Pottants Encapsulants

Most of the chapters in the third section are concerned with photovoltaic (PV) applications (conversion of light into electrical energy). Because of the diffuse nature of solar energy, the photovoltaic collection devices must be very large or else the light that strikes them must be concentrated. The first chapter in this section gives an overview of luminescent solar concentrators that can be used with the PV collectors. Most PV collectors or modules are multilayered systems containing a photovoltaic cell element. The next four chapters consider the use of various plastics as encapsulant or pottant materials in the PV modules. 

Other encapsulation applications of polymers for specific designs Include soil, ultraviolet, and abrasion-resistant front covers. The cover can serve as a transparent structural superstrata. Substrate support designs require a hard, durable front cover film to protect the relatively soft pottant from mechanical damages and excess soil accumulation. A polymeric front cover must be low In cost, highly transparent, and weather resistant to compete with glass. For applications out of the optical path between the sun and the solar cells (adhesives. Insulation, edge seals, gaskets) requirements for polymeric use In encapsulation are the same as for other applications. 

The chemical and mechanical stability of poly(n-butyl acrylate)(PnBA) to weathering, especially to solar radiation, is of interest for possible use of this material as an encapsulant/ pottant for silicon cell solar energy arrays. This application requires that the material retain an acceptable level of its desirable properties, such as transparency, elastic modulus, etc., over several years of exposure to intermittent moisture, temperatures ranging from -10 to 50 C, solar radiation, and other norms and extremes of exposure conditions. Knowledge of the dependence of changes in properties and composition of the material on exposure conditions is a requisite for establishing reasonable estimates of its prospective performance lifetime characteristics. 

Pottant. The central core of an encapsulation system Is the pottant, a transparent, polymeric material which Is the actual encapsulation media In a module. As there Is a significant difference between the thermal-expansion coefficients of polymeric materials and the silicon cells and metallic Interconnects stresses developed from the thousands of dally thermal cycles can result In fractured cells, broken Interconnects, or cracks and separations In the pottant material. To avoid these problems, the pottant material must not overstress the cell and Interconnects, and must Itself be resistant to fracture. From the results of a theoretical analysis ( ), experimental efforts O), and observations of the materials of choice used for pottants In commercial modules, the pottant must be a low-modulus, elastomeric material. 

Also, these materials must be transparent, processlble, com-merlcally available, and desirably of low cost. In many cases, the commercially available material Is not physically or chemically suitable for Immediate encapsulation use, and therefore must also be amenable to low-cost modification. The pottant materials... 

PnBA is not commercially available in a form suitable for use as an encapsulation pottant, but the n-butyl acrylate monomer is readily available at a bulk cost of about 0.45/lb. As a result of the developmental program, a 100%-pure PnBA liquid was developed that could be cast as a conventional liquid-casting resin, and that subsequently cures to a tough, temperature-stable elastomer. Modules fabricated with the PnBA elastomer have successfully passed module engineering tests. 

The analyses for structural adequacy Identified that the thermal expansion or wind deflection of photovoltaic modules can result In the development of mechanical stresses In the encapsulated solar cells sufficient to cause cell breakage. The thermal stresses are developed from differences In the thermal expansion properties of the load carrying panel, and the solar cells. However, the analysis Interestingly Identlfed that the solar cell stresses from either thermal expansion differences or wind deflection can be reduced by Increasing the thickness t of the pottant, or by using pottants with lower Young s Modulus E. 

Encapsulants are necessary for electrical Isolation of the photovoltaic circuit. They also provide mechanical protection for the solar cell wafers and corrosion protection for the metal contacts and circuit interconnect system over the 20-year design life of a photovoltaic array. The required components Include the solar cell circuit, the rigid or structural member, the pottant, and the outer cover/insulator. Surface modifications may be needed to develop strong, stable bonds at the Interfaces in the composite. If the module is to be framed, edge sealants may also be required. The functions of the Individual components and the performance requirements as they are now known are described. Costs are ccmipared where possible and candidate materials identified. 

Uses Optical fibers, pottants and encapsulants, conversion coatings, pressure-sensitive adhesives... 

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