Solar panel

From The Book of THoTH (Leaves of Wisdom)

Solar Panel (photovoltaic array)

A Solar panel is a flat collection of solar cells or solar thermal collectors used for converting solar energy into electricity or heat. The term solar panel can be applied to either solar hot water panels (usually used for providing domestic hot water) or solar photovoltaic panels (providing electricity).

1 History
2 Solar photovoltaic panels
- 2.1 Solar photovoltaic panels on spacecraft
3 Solar hot water panels
4 Theory and construction
- 4.1 Solar photovoltaic panels
5 Current development
6 See also
7 References
8 External links

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History

In 1839, during the Industrial Revolution, 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.

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 For example, single crystal Si solar cells have largely been replaced by less expensive multi-crystalline silicon solar cells, and thin film silicon solar cells have also been developed recently at lower costs of production yet (see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 backup battery, solar photovoltaics have become routine in certain low-power applications, such as powering 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 away from grid power. Large-scale incentive programs, offering financial incentives like the ability to sell excess electricity back to the public grid, have greatly accelerated the pace of solar PV installations in Spain, Germany, Japan, the United States, Australia and other countries.

Solar thermal panels are more economically viable at the moment than solar photovoltaic panels. Recently they have also seen a resurgence in development towards large scale energy production in parts of the world with significantly high insolation levels.

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Solar photovoltaic panels

Solar photovoltaic panels contain arrays of solar cells that convert light into electricity. Solar cells, or PV cells, rely on the photovoltaic effect, which describes how certain materials can convert sunlight into electricity to absorb 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 is significant when connected together as an array making up a panel.

On a bright day, the sun delivers about 1 kW/m² to the Earth's surface. Typical solar panels have an average efficiency of 12%, with the best commercially available panels at 20%, and recent prototype panels at around 30%. This would result in 200 W/m². However, not all days have bright sunlight, and therefore not enough solar energy can be captured.

At middle northern latitudes, taking the daylight cycle and weather conditions into account, on average 100 W/m² in winter and 250 W/m² in summer reach the ground. With a conversion efficiency of about 20%, one can expect to obtain between 20 and 50 watts per square meter of solar cell. The Sahara desert, with less cloud cover and better solar angle, one can obtain closer to 83 W/m². The area of unpopulated Sahara desert is over 9 million km², which if covered with solar panels would provide 750 terawatts total. The Earth's total energy is around 14 TW at any given moment (including oil, gas, coal, nuclear, and hydroelectric power).

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Solar photovoltaic panels on spacecraft

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 magnify 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), 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.

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Solar hot water panels

A laundromat in California with solar hot water panels on the roof.

A solar water heater uses the sun's energy to heat a fluid, which is used to transfer the heat to a heat storage vessel. In the home, for example, sanitary hot water would be heated and stored in a hot water cylinder. Panels on the roof have an absorber plate to which fluid circulation tubes are attached. The absorber, usually coated with a dark selective coating, assures the conversion of the sun's radiation into heat, while fluid circulating through the tubes carries the heat away where it can be used or stored. The heated fluid is pumped to a heat exchanger, which is a coil in the storage vessel or an external heat exchanger where it gives off its heat and is then circulated back to the panel to be reheated. This provides an effective way of harnessing the sun's energy.

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Theory and construction

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Solar photovoltaic panels

A solar panel on top of a parking meter

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.

Crystalline ingots are sliced into wafer-thin disks, polished to remove slicing damage, dopants are introduced into the wafers, 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.n

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Current development

Many corporations and institutions are currently developing ways to increase the practicality of solar power. While private companies conduct much of the research and development on solar energy, colleges and universities also work on solar-powered devices, especially solar-powered vehicles. Solar-powered cars have commonly appeared at many car and technology shows, and now solar boats are an application of the technology. Colleges and universities compete against each other for superiority in this field of technology. They meet in competitions such as the Solar Splash competition in North America, or the Frisian Nuon Solar Challenge in Europe.

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

In 2005, the most important issue with solar panels was cost, which was at $3-$4 USD per watt of installed power. Because of much increased demand, the price of silicon used for most panels is now experiencing upward pressure. This has caused developers to start using other materials and thinner silicon to keep cost down. Due to economies of scale solar panels get less costly as people use and buy more — as manufacturers increase production, the cost is expected to continue to drop in the years to come. As of early 2006 average cost per installed watt has decreased from $6.00 to $4.50.

Grid tied systems represented the largest growth area. In the USA, with incentives from state governments, power companies and (in 2006 and 2007) from the federal government, growth is expected to climb. Net metering programs are one type of incentive driving growth in solar panel use. Net metering allows electricity customers to get credit for any extra power they send back into the grid. This would cause role reversal, as the utility company would be the buyer, and the solar panel owner would be the seller of electricity. To spur growth of their renewable energy market, Germany has adopted an extreme form of net metering, whereby customers get paid 8 times what the power company charges them for any surplus they supply back to the grid. That large premium has made a huge demand in solar panels for that area.

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