Читать книгу Photovoltaic Module Reliability - John H. Wohlgemuth - Страница 13

While the solar cells actually produce the electricity, the module package is important for the continued operation of the solar cells. Often the costs associated with the packaging exceed the costs of the cells themselves. Historically, it is usually the package that fails first, ultimately leading to degradation of the cells, conductors, connectors and diodes resulting in failed or degraded modules. The PV module package provides for the following functions:

 Mechanical support – holding the cells in place pointing toward the sun.

 Dielectric protection – keeping the high voltage away from people and keeping current from flowing out of the array circuit (to ground or in a loop) where it has the potential to cause a fire.Table 1.2 Typical commercial module constructions.Glass Superstrate: Cry‐Si CellsGlass/encapsulant/cry‐Si cells/encapsulant/backsheetGlass/encapsulant/cry‐Si cells/encapsulant/glassGlass Superstrate: Thin Film CellsGlass/thin film cells on front glass/encapsulant/glass with edge sealGlass/thin film cells on front glass/encapsulant/backsheetGlass/encapsulant/thin film cells on back glass/substrateFlexible substratesTransparent frontsheet/encapsulant/thin film cells/flexible substrateTransparent frontsheet/encapsulant/cry‐Si cells/encapsulant/flexible substrate

 Protection of the cells, diodes and interconnects from the weather (UV, rain, humidity, hail, etc.)

 Coupling 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 encapsulant surrounds the solar cells, it helps to provide electrical isolation. So, materials used as encapsulants must have low‐bulk conductivity (or high‐bulk resistivity) to minimize flow of leakage currents. Some encapsulants are cross‐linked during module lamination to provide stability at the high temperatures at which they operate. Others, however, do not have to be cross‐linked since they are stable enough not to flow or creep at typical module operating temperatures. Typical examples of materials used as encapsulants are listed below:

 Ethylene vinyl acetate (EVA) has been used in more modules than any other encapsulant as it is reasonably priced and readily available as a formulated film for PV with primers, cross‐link agents and UV stabilizers incorporated into the film itself.

 Silicones were used in the early days of PV and worked well in the field but were abandoned due to their high costs and because liquid encapsulants were more difficult to use in manufacturing.

 Polyolefins are similar to EVA and are also available as formulated sheets. They have become more popular as a replacement for EVA in recent years.

 Ionomers were used in the past, especially by Mobil Solar and ASE Americas. They were typically used in glass/glass constructions but the particular formulation used had problems with delamination in the field probably due to less than ideal adhesion to the glass.

Backsheets: As the name implies, the backsheet is the outside material on the back or non‐sun side of the PV module. The functions of the backsheet include:

 Protecting the rest of the module from the weather – rain, snow, hail, etc.

 Screening the materials inside it from UV.

 Providing protection from the high voltage within the module, so backsheets must have high resistance and high dielectric strength.

 Providing protection from accidental exposure to the active components within the module. So, the backsheet must have high‐tensile strength and be scratch resistant.

 The backsheet is the first line of defense in case the module catches fire so its properties will impact the fire rating that a module can obtain.

 The backsheet must provide for secure bonding of junction boxes, connectors, frames and/or mounting rails.

Backsheets are usually comprised of multi‐layers of material as the different layers provide different functions. Often one of the layers (usually the center layer) is a poly(ethylene terephthalate) (PET) or polyester to provide the dielectric isolation and high resistance. The outer layer has to provide UV resistance to the layers inside and is most often composed of a fluoropolymer. The inner most layer must bond well to the encapsulant. One typical example of a multilayer backsheet is Tedlar/polyester/Tedlar.

Edge Seals: Modules that are constructed with impermeable (or extremely low permeability) front and backsheets designed to protect moisture‐sensitive PV materials, may suffer from moisture ingress from the sides. Edge Seal materials are low‐diffusivity materials that are placed around the edges of a module between the impermeable front and backsheets to prevent moisture ingress. Edge seals were borrowed from the insulated glass industry where they are used to keep moisture from penetrating between the two panes of glass. In addition to restricting moisture ingress, edge seal materials must have high electrical resistivity to provide electrical insulation as frontsheets and backsheets do. To continue to perform these functions for the lifetime of the module, edge seal materials must remain well adhered to the front and back sheets of glass. Edge seal materials are usually made of Polyisobutylene and filled with desiccants to keep moisture from penetrating throughout the useful life of the module.

Frontsheets: Frontsheets must meet all of the same requirements as backsheets, with the additional requirement of having high optical transmittance over the wavelength range that solar cells are effective, for example from about 300 nm to 1100 nm for cry‐Si. Glass is used the most as a frontsheet, but some modules are made using fluoropolymer frontsheets, particularly Tefzel or Ethylene tetrafluoroethylene (ETFE).