Abstract:
Disclosed is a method of making solar collector assemblies for photovoltaic conversion. The method comprises providing a mold for receiving encapsulant, the mold having serially arranged, alternating peaks and valleys. A respective PV solar cell is placed into each of a series of the valleys such that the light-receiving surfaces of the PV solar cells face upwards. Uncured encapsulant is delivered into the mold and onto the light-receiving surfaces, and from the light-receiving surfaces to a level at least as high as the peaks so as to form, above the light-receiving surfaces, optical concentrators for concentrating light received by the optical concentrators and directing the light to the light-receiving surfaces. The encapsulant is then cured.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of making solar collector assemblies with optical concentrator encapsulant on photovoltaic cells, and also relates to the resulting solar collector assemblies. 
     BACKGROUND OF THE INVENTION 
     The use of photovoltaic solar cells (PV solar cells) to collect solar energy and convert that energy to electricity is widely recognized. The PV solar cells are typically manufactured with impervious encapsulating layers to provide protection from environmental factors that may reduce the effectiveness of the solar cell in converting light to electricity. Such encapsulating layers may be formed from one or more of a polymer such as an acrylate-based polymer, ethylene vinyl acetate and TEDLAR-brand polyvinyl fluoride. Such encapsulating layers are used to limit the ingress of water, oxygen and other materials that may react with the cell material and cause degradation. The encapsulating layers are typically thin and provide protection only to the active PV solar cell. 
     It is also recognized that PV solar cells can operate more efficiently if there is a degree of concentration of sunlight on the PV solar cell. Concentration is typically achieved with a lens or other collection device that collects light over a wider area than is occupied by the active area of the PV solar cell. The collection devices are typically manufactured from glass or from a durable plastic and are placed over the encapsulated solar cell. The provision of the collection-and-concentration optics typically occurs as a separate step from the step of encapsulating PV solar cells, and therefore adds additional significant component of cost to solar collector assemblies. 
     Accordingly, it would be desirable to provide a streamlined, and hence more economical, method of making solar collector assemblies with encapsulated PV solar cells and an optical concentrator. 
     BRIEF SUMMARY OF THE INVENTION 
     A preferred form of the invention provides a method of making solar collector assemblies for photovoltaic conversion. The method comprises providing a mold for receiving encapsulant, the mold having serially arranged, alternating peaks and valleys. A respective PV solar cell is placed into each of a series of the valleys such that the light-receiving surfaces of the PV solar cells face upwards. Uncured encapsulant is delivered into the mold and onto the light-receiving surfaces, and from the light-receiving surfaces to a level at least as high as the peaks so as to form, above the light-receiving surfaces, optical concentrators for concentrating light received by the optical concentrators and directing the light to the light-receiving surfaces. The encapsulant is then cured. 
     Beneficially, the foregoing method provides a streamlined, and in comparison with the above-cited prior art, a more economical, method of making solar collector assemblies with encapsulated PV solar cells and an optical concentrator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following drawings, like reference numbers refer to like parts: 
         FIGS. 1A and 1B  are schematic diagrams, intended to be viewed together, of sequential method steps for encapsulating light-receiving surfaces of PV solar cells in accordance with a preferred embodiment of the invention. 
         FIG. 1C  is a schematic diagram of part of a mold and associated PV solar cell used in the method of  FIGS. 1A and 1B . 
         FIG. 1D  is a schematic diagram of a part of a mold, associated PV solar cell and encapsulant, which may be used in the method  FIGS. 1A and 1B . 
         FIG. 2  is a side view of photovoltaic cell that may be used in the present invention. 
         FIGS. 3 and 4  show upper perspective views of the mold and PV solar cell structure contained in the circle marked  FIGS. 3 ,  4  in  FIG. 1 . 
         FIGS. 5A-5C  are similar to  FIGS. 2 and 3 , but show in the foreground respective dams for containing uncured encapsulant during processing according to  FIGS. 1A and 1B . 
         FIGS. 6A and 6B  are side views of a mold for receiving PV solar Cells in different states of configuration, in accordance with an aspect of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIGS. 1A and 1B  show various steps of a preferred method of making solar collector assemblies for photovoltaic conversion, with a PV encapsulant forming an optical concentrator. By “light-receiving surface” of a PV solar cell is meant the surface of a PV solar cell that receives light for the purposes of conversion into electricity. 
     In  FIG. 1A , step  10  shows providing a mold  12  having peaks  12   a  and valleys  12   b .  FIG. 1C  shows that a “peak”  12   a  is meant to include the vertical section of the mold shown within a bracket, which starts from the top of a PV solar cell  16  when inserted into the mold. On the other hand, as best shown in  FIG. 1C , the valleys  12   b  are for receiving PV solar cells  16 . This is shown in step  14 , wherein a PV solar cell  16  is placed in a valley  12   b  in the mold  12 . Mold  12  may be composed of a silicone-type material or another material such as polyethylene or polyethylene terephthalate with flexibility and adequate rigidity for the purposes set forth herein. 
     In  FIGS. 1A and 1B , arrows  18  show movement of the mold  12  or other structures to the right, as may occur in an assembly line process. 
     Step  14  shows other PV solar cells  16  that have previously been placed into respective valleys in the mold  12 . Following step  14 , step  20  shows the delivery of uncured encapsulant from a source  24 , shown in simplified schematic form. An actual source  24  for encapsulant  22  in an uncured state would typically employ a storage tank, a delivery pipe and a controlled valve (not shown). 
     Uncured encapsulant is delivered to the upwardly facing light-receiving surfaces of PV solar cells  16 , and extends from the light-receiving surfaces to a level at least as high as the tops of peaks  12   a  of the mold  12 , and preferably above the level of the tops of peaks  12   a.    
     Following step  20 , the uncured encapsulant in the mold  12  is cured as shown in curing step  28 . In this step, an ultraviolet (UV) source  30  subjects to UV radiation encapsulant  22 , which reaches the location for curing step  28  in an uncured state. 
     Alternatively, other techniques for curing the encapsulant can be used, such as using two-part epoxy materials that react after being mixed together and cause cross-linking. Other curing techniques employ heat to cause cross-linking, and some polymer systems require the addition of water to cause cross-linking. These other techniques will be obvious and routine to those of ordinary skill in the art. 
     Preferred Uncured Encapsulant Composition 
     The encapsulant  22 , when cured, should have the properties of being highly transparent and durable. It is preferred that uncured encapsulant  22  comprise a crosslinkable polymer that has not yet been fully crosslinked, but which is still has sufficient fluidity that it can be molded as described herein. It is further preferred that the uncured encapsulant  22  contain at least one cross linking agent that becomes activated by UV radiation. The encapsulant is preferably at least 90 percent by weight polymeric material. 
     Examples of preferred compositions for the uncured encapsulant are described in U.S. Pat. Nos. 5,406,641 and 5,485,541, which are assigned to the present assignee. 
     Further, the following composition(s) are currently preferred for use as the uncured encapsulant:
         0.2% based on monomer weight of IRGACURE-brand 184 and a monomer mixture of 99.9% butyl methacrylate and 0.1% diethyleneglycol dimethacrylate.   0.2% based on monomer weight of IRGACURE-brand 184 and a monomer mixture of 49.95% butylmethacrylate and 49.95% 2-ethylhexyl methacrylate with 0.1% diethyleneglycol dimethacrylate.
 
The composition of IRGACURE-brand 184 is 1-Hydroxy-cyclohexyl-phenyl-ketone. IRGACURE-brand 184 is manufactured by Ciba Specialty Chemicals, which is now part of BASF headquartered in Ludwigshafen am Rhein, Germany.
       

     Applying Coverglass and Removing Mold 
     Preferably, during encapsulant curing step  28 , a cover-plate  36  is applied to the top of encapsulant  22 , so that the cover-plate seals the encapsulant from rain or other aspects of the environment that could damage the encapsulant. Cover-plate  36  may be a glass such as borosilicate or soda lime, or other material such as plastic having the ability to seal the encapsulant from rain or other aspects of the environment that could damage the encapsulant. Cover-plate  26  and any optional coating on cover-plate  36  may be impervious to UV radiation. If cover-plate  36  is impervious to UV radiation, it should be applied to the encapsulant as illustrated in step  28  | FIG. 1B ) late in the curing step  28 , or used with a UV-activated cross-linking agent in the encapsulant tuned to a different wavelength than the wavelengths blocked by the cover-plate  36 . For instance, if the UV-activated cross-linking agents are tuned to 380 nm and the cover-plate  36  blocks UV below 350 nm, then the cover-plate will not stop the curing of encapsulant underneath the cover-plate. 
     After applying coverglass  36  to encapsulant  22  and at a time when the encapsulant is sufficiently cured (e.g., cross-linked) so that that the encapsulant does not lose its molded shape, a mold-removal step  38  may follow. In the mold-removal step, the mold  12  is removed from the underneath of the integrated structure formed by coverglass  36 , encapsulant  12  and PV solar cells  16 . This would expose an electrode on the bottom of each PV solar cell  16 , which can be best seen in  FIG. 2  as a planar electrode  16   a  of a PV solar cell  16 . 
     The PV solar cell  16  of  FIG. 2  has semiconductor layers  16   b ,  16   c  and  16   d , an optional antireflective coating  16   e , and upper electrode grid  16   f . Layers  16   b ,  16   c  and  16   d  may comprise, for example, Silicon (Si), Gallium Arsenide Phosphide (GaAsP) and Gallium Indium Phosphide (GaInP), respectively, in a three-junction cell. PV solar cell  16  may have fewer semiconductor layers, such as Gallium Arsenide Phosphide (GaAsP) and Silicon (Si) in a two-junction cell, and other constructions of PV solar cells will be obvious and routine to those of ordinary skill in the art. Antireflective coating  16   e  preferably comprises rutile titania or other coatings made of nanostructured low-index materials or a one-sided low index fluoropolymer for durability and optical performance. 
     Mold-removal step  38  would be omitted if mold  12  is not intended to be removed from the PV solar cells  16  and encapsulant  22 . However, the respective portions of a non-removable mold  12  that underlie PV solar cells  16  would typically need suitable openings (not shown) to allow access to the lower electrode  16   a  of the PV solar cell  16 , shown in  FIG. 2 . 
     After the optional mold-removal step  38 , the PV solar cells  16 , encapsulant  22  and coverglass  36  may be mounted on a circuit board  40  to allow electrical connection from the lower electrode  16   a  of PV solar cell  16  ( FIG. 2 ) to a printed circuit (not shown) on the board  40 . Circuit board  40  also provides mechanical support to the PV solar cells  16 , encapsulant  22  and coverglass  36 . 
     One- and Two-Dimensional Arrays of PV solar cells 
       FIG. 3  shows a one-dimensional array of PV solar cells  17   a  in a mold  13   a , whereas  FIG. 4  shows a two-dimensional array of PV solar cells  17   b  in a mold  13   b . Different reference numbers for PV solar cells and the molds are used compared with those used in the dashed-line circle in  FIG. 1A  marked  FIGS. 3 ,  4 , to emphasize the different possible geometries of the PV solar cells and molds in  FIGS. 3 and 4 . In particular, the PV solar cells  17   a  in  FIG. 3  have elongated, rectangular shapes, whereas the PV solar cells  17   b  in  FIG. 4  are preferably square. 
       FIGS. 3 and 4  also show curved shapes to peaks  13   a  and  13   b , respectively, keeping in mind that “peaks” are defined with reference to  FIG. 1C , described above. The curved shapes of peaks  13   a  and  13   b  are merely diagrammatic, and the actual shapes are preferably optimized so that the encapsulant placed in the mold will form non-imaging concentrators of light. With reference to  FIG. 1D , one of the useful attributes of a non-imaging concentrator is a so-called angle-to-area conversion of light, whereby high angle light at the inlet areas of each light concentrator  46  is “converted” to smaller angle light at the smaller end-faces of each concentrator, which ends at the top of a PV solar cell  16 . Smaller angle light at the tops of the PV solar cells  16  may more easily pass into the PV solar cells and be converted to electricity. Preferably, each light concentrator  46  is shaped so as to obtain at least about 80 percent of the photons that would be obtained from an ideally shaped non-imaging concentrator having the same input area (top of light concentrator  46  in  FIG. 2 ) and the same output area (bottom of light concentrator in  FIG. 2 ). 
     With reference to  FIG. 1D , preferably each concentrator can concentrate light by a factor of between about 7 and 12. Concentration of at least about 2 to 3 is desired. 
       FIG. 5A  shows an integrated dam  48  on the foreground side of mold  12 , which prevents uncured encapsulant from leaking from the mold. Naturally, the background side of the mold (not shown) would have a similar dam. 
       FIG. 5B  shows a re-usuable dam  50 , which can be mechanically pressed against the foreground side of mold  12 . At some point during the curing process of encapsulant  22 , the encapsulant will develop some adhesive properties so that it can then hold re-usable dam  50  by itself, or with a lesser amount of mechanical pressure than when the encapsulant is initially delivered into the mold. A similar re-usable dam (not shown) could be used for the background side of the mold (not shown). 
       FIG. 5C  shows a mold  12  with peaks  13   c  for a two-dimensional array of PV solar cells  16 . For the mold shown, a short dam  52  could be used to make sure that the encapsulant ( 22 ,  FIGS. 1A and 1B ) can rise to above the level of peaks  13   c , if desired. 
     An optional aspect of the present method is illustrated in  FIGS. 6A and 6B . Thus, a mold  12  of normal shape in  FIG. 6A  can be folded in accordion style, by pressing upwardly as indicated by arrows  54 , along fold lines at the location of the arrows. The resulting, reduced-size mold  12  is shown in  FIG. 6B . This allows for reduction in the volume necessary for storing mold to be used in step  14  of  FIG. 1A , when PV solar cells  16  are inserted into the valleys  12   b  of the mold. 
     While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true scope and spirit of the invention.