Batteries: thermoelectric and photoelectric – Photoelectric – Panel or array
Reexamination Certificate
2002-06-12
2003-12-09
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Panel or array
C136S244000, C257S433000, C257S788000, C257S789000, C257S790000, C438S066000, C438S080000, C438S064000, C438S067000, C427S074000
Reexamination Certificate
active
06660930
ABSTRACT:
This invention was developed under DOE Subcontract No. ZAX-8-17647-10.
FIELD OF THE INVENTION
This invention relates to the manufacture of photovoltaic solar cell modules and more particularly to provision of solar cell modules having an improved backskin.
BACKGROUND OF THE INVENTION
A common form of solar cell module is made by interconnecting individually formed and separate solar cells, e.g., crystalline silicon solar cell, and then mechanically supporting and protecting the cells against environmental degradation by integrating the cells into a laminated solar cell module. The laminated modules usually comprise a stiff transparent protective front panel or sheet, and a rear panel or sheet typically called a “backskin”. Disposed between the front and back sheets so as to form a sandwich arrangement are the interconnected solar cells and an encapsulant. A necessary requirement of the encapsulant (or at least that portion thereof that extends between the front sides of the cells and the transparent front panel) is that it be transparent to solar radiation. The typical mode of forming the laminated module is to assemble a sandwich comprising in order a transparent panel, e.g., a front panel made of glass or a transparent polymer, a front layer of at least one sheet of encapsulant, an array of solar cells interconnected by electrical conductors (with the front sides of the cells facing the transparent panel), a back layer of at least one sheet of encapsulant, a sheet of scrim to facilitate gas removal during the lamination process, and a backskin or back panel, and then bonding those components together under heat and pressure using a vacuum-type laminator. The back layer of encapsulant may be transparent or any other color, and prior art modules have been formed using a backskin consisting of a thermoplastic polymer, glass or some other material.
Although the lamination process seals the several layered components together throughout the full expanse of the module, it is common practice to apply a protective polymeric edge sealant to the module so as to assure that moisture will not penetrate the edge portion of the module. The polymeric edge sealant may be in the form of a strip of tape or a caulking-type compound. Another common practice is to provide the module with a perimeter frame, usually made of a metal like aluminum, to provide mechanical edge protection. The foregoing prior art techniques are disclosed or suggested in U.S. Pat. No. 5,741,370, issued Apr. 21, 1998 to Jack I. Hanoka for “Solar Cell Modules With Improved Backskin And Methods For Forming Same”. That patent also discloses the concept of eliminating the back layer of encapsulant and bonding a thermoplastic backskin directly to the interconnected solar cells.
Heretofore a large number of materials have been used or considered for use as the encapsulant in modules made up of individual silicon solar cells. Until at least about 1995, ethylene vinyl acetate copolymrer (commonly known as “EVA”) was considered the best encapsulant for modules comprising crystalline silicon solar cells. However, EVA has certain limitations: (1) it decomposes under sunlight, with the result that it discolors and gets progressively darker, and (2) its decomposition releases acetic acid which in turn promotes further degradation, particularly in the presence of oxygen and/or heat.
U.S. Pat. No. 5,478,402, issued Dec. 20, 1995 to J. Hanoka, discloses use of an ionomer as a cell encapsulant substitute for EVA. Relevant information contained in that patent is incorporated herein by reference. The use of ionomer as an encapsulant is further disclosed in U.S. Pat. No. 5,741,370, supra. The term “ionomer” and the type of resins identified thereby are well known in the art, as evidenced by Richard W. Rees, “Ionic Bonding In Thermoplastic Resins”, DuPont Innovation, 1971,2(2), pp. 1-4, and Richard W. Rees, “Physical Properties And Structural Features Of Surlyn® lonomer Resins”, Polyelectrolytes, 1976, C, 177-197. Ionomers may be formed by partial neutralization of ethylene-methacrylic acid copolymers or ethylene-acrylic acid copolymers with organic bases having cations of elements from Groups I, II, or III of the Periodic Table, notably, sodium, zinc, aluminum, lithium, magnesium and barium. Surlyn® ionomers have been identified as copolymers of ethylene and methacrylic acid that typically have a melting point in the range of 83°-95° C.
Although it is known to use a rear panel or backskin that is made of the same material as the front panel, a preferred and common practice is to make it of a different material, preferably a material that weights substantially less than glass. e.g., a material such as TEDLAR® (the trade name for a polyvinyl fluoride polymer made by E.I. DuPont de Nemeurs Co.). Heretofore a popular and widely used backskin material has been a TEDLAR/polyester/ethylene vinyl acetate laminate. However, TEDLAR and TEDLAR laminates are not totally impervious to moisture, and as a consequence over time the power output and/or the useful life of modules made with this kind of backskin material is reduced due to electrical shorting resulting from absorbed moisture.
U.S. Pat. No. 5,741,370, supra, suggests that manufacturing and module mounting costs could be reduced by using as the backskin material a thermoplastic olefin comprising a combination of two different ionomers, e.g., a sodium ionomer and a zinc second ionomer, with that combination being described as producing a synergistic effect which improves the water vapor barrier property of the backskin material over and above the barrier property of either of the individual ionomer components. Significantly the patent discloses shows use of an ionomer encapsulant with the dual ionomer backskin.
However, the water absorption characteristics of all sodium ionomers are not identical. The same is true, but to a lesser extent, of zinc ionomers. More importantly, the water absorption characteristics of zinc ionomers tend to be substantially less, than those of sodium ionomers. Significantly modules made using sodium based ionomers as encapsulants, as taught by U.S. Pat. No. 5,478,402, tend to degrade with time, often in a matter of months, with portions of the encapsulant changing color. This discoloration, which occurs at multiple points along the length and breadth of the module, reduces the ionomer's light transmissibility, thereby lowering the module's energy conversion efficiency and power output, as well as rendering it less appealing from an aesthetic viewpoint. Degradation of the ionomer destroys the electrical insulation resistance, causing electrolytic corrosion plus a loss of required safety provisions. Such sodium ionomer degradation is known to result from moisture absorption, and also from another cause related to the solar cell interconnections.
As disclosed in copending U.S. patent application Ser. No. 10/35,107, pending filed Dec. 27, 2001 by R. C. Gonsiorawski for “Encapsulated Photovoltaic Modules And Method Of Manufacturing Same”, the teachings of which are incorporated herein by reference, sodium ionomer degradation may be induced by solder flux residues. It should be noted that the conductors (tabbing) used to interconnect individual solar cells are secured in place by a solder. The solder may be applied separately or the conductors may be pre-tined, i.e., provided with a solder layer, to facilitate soldering. In both cases, the solder or soldering process includes a flux composition for removing oxide films and ensuring adequate wetting of surfaces. Commonly it is preferred to use pre-tinned conductors which also have been coated with a suitable flux composition. The fluxes commonly comprise an inorganic or organic acid or acid salt, e.g., a carboxylate or a benzoate compound such as the one sold under the name “Pentoate”. Although the flux compositions are designed to be eliminated by vaporization directly or via decomposition during the soldering process, in practice some flux residue may continue to exist at the soldered connection points in contact
Diamond Alan
Pandiscio & Pandiscio
RWE Schott Solar Inc.
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