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![[image of flower]](../../ima/flowXS_01.gif)
Photovoltaic Cell |
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Chronology
A solar cell (photovoltaic cell) is a device that converts light energy into electrical 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 light source is unspecified. the device needs to fulfill only two functions: photogeneration of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity, i.e., carry electrons through a metal contact into a wire or other circuit. This conversion is called the photovoltaic effect, and the field of research related to solar cells is known as photovoltaics. Wiki n.p. Solar cells have many applications. They have long been used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth-orbiting satellites and space probes, consumer systems, e.g. handheld calculators or wrist watches, remote radiotelephones and water pumping applications. More recently, they are starting to be used in assemblies of solar modules (photovoltaic arrays) connected to the electricity grid through an inverter, often in combination with a net metering arrangement. Wiki n.p. The photovoltaic effect was first recognised in 1839 by French physicist, Alexandre-Edmond Becquerel. However, it was not until 1883 that the first solar cell was built by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient. Russell Ohl patented the modern solar cell in 1946. Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells. The modern age of solar power technology arrived in 1954 when Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light. This resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6%. This milestone created interest in producing and launching a geostationary communications satellite by providing a viable power supply. Russia launched the first artificial satellite in 1957, and the United States' first artificial satellite was launched in 1958. Russian Sputnik 3 ("Satellite-3"), launched on 15 May 1958 was the first satellite to use solar arrays. This was a crucial development which diverted funding from several governments into research for improved solar cells. Wiki n.p. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon. Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. The complementary positive charges that are also created are called holes and flow in the direction opposite of the electrons in a silicon solar panel. An array of solar panels converts solar energy into a usable amount of direct current (DC) electricity. Also, the DC current enters an inverter. The inverter turns DC electricity into 120 or 240-volt AC (alternating current) electricity needed for home appliances. The AC power enters the utility panel in the house. The electricity is then distributed to appliances or lights in the house. The electricity that is not used will be recycled and reused in other facilities. Wiki n.p. Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 40.7% with multiple-junction research lab cells. Solar cell energy conversion efficiencies for commercially available multicrystalline Si solar cells are around 14-16%. The highest efficiency cells have not always been the most economical; for example, a 30% efficient multijunction cell based on exotic materials such as gallium arsenide or indium selenide and produced in low volume might well cost one hundred times as much as an 8% efficient amorphous silicon cell in mass production, while only delivering about four times the electrical power. To make practical use of the solar-generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected PV systems). In stand alone systems, batteries are used to store the energy that is not needed immediately. All solar cells require a light absorbing material contained within the cell structure to absorb photons and generate electrons via the photovoltaic effect. The materials used in solar cells tend to have the property of preferentially absorbing the wavelengths of solar light that reach the earth surface; however, some solar cells are optimized for light absorption beyond Earth's atmosphere as well. Light absorbing materials can often be used in multiple physical configurations to take advantage of different light absorption and charge separation mechanisms. By far the most prevalent bulk material for solar cells is crystalline silicon (abbreviated as a group as c-Si), also known as "solar grade silicon". Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, or wafer. Wiki n.p. A photovoltaic array is a linked collection of photovoltaic modules, which are in turn made of multiple interconnected solar cells. The cells convert solar energy into direct current electricity via the photovoltaic effect. The power that one module can produce is seldom enough to meet requirements of a home or a business, so the modules are linked together to form an array. Most PV arrays use an inverter to convert the DC power produced by the modules into alternating current that can plug into the existing infrastructure to power lights, motors, and other loads. The modules in a PV array are usually first connected in series to obtain the desired voltage; the individual strings are then connected in parallel to allow the system to produce more current. Solar arrays are typically measured by the energy they produce, in watts, kilowatts, or even megawatts. Wiki n.p. In urban and suburban areas, photovoltaic arrays are commonly used on rooftops to offset power use; often the building will have a preexisting connection to the power grid, in which case the energy produced by the PV array will be sold back to the utility in some sort of net metering agreement. In more rural areas, ground-mounted PV systems are more common. The systems may also be equipped with a battery backup system to compensate for a potentially unreliable power grid. In agricultural settings, the array may be used to directly power DC pumps, without the need for an inverter. In remote settings such as mountainous areas, islands, or other places where a power grid is unavaliable, solar arrays can be used as the sole source of electricity. Satellites and small boats use solar arrays for their power. Together with a storage battery, photovoltaics have become commonplace for certain low-power applications, such as street lights, signal buoys or devices in remote areas or simply where connection to the electricity mains would be impractical. Many yachts and land vehicles use them to charge on-board batteries. Wiki n.p. At high noon on a cloudless day at the equator, the power of the sun is about 1 kW/mē, on the Earth's surface, to a plane that is perpendicular to the sun's rays. As such, PV arrays can track the sun through each day to greatly enhance energy collection. However, tracking devices add cost, and require maintenance, so it is more common for PV arrays to have fixed mounts that tilt the array and face due South in the Northern Hemisphere (in the Southern Hemisphere, they should point due North). The tilt angle, from horizontal, can be varied for season, but if fixed, should be set to give optimal array output during the peak electrical demand portion of a typical year. For large systems, the energy gained by using tracking systems outweighs the added complexity, since trackers can increase efficiency by 30% or more. PV arrays that approach or exceed one megawatt often use solar trackers. Wiki n.p. |
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