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frame and glass on the front.]] In the field of Photovoltaics , a photovoltaic module is a packaged interconnected assembly of photovoltaic cells, also known as Solar Cell s. An installation of photovoltaic modules or panels is known as a Photovoltaic Array . Photovoltaic cells typically require protection from the environment. For cost and practicality reasons a number of cells are connected electrically and packaged in a photovoltaic module, while a collection of these modules that are mechanically fastened together, wired, and designed to be a field-installable unit, usually with a Glass covering and a frame and backing made of Metal , Plastic or Fiberglass , are known as a '''photovoltaic panel''' or simply '''solar panel'''. A photovoltaic installation typically includes an array of photovoltaic modules or panels, an Inverter , Batteries (for off grid) and interconnected wiring. THEORY AND CONSTRUCTION Crystalline Silicon and Gallium Arsenide are typical choices of Material s for solar cells. Gallium arsenide crystals are grown especially for photovoltaic use, while silicon crystals are available in less-expensive standard Ingot s. These ingots are produced mainly for consumption in the Microelectronics industry. Polycrystalline Silicon has lower conversion efficiency but also lower cost. During the manufacturing process, crystalline silicon ingots are sliced into ed onto a substrate (either a rigid panel or a flexible blanket). Electrical connections are made in series to achieve a desired output voltage and/or in parallel to provide a desired amount of current source capability. The cement and the substrate must be thermally conductive, because the cells heat up from absorbing Infrared energy that is not converted to electricity. Since cell heating reduces the operating efficiency it is desirable to minimize the heating. The resulting assemblies are called solar or photovoltaic panels. Depending on construction the photovoltaic can cover a range of frequencies of light and can produce electricity from them, but cannot cover the entire solar spectrum. Hence much of incident Sunlight energy is wasted when used for solar panels, although they can give far higher efficiencies if illuminated with monochromatic light. Some more advanced multispectrum photovoltaic arrays have several different cells tuned to different frequency ranges. This can raise the solar efficiency by several times, but can be far more expensive to produce. Sunlight conversion rates can vary from 5-12% in general,1 however some rates have been as high as 30% due to advances in technologies. CRYSTALLINE SILICON MODULES The most common design of modules contains cells connected using conductive ribbons into one or more 'strings'. The strings are sandwiched between a solar glass frontside and a flexible and durable polymer backsheet, using a polymeric encapsulant. The encapsulant is melted and crosslinked in a vacuum laminator. The strings are electrically terminated into a Junction Box usually glued to the back of the module. A frame made of aluminium profile is fitted around the edges. Crystalline silicon modules have a sunlight conversion rate of 10-12%. RIGID THIN-FILM MODULES In rigid thin film modules, the cell is created directly on a glass substrate or superstrate, and the electrical connections are created ''in situ'', a so called "monolithic integration". The substrate or superstrate is laminated with an encapsulant to a front or back sheet. The main cell technologies in this category are CdTe , Amorphous Silicon , micromorphous silicon (alone or tandem), or CIGS (or variant). Amorphous silicon has a sunlight conversion rate of 5-7%. FLEXIBLE THIN-FILM MODULES Flexible thin film cells are created by depositing the photoactive layer and other necessary layers on a flexible substrate. If the substrate is an insulator (e.g. polyester or polyimide film) then monolithic integration can be used. If a conductor then monolithic integration cannot be used, and another technique for electrical connection used. The cells are converted to a module by lamination to a transparent colourless fluoropolymer on the front side (typically ETFE or FEP) and a polymer suitable for bonding to the final substrate on the other side. The only commercially available (in MW quantities) in a flexible module is amorphous silicon triple junction (from Unisolar). REFERENCES SEE ALSO
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