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Photovoltaics
The word photovoltaic is made from two words: the Greek word photo, meaning light, and volt, wich is the unit used to measure the electrical potential. Photovoltaics is the field of technology and research related to the generation of direct current (DC) from semiconductors when they are illuminated by photons. So, in short, photovoltaic means electricity from light.
Photovoltaic generation of power is caused by radiation, in form of photons, that separates negative and positive charge carriers in an absorbing material. If an electric field is present, then these charges can produce electric current for use in an external circuit. Such fields exists permanently at junctions in semiconductor materials.
The dominant semiconductor material used to create photovoltaic cells is crystalline silicon. One of the reasons for crystalline silicon to be dominant in photovoltaics was that silicon was already in use in microelectronics. Another reason is that silicon is an abundant material.
The devices, made from semiconductor materials, that produce electricity are commonly known as solar cells or photovoltaic cells. These cells come in many shapes and sizes, from smaller than a coin to several inches across. They are the basic units of a photovoltaic generator.
Because all silicon type solar cells generate, in normal operation, about 0.5V, they are connected in series to bring the voltage to a useful level. Arrangements of many solar cells (usually 30 to 36) wired in series and supported inside a metal frame are called solar cells modules. The modules can be grouped together to form solar arrays.
There are several varieties of silicon-type solar cells and solar modules:
Other technologies used for creation of solar cells are:
- Thin film cadmium telluride
- Gallium-arsenide cells
Solar Cells Advantages
- The fuel source, the sun, is vast and essentially infinite.
- Low operation and maintanance costs.
- Solar modules are very reliable. Because there are no moving parts the modules have a life span of more then 20 years.
Solar Cells Disadvantages
- High installation costs.
- Poor reliability of auxiliary elements (batteries).
History of Photovoltaics
Although the photovoltaic effect was first discovered in 1839 by the french physicist Edmond Becquerel it took more then a century to understand the process. Eventually scientists discovered that the light energy is converted into electric energy at the atomic level.
The Vanguard I satellite. The first
solar-powered satelliteThe earliest significant application of solar cells was
the back-up power source to the Vanguard I satellite in 1958. Because of his size, just
152 mm (6 in) in diameter and weighing just 1.4 kg (3 lb), Vanguard 1 was described by
then-Soviet Premier Nikita Khrushchev as, "The grapefruit satellite". The
solar cells allowed it to continue transmitting for over a year after its chemical
battery was exhausted. The successful operation of solar cells on this mission was
duplicated in many other Soviet and American satellites. By the late 1960s,
photovoltaic panels had become the established source of power for the satellites.
After the successful application of solar panels on the Vanguard satellite it still was
not until the energy crisis, in the 1970s, that photovoltaic solar panels gained use
outside of back up power suppliers on spacecraft. Photovoltaics went on to play an
essential part in the success of early commercial satellites such as Telstar, and they
remain vital to the telecommunications infrastructure today.