|
|
|
|
Photovoltaic module
|
| |
|
| |
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.
Discussion
Ask a question about 'Photovoltaic module' Start a new discussion about 'Photovoltaic module' Answer questions from other users |
Encyclopedia
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.
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 commonly used semi-conductor.
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 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 sometimes cannot cover the entire solar spectrum (specifically, ultraviolet, infrared and low or diffused light). 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%. Also, the use of infrared photovoltaic cells can increase the efficiencies, producing power at night.
Sunlight conversion rates (module efficiencies) can vary from 5-18% in commercial production (solar panels), that can be lower than cell conversion.
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 modules 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. As of January 14, 2009 a World Record efficiency level of 41.1% has been reached. .
Thin-film modules
Third generation solar cells are advanced thin-film cells.They produce high-efficiency conversion at low cost.
Rigid thin-film modules
In rigid thin film modules, the cell and the module are manufactured in the same production line.
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.
The main cell technologies in this category are CdTe, or a-Si, or a-Si+uc-Si|Tandem , or CIGS (or variant). Amorphous silicon has a sunlight conversion rate of 6-10%.
Flexible thin-film modules
Flexible thin film cells and modules are created on the same production line 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 it is a conductor then another technique for electrical connection must be used.
The cells are assembled into modules by laminating them 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) flexible module uses amorphous silicon triple junction (from Unisolar).
So-called inverted metamorphic (IMM) multijunction solar cells made on compound-semiconductor technology are just becoming commercialized in July 2008. The University of Michigan's solar car that won the North American Solar challenge in July 2008 used IMM thin-flim flexible solar cells.
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 .
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.
Standards
Standard 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).
See also
External links
|
| |
|
|
