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Photovoltaic array

2007 Schools Wikipedia Selection. Related subjects: Engineering

   A photovoltaic module that is composed of individual PV cells. A PV
   array is a linked assembly of modules.
   Enlarge
   A photovoltaic module that is composed of individual PV cells. A PV
   array is a linked assembly of modules.

   A photovoltaic array is a linked collection of photovoltaic modules,
   one of which is shown in the picture to the right. Each photovoltaic
   (PV) module is made of multiple interconnected PV cells. The cells
   convert solar energy into direct-current electricity. PV modules are
   sometimes called solar panels, although that term better applies to
   solar-thermal water or air heating panels. Photovoltaic modules
   distinguish themselves from solar cells in that they are conveniently
   sized and packaged in weather-resistant housings for easy installation
   and deployment in residential, commercial, and industrial applications.
   The application and study of photovoltaic devices is known as
   photovoltaics.

   PV cells operate via the photovoltaic effect which describes how
   certain materials can convert sunlight into electricity; they absorb
   some of the energy of the Sun and cause current to flow between two
   oppositely charged layers. Individual solar cells provide a relatively
   small amount of power, but electrical output can be significant when
   connected together. The cells, modules, and arrays can be connected in
   series or parallel, or typically a combination, to create a desired
   peak voltage output.

History

   In 1839, during the Industrial Revolution, Alexandre Edmond Becquerel,
   father of the Nobel Laureate Henri Becquerel, discovered the
   photovoltaic effect which explains how electricity can be generated
   from sunlight. He claimed that "shining light on an electrode submerged
   in a conductive solution would create an electric current."^ . However,
   despite extensive research and developments after this discovery,
   photovoltaic power continued to be very inefficient. As such,
   photovoltaic cells were used mainly for the purposes of measuring
   light. Over 100 years later, in 1941, Russell Ohl invented the solar
   cell, following the invention of the transistor.

Applications

   The solar panels on this small yacht at sea can charge the 12 volt
   batteries at up to 9 amperes in full, direct sunlight.
   Enlarge
   The solar panels on this small yacht at sea can charge the 12 volt
   batteries at up to 9 amperes in full, direct sunlight.

   Solar photovoltaic panels are frequently applied in satellite power.
   However, costs of production have been reduced in recent years for more
   widespread use through production and technological advances. For
   example, single crystal silicon solar cells have largely been replaced
   by less expensive multicrystalline silicon solar cells, and thin film
   silicon solar cells have also been developed recently at lower costs of
   production yet (see Solar cell). Although they are reduced in energy
   conversion efficiency from single crystalline Si wafers, they are also
   much easier to produce at comparably lower costs.

   Together with a storage battery, photovoltaics have become commonplace
   for certain low-power applications, such as signal buoys or devices in
   remote areas or simply where connection to the electricity mains would
   be impractical. In experimental form they have even been used to power
   automobiles in races such as the World solar challenge across
   Australia. Many yachts and land vehicles use them to charge on-board
   batteries.

PV performance

   Larger solar arrays can provide electricity to habitations in isolated,
   sunny areas.
   Enlarge
   Larger solar arrays can provide electricity to habitations in isolated,
   sunny areas.

   At high noon on a cloudless day at the equator, the sun delivers about
   1 kW/m², on the Earth's surface, to a plane that is perpendicular to
   the sun's rays. As such, PV arrays should 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.

   Other factors affect PV performance. Accounting for clouds, and the
   fact that most of the world is not on the equator, and that the sun
   sets in the evening, the correct measure of solar power is insolation:
   the average number of kilowatt-hours per square meter per day. For the
   weather and latitudes of the United States and Europe, typical
   insolation ranges from 4 KWh/m²/day in northern climes to 6.5
   KWh/m²/day in the sunniest regions. Typical solar panels have an
   average efficiency of 12%, with the best commercially available panels
   at 20%. Thus, a photovoltaic installation in the southern latitudes of
   Europe or the United States may expect to produce 1KWh/m²/day. A
   typical "150 Watt" solar panel is about a square meter in size: such a
   panel may be expected to produce 1 KWh every day, on average, after
   taking account the weather and the latitude.

   In the Sahara desert, with less cloud cover and a better solar angle,
   one can obtain closer to 83 W/m². The unpopulated area of the Sahara
   desert is over 9 million km², which if covered with solar panels would
   provide 750 terawatts total. The Earth's current energy comsumption is
   around 13.5 TW at any given moment (including oil, gas, coal, nuclear,
   and hydroelectric power).

   Photovoltaic cells' electrical output is extremely sensitive to
   shading. When even a small portion of a cell, module, or array is
   shaded, while the remainder is in sunlight, the output falls
   drammatically due to internal 'short-circuiting' -- the electrons
   reversing course through the shaded portion of the P-N junction. As
   such it is extremely important that PV arrays not be shaded, at all, by
   trees, architectural features, flag poles, or other obstructions.

   Module output and life are also degraded by increased temperature.
   Allowing ambient air to flow over, and if possible behind, PV modules
   reduces this problem. However, effective module lives are typically 20
   years or so, so replacement costs should be considered as well.

Solar photovoltaic panels on spacecraft

   Solar panels on the Stardust spacecraft (NASA image)
   Enlarge
   Solar panels on the Stardust spacecraft (NASA image)

   Solar panels can be used on spacecraft, particularly when they are in
   the inner part of the solar system. They have been designed to pivot on
   spacecraft, so that they will always be in the direct path of solar
   rays. In order to optimise the amount of energy generated, solar panels
   on spacecraft can be equipped with a Fresnel lens, which concentrates
   sunlight. Because of these efforts to maximize electric production, and
   the fact that the Sun is mostly the only source of energy, the
   construction of solar cells on spacecraft could be one of the highest
   costs. When journeying to outer parts of the solar system (or beyond),
   nuclear reactors or radioisotope thermal generators are preferred, as
   the Sun's rays are too weak at such massive distances to power a
   spacecraft.

   The ESA is researching the possibility of solar power satellites that
   would generate electricity in space and then beam it to Earth via laser
   or microwaves. In addition, solar power is being considered to be used
   as a propulsion mechanism in lieu of chemical propulsion.

Theory and construction

   A solar panel on top of a parking meter. Note that this particular
   installation is shaded, and may not perform as desired.
   Enlarge
   A solar panel on top of a parking meter. Note that this particular
   installation is shaded, and may not perform as desired.

   Crystalline silicon and gallium arsenide are typical choices of
   materials for solar cells. Gallium arsenide crystals are grown
   especially for photovoltaic use, while silicon crystals are available
   in less-expensive standard ingots. These ingots are produced mainly for
   consumption in the microelectronics industry. Polycrystalline silicon
   has lower conversion efficiency but also lower cost.

   During the manufacturing process, crystalline silicon ingots are sliced
   into wafer-thin disks, polished to remove slicing damage, dopants are
   introduced into the soup, and metallic conductors are deposited onto
   each surface: a thin grid on the sun-facing side and usually a flat
   sheet on the other.^ Solar panels are constructed of these cells cut
   into appropriate shapes, protected from radiation and handling damage
   on the front surface by bonding on a cover glass, and cemented onto a
   substrate (either a rigid panel or a flexible blanket). Electrical
   connections are made in series or in parallel to determine total output
   voltage. The cement and the substrate must be thermally conductive,
   because the cells heat up from absorbing infrared energy that is not
   converted to electricity. Since cell heating reduces the operating
   efficiency it is desirable to minimize the heating. The resulting
   assemblies are called solar panels or solar arrays.

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