Pottants

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. 

These data can serve as a preliminary basis for evaluation of PnBA as a pottant material with prediction of structure-property relationships for extended exposure conditions. On the basis that one hour exposure in the QUV corresponds to one day exposure in practice, the present study is predictive of a ten year period of use. 

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... 

In a fabricated module, the pottant provides three critical functions for module life and reliability ... 

For expected temperature levels In operating modules, = 60°C In a rack-mounted array and possibly up to 80°C on a rooftop, three generic classes of transparent polymers are generally resistant to the above weathering actions silicones, fluorocarbons, and PMMA acrylics. Of these three, only silicones, which are expensive, have been available as low-modulus elastomers suitable for pottant application. 

The situation Is different for a substrate module however, which will employ a weatherable plastic-film front cover. Because all plastic films are permeable to oxygen and water vapor (the only difference Is permeation rate), the pottant is exposed to oxygen and water vapor, and also to UV If the plastic film is non-UV screening. Because Isolation of the pottant from oxygen and water vapor is not practically possible in this design option. It becomes a requirement that the pottant be intrinsically resistant to hydrolysis and thermal oxidation, but sensitivity to UV is... 

This EVA pottant has undergone extensive Industrial evaluation, and manufacturers of photovoltaic (PV) modules have reported certain advantages of EVA when compared to polyvinyl butyral (PVB), a laminating film material In common use within the PV module industry. The reported advantages are ... 

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. 

In addition to weatherablllty, the front cover must also function as a UV screen, to protect underlying pottants that are sensitive to degradation by UV photooxldatlon or UV photolysis. 

Back covers function to provide necessary back side protection for substrates, such as for example corrosion protection for low-cost mild steel panels, or humidity barriers for moisture sensitive panels. For superstrate designs, the back covers provides a tough overlay on the back surface of the soft, elastomeric pottant. If the back cover for a superstrate design is selected to be a metal foil, an additional Insulating dielectric film should be Inserted In the module assembly between the cells and the metal foil, as shown In Figure 1. Candidate back cover films are listed In Table... 

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. 

In other words, the analysis Indicates that the load carrying panel can be considered to be the generator of stress, and that the pottant acts to dampen the transmission of the stress to the cells. The pottants ability to dampen transmitted stress Is directly related to the ratio of Its thickness to modulus, t/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. 

Thin Film Cells. Future, lower cost solar cell materials will likely be more flexible than crystalline silicon and therefore may not require a rigid member In the module lay up. They will still need electrical Isolation and protection fradhesive layers as well as outer covers/insulators. 

The pottant Is the soft, elastomeric, vibration-damping material that Immediately surrounds both sides of fragile solar cell wafers and their electrical contacts and Interconnects. It protects the cells from stresses due to thermal expansion differences and external Impact. It Isolates them electrically and helps protect their metallic contacts and Interconnects from corrosion. 

Mechanical Requirements. The pottant material should have a relatively low modulus (< 2000 psl at 25°C). The maximum tolerable modulus depends on the difference In expansion coefficients of the cells and the rigid member and on the thickness of the layer between them. Relatively high modulus rubbers could be used but would require inordinately thick and thus expensive layers to damp out the expansion differences. For example, with an 1/8-in.-thick glass superstrata and silicon cells, which will take 5000 psl maximum linear stress, a pottant of 1000 psl modulus needs to be a minimum of only 1.5 mils each side for a 1 1 safety factor in service with a 2500 psl modulus the minimum is - 3.5 mils, etc. A safety factor higher than 1 1 Is highly desirable. 

In a vacuum lamination process, the steeper the melt viscosity/temperature curve for sheet pottant material, the better. The layers need to be dry and non-tacky during the initial evacuation step so as not to trap air between them. At the same time, the pottant must then melt to as fluid a state as possible in order to effectively penetrate and wet all the irregularities of the cell circuit. 

Electrical Reoulrements. The pottant should contain no plasticizer because plasticizer can reduce the volume resistivity of a polymer drastically. It reduces the resistivity of PVB by 5 orders of magnitude In some formulations. PVB with 40 dlester plasticizer measures only 10 ohm-cm In laminated form at room temperature whereas It measures 10 ohm-cm with the plasticizer driven out. Volume resistivities of 10 2 ohm-cm or less will conduct small amounts of current fairly readily, albeit slowly. (For example, a resolved 5 line palr/mm charge Image has been observed to blur within the first few seconds when placed on the surface of a film or Immersed In a llould of lO I-IO ohm-cm resistivity. The same Image on or In 101 ohm-cm material will not blur for several hours. On a ohm-cm material an Image will last unblurred from weeks to months.)... 

The volume resistivity of an unplastlclzed pottant material such as EVA is 10 ohm-cm. The module current leakage at 1.5 kV with EVA is an order of magnitude lower than with plasticized PVB at 25-30°C and there appears to be no rise in leakage at 50-60°C. (See Figure H and Table I.) Similarly, the current leakage of modules containing plasticized PVB can be blocked by the Insertion of an additional high volume resistivity layer such as polyethylene terephthalate film as discussed below, which is resistant to the solvation effect of dlester plasticizers. 

Chemical Requirements. The pottant must be stable that is, chemically resistant to oxidation and hydrolysis unless protected in a hermetic package, to reduction by metals, and to outgassing of dissolved gases or liquids or decomposition products under normal operating conditions of -itO°C to +90°C for 20 years. The need for chemical stability is especially stringent when a lower cost non-hermetic design is used. Even when a hermetic package is... 

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