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Photovoltaic modules

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 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.

In order to use the cells in practical applications, they must be:
  • connected electrically to one another and to the rest of the system.
  • protected from mechanical damage during manufacture, transport and installation and use (in particular against hail impact, wind and snow loads). This is especially important for wafer-based silicon cells which are brittle.
  • protected from moisture, which corrodes metal contacts and interconnects, (and for thin-film cells the transparent conductive oxide layer) thus decreasing performance and lifetime.
  • electrically insulated including under rainy conditions.
  • mountable on a substructure or building integrated.
  

Most modules are rigid, but there are some kinds of 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.
PV modules are actually more efficient at lower temperatures, so to ensure that they do not overheat, it is essential that they be mounted in such a way as to allow air to move freely around them. This is a particularly important consideration in locations that are prone to extremely hot midday temperatures. The ideal PV generating conditions are cold, bright, sunny days. However, by this token photovoltaic cells not only use the direct component of the light, but also produce electricity when the sky is overcast. - it uses indirect light as well as direct light from the sun.
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.

 

Photovoltaic modules usually hold about 40 cells; about 10 of these modules are mounted in PV arrays - a collection of photovoltaic modules electrically wired together in one structure to produce a specific amount of power - that can measure up to several meters on a side. These flat-plate PV arrays can be mounted at a fixed angle facing south, or they can be mounted on a tracking device that follows the sun, allowing them to capture the most sunlight over the course of a day. About 10 to 20 PV arrays can provide enough power for a household; for large electric utility or industrial applications, hundreds of arrays can be interconnected to form a single, large PV system.
Module performance are generally rated under Standard Test Conditions (STC) :
irradiance of 1,000 W/m², solar spectrum of AM 1.5 and module temperature at 25ºC. Electrical characteristics includes nominal power (Pmax, measured in W), open circuit voltage (Voc), short circuit current (Isc, measured in Amperes), maximum power voltage (Vmpp), maximum power current (Impp) and module efficiency (%).
In kWp, kW is kilowatt and the p means “peak” as peak performance. The “p” however does not show the peak performance, but rather the maximum output according to STC.
Crystalline silicon modules offer guarantee for 10 years the 90% of rated power output and 25 years at 80%.
Standards generally used in photovoltaic panels there are:
  • IEC 61215 (crystalline silicon performance), 61646 (thin film performance) and 61730 (all modules, safety)
  • TÜV Safety Class II (now replaced by IEC 61730)
  • UL 1703
  • CE mark
  • Electrical Safety Tester (EST) Series (EST-460, EST-22V, EST-22H, EST-110).
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