In the field of photovoltaics, a photovoltaic module is a packaged interconnected assembly of photovoltaic cells, also known as solar cells. An installation of photovoltaic modules or panels is known as a photovoltaic array or a solar panel. 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, sometimes 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 interconnection wiring.
The majority of modules use wafer-based Crystalline silicon cells or a thin film cell based on cadmium telluride or silicon (see photovoltaic cells for details) crystalline silicon, which is commonly used in the wafer form in photovoltaic (PV) modules, is derived from silicon, a relatively multi-faceted element.
In order to use the cells in practical applications, they must be:
Most modules are rigid, but there are some flexible modules available, based on thin film cells.
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. Diodes are included to avoid overheating of cells in case of partial shading.
Since cell heating reduces the operating efficiency it is desirable to minimize the heating. Very few modules incorporate any design features to decrease temperature, however installers try to provide good ventilation behind the module,
New designs of module include concentrator modules in which the light is concentrated by an array of lenses or mirrors onto an array of small cells. This allows the use of cells with a very high cost per unit area (such as gallium arsenide) in a cost-competitive way.
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. Another design concept is to split the light into different wavelength ranges and direct the beams onto different cells tuned to the appropriate wavelength ranges. This is projected to raise efficiency to 50%. Sunlight conversion rates (module efficiencies) can vary from 5-18% in commercial production.
A group of researchers at MIT has recently developed a process to improve the efficiency of luminescent solar concentrator (LSC) technology, which redirects light along a translucent material to PV-modules located along its edge. The researchers have suggested that efficiency may be improved by a factor of 10 over the old design in as little as three years (it has been estimated that this will provide a conversion rate of 30%). 3 of the researchers involved have now started their own company, called Covalent Solar, to manufacture and sell their innovation in PV-modules.
The current market leader in efficient solar energy is SunPower, whose solar panels have a conversion ratio of 19.3%. However, a whole range of other companies (HoloSun, Gamma Solar, NanoHorizons) are emerging which are also offering new innovations in photovoltaic modules, with an average conversion ratio of around 18%. These new innovations include power generation on the front and back sides and increased outputs, however, most of these companies have not yet produced working systems from their design plans, and are mostly still actively improving the technology.
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, usually another sheet of glass.
So-called Inverted Metamorphic (IMM) multi-junction solar cells made on compound-semiconductor technology is just be comming commercialized in July 2008. The University of Michigan's solar car won the North American Solar challenge in July 2008 used IMM thin-flim flexible solar cells.
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