Photovoltaic Module Reliability. John H. Wohlgemuth

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СКАЧАТЬ of the maximum amount of light energy possible into the solar cells (at all angles at the wavelengths that the cells can utilize).

       Cooling of the cells to minimize their temperature increase.

There are really just a few types of module constructions that make up the vast majority of commercial PV modules. Table 1.2 provides a list of the types of typical commercial module constructions. A vast majority of PV modules use glass as the front surface because of its excellent optical properties and as we will see in Chapter 2 as the main structural support because of the low thermal expansion coefficient of glass.

The first construction for cry‐Si modules (Glass/encapsulant/c‐Si cells/encapsulant/backsheet) has certainly been used on more modules than any other and still remains the most popular in the industry. Figure 1.3a shows a cross‐sectional drawing of a Glass/encapsulant/cry‐Si cells/encapsulant/backsheet module construction. The second construction for c‐Si modules substitutes a second glass layer for the standard backsheet as shown in cross‐section in Figure 1.3b. This type of construction is becoming more popular especially for use in bifacial designs (modules that produce electricity from light that falls on both sides, not just the front). The third construction for c‐Si modules is a flexible design. This is shown in Figure 1.3c. Flexible modules are usually designed as portable power supplies to be carried and deployed when needed. They are not designed for continuous outdoor exposure.

Thin film cells are deposited onto a foreign substrate. These substrates can be glass where the cells are deposited right side up or upside down depending on the technology of the particular thin film material being used. Figures 1.4a and 1.4b show the cross section of these two types of module constructions. In Figure 1.4a, the thin film is deposited on the backside of the front glass. This is typical of how CdTe and a‐Si modules are fabricated. Figure 1.4a has been drawn with edge seals as this is typically how CdTe modules are fabricated today. The edge seals are designed to keep moisture from reaching the active cell area for the lifetime of the product (typically warrantied by the manufacturer for 25 years). In Figure 1.4b, the thin film is deposited on the front side of the back glass. This is typical of how CIS and CIGS modules are fabricated. Figure 1.4b has also been drawn with edge seals, but edge seals are not as prevalent in these types of modules. In this case, the superstrate can also be made of glass though other materials are often used.

Figure 1.3a Cross‐sectional drawing of glass/encapsulant/cry‐Si cells/encapsulant/backsheet module.

Figure 1.3b Cross‐sectional drawing of glass/encapsulant/cry‐Si cells/encapsulant/glass module.

Figure 1.3c Cross‐sectional drawing of flexible cry‐Si module.

In some cases, thin films are deposited in large areas and then cut to cell size afterwards. Basically, creating wafers out of thin films which then have to be electrically connected in series just like cry‐Si cells. Figure 1.4c shows the typical construction used for such thin film modules, although any of the constructions used for cry‐Si wafers could also be used to package these thin film “wafers.” Some CIGS modules have been made with these types of cells. They are particularly of interest for fabrication of large‐area flexible modules.

Figure 1.4a Cross‐sectional drawing of front glass/thin film cells/encapsulant/back glass modules.

Figure 1.4b Cross‐sectional drawing of front glass/encapsulant/thin film cells/substrate modules.

Figure 1.4c Cross‐sectional drawing of module structures for thin film wafer like cells.

      There are only a handful of materials that appear in these drawing so let's briefly take a look at the properties required and those typically selected for use in PV modules.

      Glass: When glass is used as the superstrate, one of the properties of interest is the optical transmittance over the wavelength range, that solar cells can effectively use the photons, from about 300 nm to 1100 nm for cry‐Si for example. To maximize performance without significantly increasing the cost, most cry‐Si modules and some thin‐film modules are built using low iron glass which has better transmittance than the standard soda lime (window) glass. Some thin‐film PV modules do use regular soda lime glass to keep the cost down. In addition, most cry‐Si modules use tempered or heat‐strengthened glass to provide added strength to withstand wind and snow loads as well as hail impact. Some thin‐film modules can't use heat‐strengthened glass because the thin‐film deposition process occurs at such a high temperature that the heat strengthening would be removed from the glass. In this case, the modules are usually built with double glass (glass on front and back) to provide the strength necessary to survive in the field.

      Encapsulant: The encapsulant is the material that surrounds the cells and the “glue” that holds the whole package together. The encapsulant should provide good adhesion to all of the other components within the module so that everything in the package stays stuck together for 25 or 30 years. This is usually assisted by addition of a primer into the encapsulant formulation itself. Of course, any of the encapsulant material that is used in front of active solar cells must be optically transparent and resistant to UV exposure. Since the СКАЧАТЬ