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Solar panels are devices that collect sunlight and convert it into either electricity or heat. The sun's energy is a renewable resource because of its abundance and contact replenishing. Solar panels do not emit any pollutants while converting the sun's energy, and are thus considered to be a green resource. Solar panels can be placed directly onto a building, or placed on a stand.
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There are two main types of solar panels, photovoltaic's, which convert light into electricity and solar thermals, which converts light into heat.
Solar panels were in use over one hundred years ago for hot water heating. Solar cells are a way to help have a cleaner future. In the field of photovoltaics, a photovoltaic module or photovoltaic panel 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. 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.
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Theory and construction
The majority of modules use wafer-based crystalline silicon cells or a thin-film cell based on cadmium telluride or silicon. 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:
- 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 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 |
Module performance and lifetime
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 [3].
Crystalline silicon modules offer for 10 years the 90% of rated power output and 25 years at 80%. 2 million were sold in 2004. 4 million were sold in 2005 and 7 million were sold in 2006. In 2007 8 million were sold. |
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Standards
Standards generally used in photovoltaic panels:
- 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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