sabato 20 settembre 2008

Solar cell

A solar cell or photovoltaic cell is a device that converts solar energy into electricity by the photovoltaic effect. Photovoltaics is the field of technology and research related to the application of solar cells as solar energy. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the source is unspecified.
Assemblies of cells are used to make solar modules, which may in turn be linked in photovoltaic arrays.

Solar cells have many applications. Individual cells are used for powering small devices such as electronic calculators. Photovoltaic arrays generate a form of renewable electricity, particularly useful in situations where electrical power from the grid is unavailable such as in remote area power systems, Earth-orbiting satellites and space probes, remote radiotelephones and water pumping applications. Photovoltaic electricity is also increasingly deployed in grid-tied electrical systems.

Similar devices intended to capture energy radiated from other sources include thermophotovoltaic cells, betavoltaics cells, and optoelectric nuclear battery.

Photovoltaic Panels

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.

Theory and construction

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:

  • 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

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. [1] 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. [2]

Finally, a whole range of other companies (eg HoloSun, Gamma Solar, NanoHorizons, ...) are emerging which are also offering new innovations in the modules. [3] These new innovations include power generation at the front and back side, 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.

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 CdTeamorphous silicon, micromorphous silicon (alone or tandem), or CIGS (or variant). Amorphous silicon has a sunlight conversion rate of 5-9%.

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) 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.

Solar Energy, What is ?

Solar energy refers to the utilization of the radiant energy from the SunSolar power is used interchangeably with solar energy, but refers more specifically to the conversion of sunlight into electricity by photovoltaics, concentrating solar thermal devices, or by an experimental technology such as a solar chimney or solar pond.

Solar energy and shading are important considerations in building design. Thermal massis used to conserve the heat that sunshine delivers to all buildings. Daylightingtechniques optimize the use of light in buildings. Solar water heaters heat swimming pools and provide domestic hot water. In agriculture, greenhouses expand growing seasons and pumps powered by solar cells (known as photovoltaics) provide water for grazing animals. Evaporation ponds are used to harvest salt and clean waste streams of contaminants.

The parabolic dish engine system, which concentrates solar power

Solar distillation and disinfection techniques produce potable water for millions of people worldwide. Simple applications include clotheslines and solar cookers which concentrate sunlight for cooking, drying and pasteurization. More sophisticated technologies concentrate sunlight for high-temperature material testing, metal smelting and industrialchemical production. A range of experimental solar vehicles provide ground, air and sea transportation.