central support region, which facilitates close contact to curved lower surface of the solid optical element during assembly. The wiring layers of the heat spreader are optionally used to help direct heat to the optical element. In one embodiment, the primary mirror includes a thin silver reflective layer, a copper anti-migration layer disposed on the silver layer, and a barrier paint layer disposed on the anti-migration layer. The heat spreader is then secured to the barrier paint layer by way of a suitable adhesive (e.g., EVA), and a protective shell (e.g., Tedlar) is secured to the backside of the flexible substrate using the same adhesive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, where: 
         FIG. 1  is an exploded perspective view showing a CPV device according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional side view showing the CPV device of  FIG. 1  during operation; 
         FIG. 3  is an exploded perspective view showing a CPV device according to another embodiment of the present invention; 
         FIG. 4  is a cross-sectional side view showing CPV device of  FIG. 3  in additional detail; 
         FIG. 5  is a perspective view showing a heat spreader substrate utilized in the CPV device of  FIG. 3 ; 
         FIG. 6  is an assembled perspective view showing the CPV device of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The present invention relates to managing the heat generated at and around the PV cell in a solid dielectric solar concentrator, such as that disclosed in co-owned and co-pending U.S. patent application Ser. No. 11/110,611 entitled “CONCENTRATING SOLAR COLLECTOR WITH SOLID OPTICAL ELEMENT”, which is incorporated herein by reference in its entirety. In particular, the present invention relates to a passive heat management system that avoids the production and maintenance costs of conductive and fluid cooled systems by facilitating the radiation of more than 30% of the generated heat from the front aperture surface of the solid optical element. 
     In considering the radiation balance of the CPV device, it is important to recognize that the blackbody temperature of the sky is typically on the order of −40 degrees Celsius (−40° C.). The blackbody temperature of the ground is typically about 4° C. above the ambient temperature. It is therefore desirable to provide a thermal path to the front surface of the device so it can radiate heat skyward. 
     Quantitatively, the net radiation flux per unit area from the front surface of the concentrator can be expressed as: 
         Q   f =ε f   σT   f   4 −(1 −R   f )σ T   s   4   Equation 1
 
     where ε f  is the emissivity of the front surface (typically 0.85 for low iron glass), σ is the Stefan-Boltzmann constant (5.67×10 −8  Watts/m 2  Kelvin 4 ), T f  is the absolute temperature of the front surface, R f  is the reflectivity of the front surface (typically about 8%) and T s  is the blackbody temperature of the sky (about −40 Celsius). 
     The radiation out the back of the concentrator can be expressed similarly as: 
         Q   b =ε b   σT   b   4 −(1 −R   b )σ T   g   4   Equation 2
 
     where ε b  is the emissivity of the back surface (typically 0.9 for plastic laminated Tedlar), T b  is the absolute temperature of the concentrator&#39;s back surface, R b  is the reflectivity of the concentrator&#39;s back surface (typically about 10% for Tedlar in the infrared) and T g  is the blackbody temperature of the ground or rooftop (typically about 4 degrees Celsius above ambient). 
     What is apparent from Equations 1 and 2 is that the front surface radiates into a much colder bath than the back surface. In flat plate PV systems, more than twice as much heat is typically lost out the front of the panel than out the rear. It is a useful aspect of this invention to create a concentrating PV system that mimics this advantageous heat loss mechanism. 
       FIG. 1  is an exploded perspective view showing an internal mirror, Cassegrain-type concentrator photovoltaic (CPV) device  100  according to a simplified embodiment of the present invention. Concentrating solar collector  100  generally includes an optical element  110 , a photovoltaic (PV) cell  120 , a primary mirror  130 , a secondary mirror  140 , and a heat spreader  150 . 
     Optical element  110  is a solid, disk-like, light-transparent structure including an upper layer  111 , a relatively large convex surface  112  protruding from a lower side of upper layer  111 , a substantially flat aperture surface  115  disposed on an upper side of upper layer  111 , and a relatively small concave (curved) surface (depression)  117  defined in aperture surface  115  (i.e., extending into upper layer  111 ). In order to minimize material, weight, thickness and optical adsorption, upper layer  111  may be vanishingly small. In one embodiment, optical element  110  is molded using a low-iron glass (e.g., Optiwhite glass produced by Pilkington PLC, UK) structure according to known glass molding methods. Alternatively, clear plastic may be machined and polished to form single-piece optical element  110 , or separate pieces by be glued or otherwise secured to form optical element  110 . In a preferred embodiment, optical element  110  is 5 to 12 mm thick and 20 to 40 mm wide. This thickness helps to ensure that the heat conduction path from the backside convex surface  112  to aperture surface  115  does not become too resistive as it would be if optical element  110  were either thicker or hollow. 
     PV cell  120  is located in a central first side (cavity) region  113  that is defined in the center of convex surface  112 . PV cell  120  is connected by way of suitable conductors  122  and  124  (indicated in  FIG. 2 ), for example, to the PV cells of adjacent CPV devices (not shown) using known techniques. Suitable photovoltaic (concentrator solar) cells are produced, for example, by Spectrolab, Inc. of Sylmar, Calif., USA. 
     Primary mirror  130  and secondary mirror  140  are respectively disposed on convex surface  112  and concave surface  117 . Primary mirror  130  and secondary mirror  140  are shaped and arranged such that, as shown in  FIG. 2 , light beams LB traveling in a predetermined direction (e.g., perpendicular to aperture surface  115 ) that enters optical element  110  through a specific region of aperture surface  115  is reflected by a corresponding region of primary mirror  130  to an associated region of secondary mirror  140 , and from the associated region of secondary mirror  140  to PV cell  120  (e.g., directly from secondary mirror  140  to PV cell  120 , or by way of a reflective or refractive surface positioned between secondary mirror and PV cell  120 ). As used herein, directional terms such as “upper”, “lower”, “above” and “below” are intended to provide relative positions for purposes of description, and are not intended to designate an absolute frame of reference. In one embodiment, primary mirror  130  and secondary mirror  140  are fabricated by sputtering or otherwise depositing a reflective mirror material (e.g., silver (Ag) or aluminum (Al)) directly onto convex surface  112  and concave surface  117 , thereby minimizing manufacturing costs and providing superior optical characteristics. By sputtering or otherwise forming a mirror film on convex surface  112  and concave surface  117  using a known mirror fabrication technique, primary mirror  130  substantially takes the shape of convex surface  112 , and secondary mirror  140  substantially takes the shape of concave surface  117 . As such, optical element  110  is molded or otherwise fabricated such that convex surface  112  and concave surface  117  are arranged and shaped to produce the desired mirror shapes. Note that, by forming convex surface  112  and concave surface  117  with the desired mirror shape and position, primary mirror  130  and secondary mirror  140  are effectively self-forming and self-aligning, thus eliminating expensive assembly and alignment costs associated with conventional concentrating solar collectors. Further, because primary mirror  130  and secondary mirror  140  remain affixed to optical element  110 , their relative position is permanently set, thereby eliminating the need for adjustment or realignment that may be needed in conventional multiple-part arrangements. In one embodiment, primary mirror  130  and secondary mirror  140  are formed simultaneously using the same (identical) material or materials (e.g., plated Ag), thereby minimizing fabrication costs. Further, by utilizing the surfaces of optical element  110  to fabricate the mirrors, once light enters into optical element  110  through aperture surface  115 , the light is only reflected by primary mirror  130 /convex surface  112  and secondary mirror  140 /concave surface  117  before reaching PV cell  120 . As such, the light is subjected to only one air/glass interface (i.e., aperture surface  115 ), thereby minimizing losses that are otherwise experienced by conventional multi-part concentrating solar collectors. The single air/glass interface loss can be further lowered using an antireflection coating on aperture surface  115 . Although it is also possible to separately form primary mirror  130  and secondary mirror  140  and then attach the mirrors to convex surface  112  and concave surface  117 , respectively, this production method would greatly increase manufacturing costs and may reduce the superior optical characteristics provided by forming mirror films directly onto convex surface  112  and concave surface  117 . 
     Heat spreader  150  includes a central portion  151  and a curved peripheral portion  152  extending outward from central portion  151 . Heat spreader  150  includes a material having relatively high thermal conductivity, and includes a thickness selected such that a lateral thermal resistance TR 1  of heat spreader  150  (i.e., measured in a radial direction from central portion  151  to the outer edge of peripheral portions  152 ) is less than a transverse thermal resistance TR 2  of optical element  110  (i.e., measured from the convex surface  112  to the aperture surface  115 ). In one practical embodiment, many small CPV devices  100  are arrayed together in order to keep the volume of glass from becoming excessively large, and to keep the amount of power per PV cell manageable without active cooling. In the preferred embodiment, low-iron glass having a thickness of 5 to 12 mm is used for optical element  110 , and heat spreader  150  includes a copper heat-distributing layer having a thickness of 70 microns (i.e., two ounce copper), which provides a thermal resistance TR 1  that is greater than a thermal resistance TR 2  of optical element  110 . At this thickness, a lateral thermal resistance of the copper heat-distributing layer is greater than the transverse thermal resistance of the optical element. 
     As indicate in  FIG. 2 , central portion  151  of heat spreader  150  is disposed over cavity  113 , and curved peripheral portion  152  is formed on or otherwise secured to the back (non-reflecting) surface of primary mirror  130 . PV cell  120  is mounted on an inside surface of central portion  151  such that PV cell  120  is disposed inside cavity  113 . A gap filling transparent adhesive  128 , such as silicone (e.g., polydiphenylsiloxane or polymethylphenylsiloxane), is also disposed inside cavity  113  over PV cell  120 , and serves to minimize the disruptive break in the refractive indicies between the outside surface of cavity  113  and PV cell  120 . Note that a central opening  131  is defined in primary mirror  130  to facilitate the passage of light through cavity  113  to PV cell  120 . In one embodiment, PV cell  120  is mounted onto central region  151  by way of a heat slug  127 . In another embodiment, one or more openings are formed in central region  151  and heat slug  127  to facilitate the passage of current from PV cell  120 , e.g., by way of conductors  122  and  124 . In another embodiment, current is transmitted to and from PV cell  120  by way of heat spreader  150  or primary mirror  130  in a manner similar to that disclosed in co-owned and co-pending U.S. patent application Ser. No. 11/110,611 (cited above). 
     Although primary mirror  130  and heat spreader  150  are illustrated as separate layers in  FIGS. 1 and 2 , in one embodiment a single layer may be formed on convex surface  112  that serves the functions of both primary mirror  130  and heat spreader  150 . That is, mirror surfaces are typically formed using a thin 500 Angstrom Ag layer and one or more protective layers that may include a thin 1000 Angstrom Cu anti-migration layer and/or a barrier paint layer. Such conventional mirror surfaces exhibit a relatively high lateral thermal resistance that is insufficient for adequately distributing heat from PV cell  120  such that optical element  110  achieves uniform heat distribution. Hence, a relatively thick layer of a material (e.g., copper) exhibiting high thermal conductivity is formed over the backside of the mirror surface to provide the needed heat distribution. While these two separate layers are needed to provide both an optimal reflective surface and adequate heat transfer, it may be possible to utilize a single (e.g., silver or copper) layer to perform both the reflective and heat transfer functions. However, at this time, forming silver to the thickness needed to facilitate sufficient heat transfer is economically infeasible, and depositing copper using known techniques is considered to form an inadequate mirror surface. 
       FIG. 2  is a side view showing concentrating solar collector  100  during operation. Similar to conventional concentrating solar collectors, a collector positioning system (not shown; for example, the tracking system used in the MegaModule™ system produced by Amonix, Incorporated of Torrance, Calif., USA) is utilized to position concentrating solar collector  100  such that light beams LB (e.g., solar rays) are directed into aperture surface  115  in a desired direction (e.g., perpendicular to aperture surface  115 . PV cell  120  is disposed substantially in a concentrating region F, which designates the region at which light beams LB are concentrated by primary mirror  130 , secondary mirror  140  and any intervening optical structures (e.g., a dielectric flux concentrator). To facilitate the positioning of concentrating region F in central region  113 , convex surface  112 , primary mirror  130 , concave surface  117 , and secondary mirror  140  are centered on and substantially symmetrical about an optical axis X that extends substantially perpendicular to aperture surface  115  (i.e., the curved portions of convex surface  112  and concave surface  117  are defined by an arc rotated around optical axis X). 
     In accordance with the present invention, waste heat generated at focal point F (i.e., heat generated by solar energy that is not converted to electricity by PV cell  120 ) is transmitted via central portion  151  (by way of heat slug  127 , when present) by conductive heat transfer to peripheral portion  152 , as indicated by dashed line arrows CH 1  in  FIG. 2 . For the purposes of this invention, the use of the term focal point refers both to concentration by imaging and non-imaging elements. The heat transferred to peripheral portions  152  in this manner is passed into optical element  110  via primary mirror  130  and convex surface  112 , and are transmitted by conductive heat transfer to aperture surface  115 , as indicated by dashed line arrows CH 2  in  FIG. 2 . From aperture surface  115 , the heat is radiated into space, as indicated by the wavy dashed line arrows RH. 
       FIG. 3  is a top-side exploded perspective view showing a CPV device  200  according to another embodiment of the present invention. Similar to concentrating solar collector  100 , concentrating solar collector  200  includes an optical element  210 , a photovoltaic cell  220 , a primary mirror  230  formed on a convex surface  212  of optical element  210 , a secondary mirror  240  formed on a concave surface  217  of optical element, and a heat spreader  250 . 
     As indicated in  FIG. 3 , optical element  210  includes six contiguous facets  219  located around a peripheral edge of aperture surface  215 . This six-sided arrangement facilitates the formation of large arrays of concentrating solar collectors  200  in a highly space-efficient manner, as discussed in additional detail in co-owned and co-pending U.S. patent application Ser. No. 11/110,611 (cited above). In other embodiments, less space-efficient concentrating solar collector arrays may be produced using concentrators having other peripheral shapes (e.g., the circular peripheral shape of concentrator  100 , described above). A central region (cavity)  213  is defined in (e.g., molded into) convex surface  212  for receiving PV cell  220 . 
       FIG. 4  is a simplified, partially exploded cross-sectional side view showing the various components of CPV device  200  in additional detail. 
     In one embodiment, a fabrication process for producing CPV device  200  begins by forming primary mirror  230  and secondary mirror  240  on optical element  210 . First, highly reflective (mirror) material layers  232  and  242  (e.g., silver) are deposited on convex surface  212  and concave surface  217 , respectively. The silver can be applied by various techniques including liquid silvering which is commonly used to produce mirrors on glass for architectural applications. The silver can also be applied by known sputtering techniques such as DC magnetron sputtering. 
     Next, anti-migration layers  234  and  244  (e.g., copper) are deposited over highly reflective material layers  232  and  242 , respectively. In liquid immersion or spray techniques, this process typically uses an electroless Cu process. In a sputter process, metals such as titanium or inconel are used to cap and protect the silver from tarnishing. Next, optional barrier paint layers  236  and  246  are formed over anti-migration layers  234  and  244  respectively. The barrier paint is typically applied by a spray coating process and then baked to both dry and harden the paint layer. 
     Next, an inner adhesive layer  260  (e.g., EVA adhesive produced by Dupont) is deposited onto barrier layer  236 , and a transparent adhesive  228  is deposited into cavity  213 . For example, the cavity  213  can be filled with the adhesive in its uncured state prior to the lamination process. Care should be exercised when applying inner adhesive  260  to ensure none of it enters cavity  213 . In an alternative embodiment, adhesive  260  is adhered to heat spreader  250  instead of optical element  210 . Adhesive layer  260  has a nominal thickness of approximately 100 microns. Additional details regarding lamination of the various layers of CPV device  200  are disclosed in co-owned and co-pending U.S. patent application Ser. No. ______, entitled “LAMINATED SOLAR CONCENTRATING PHOTOVOLTAIC DEVICE” [Atty Docket No. 20060351-US-NP (XCP-071)], which is co-filed with the present application and incorporated herewith by reference in its entirety. 
     Heat spreader  250  is produced and assembled with PV cell  220  prior to being mounted onto adhesive layer  260 . In accordance with another aspect of the present invention, heat spreader  250  is a multilayered substrate (referred to in the industry as “flex”) including one or more layers of a conductive layer  250 B (e.g., copper or other metal) faulted on a flexible substrate  250 A (e.g., a polyimide film such as Kapton® produced by DuPont Electronics, 0.5 mm thickness). Kapton flex that is suitable for the production of heat spreader  250  is available from 3M Corporation (St. Paul, Minn., USA). As shown in  FIG. 5 , heat spreader (flex)  250  is cut or otherwise patterned from a flat sheet to include a central portion  251  and multiple peripheral portions (radial arms)  252  that extend radially from central portion  251  and are separated by slits  254 . PV cell  220  will typically have a top (illuminated side) electrical contact and a bottom electrical contact. PV cell  220 , which is mounted on and in mechanical and electrical contact with heat spreader  250 , may have its top electrical contact electrically connected to a heat slug which is in turn electrically connected to one electrical portion of the flex. The bottom electrical contact is electrically connected to a second electrical portion of the flex. In one embodiment, where there are multiple electrical paths in the thermal conductive layer  250 B, both the base and emitter contacts of PV cell  220  are electrically connected to thermal conductive layer  250 B. In an array of power units, a portion of conductor layer  250 B may be used to carry current from PV cells  220  using series or parallel connections. The connections between PV cell  220  and thermal conductive layer  250 B may either be direct, or through an intermediate package or heat slug. In an alternative embodiment, the copper conductive layer may be replaced with another metal or alloy (e.g., Alloy 42 (Fe—Ni alloy) exhibits a better CTE match to optical element  210 , but is not as good of an electrical or thermal conductor. A further improvement is to form the heat spreader out of a bonded stack of metals, for example copper and Alloy 42. Such a structure has superior thermal expansion characteristics compared to copper without compromising electrical conductivity. 
     In accordance with another aspect of the present invention, heat spreader  250  is conformally attached to primary mirror  230  by way of adhesive layer  260  such that thermal conductive layer  250 B is in good mechanical and thermal contact with optical element  210 . Ordinarily, as indicated in  FIG. 5 , flex is processed in sheet or roll form, so it is inherently flat. By patterning peripheral portions  252 A and  252 B of heat spreader  250  in the manner shown in  FIG. 5 , both flexible substrate  250 A and thermal conductive layer  250 B conform to curved convex surface  212  when heat spreader  250  is mounted onto inner adhesive layer  260 , as illustrated in  FIGS. 3 and 6 , thereby facilitating contouring of heat spreader  250  to provide close thermal contact between thermal conductive layer  250 B and optical element  210 . Holes may be punched through peripheral portions  252  to facilitate the communication between adhesive layers  260  and  275 . 
     In alternative embodiments, heat spreader  250  may be implemented using stamped metal shim stock that is utilized to perform both heat transfer and electrical conduction functions. When multiple CPV devices of an array are parallel-wired, it may be feasible to make a stamped or formed part that includes the heat slug, spreader, and wiring, and has the emitter and base leads tied together outside the array so they can be trimmed and separated after lamination. The PV cells could slip into a “sandwich” which nests the cell from the front and makes contact to the back in a structure which goes through one solder reflow step to make both contacts. However, this arrangement might act like a guillotine and break cells when pressure is applied. An alternative embodiment is to form the heat slug, spreader and one side of the parallel wiring of an array of cells within the concentrator from a single stamped or formed metal part. The other side of the parallel wiring could be provided for example with a piece of flex. Additional details regarding the use of a heat slug and other packaging features are disclosed in co-owned and co-pending U.S. patent application Ser. No. ______, entitled “SOLAR CONCENTRATING PHOTOVOLTAIC DEVICE WITH RESILIENT CELL PACKAGE ASSEMBLY” [Atty Docket No. 20060466-US-NP (XCP-070)], which is co-filed with the present application and incorporated herewith by reference in its entirety. 
     In another alternative embodiment, a double-sided heat spreader arrangement that includes copper on both sides of Kapton substrate. This would make the structure more complex, but would eliminate a Kapton/EVA interface. 
     A protective plastic shell layer  270  (e.g., Tedlar® produced by DuPont with 150 micron thickness) is then secured onto the exposed surface of flexible substrate  250 A using an outer (e.g., EVA) adhesive layer  275 . Because Kapton is an inert material, suitable adherence to EVA may require surface preparation. For example, the surface may be prepared using a plasma treatment of the Kapton surface or a silane coupling agent applied to the Kapton prior to assembly. In one embodiment, the flex substrate may have a layer of EVA applied directly after this surface treatment before the components of the stack are assembled together for lamination. 
     CPV device  200  exploits the discovery that the thermal resistance of the flex conductive (e.g., copper) in the lateral direction is comparable to the thermal resistance of the optical element glass in the vertical direction. As a result of this for the proposed concentrator that has a glass thickness of 5 to 12 mm and a copper layer of 70 microns, neither part of the structure becomes a severe bottleneck for heat transfer from aperture surface  215 . Adequate heat spreading ensures that radiative and convective cooling occurs over wide surface areas on the front and back of CPV device  200 . This results in a more uniform surface temperature and a colder junction temperature for the PV cell. A thermal model of CPV device  200  during regular operating conditions for a cell with 35% electrical conversion efficiency in a 300° K ambient indicates the junction temperature rises less than 30° C. above ambient. In spite of the fact that this device concentrates the sun several hundred times and uses only passive cooling, the junction temperature of the cell rises only about 5° C. higher above the ambient than a conventional flat plate module collecting sunlight without any concentration. For the invention described herein, during normal operating conditions, the heat flow calculations predict that 67% or about two-thirds of the heat flowing out of the concentrator passes through the top surface. 
     Although the present invention has been described with respect to certain specific embodiments, it will be clear to those skilled in the art that the inventive features of the present invention are applicable to other embodiments as well, all of which are intended to fall within the scope of the present invention. For example, the primary and secondary mirrors may be preformed and then mounted to the optical element using a suitable adhesive, but this approach may substantially increase production costs. In yet another alternative embodiment, the curved surface utilized to form the secondary mirror may be convex instead of concave, thus being in the form of a classical Gregorian type system. In yet another alternative embodiment, the curved surfaces utilized to form the primary and secondary mirrors may be elliptical, ellipsoidal, spherical, or other curved shape.