A solar cell or photovoltaic cell is a device that converts light into electricity using the photovoltaic effect. The first working solar cells were constructed by Charles Fritts in 1883. These prototype cells were made of selenium and achieved efficiencies of around 1%. Following the fundamental work of Russell Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and Daryl Chapin created the silicon solar cell in 1954.
Until recently, their use has been limited because of high manufacturing costs. One cost effective use has been in very low-power devices such as calculators with LCDs. Another use has been in remote applications such as roadside emergency telephones, remote sensing, cathodic protection of pipe lines, and limited "off grid" home power applications. A third use has been in powering orbiting satellites and spacecraft.
To take advantage of the incoming electromagnetic radiation from the sun, solar panels can be attached to each house or building. The panels should be mounted perpendicular to the arc of the sun to maximize usefulness. The easiest way to use this electricity is by connecting the solar panels to a grid tie inverter. However, these solar panels may also be used to charge batteries or other energy storage device. Solar panels produce more power during summer months because they receive more sunlight.
Total peak power of installed PV is around 6,000 MW as of the end of 2006. Installed PV is projected to increase to over 9,000 MW in 2007.
Declining manufacturing costs (dropping at 3 to 5% a year in recent years) are expanding the range of cost-effective uses. The average lowest retail cost of a large photovoltaic array declined from $7.50 to $4 per watt between 1990 and 2005. With many jurisdictions now giving tax and rebate incentives, solar electric power can now pay for itself in five to ten years in many places. "Grid-connected" systems - those systems that use an inverter to connect to the utility grid instead of relying on batteries - now make up the largest part of the market.
In 2003, worldwide production of solar cells increased by 32%. Between 2000 and 2004, the increase in worldwide solar energy capacity was an annualized 60%. 2005 was expected to see large growth again, but shortages of refined silicon have been hampering production worldwide since late 2004. Analysts have predicted similar supply problems for 2006 and 2007.
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 (electrical). In remote settings such as mountainous areas, islands, or other places where a power grid is unavailable, solar arrays can be used as the sole source of electricity, usually by charging a storage battery. Satellites use solar arrays for their power. In particular the International Space Station uses multiple solar arrays to power all the equipment on board. 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.
A solar panel on top of a parking meter. Note that this particular installation is shaded, and may not perform as desired.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 (trackers can increase efficiency by 30% or more). PV arrays that approach or exceed one megawatt often use solar trackers. 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/mē/day in northern climes to 6.5 kWh/mē/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 1 kWh/mē/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 into account the weather and the latitude. In the Sahara desert, with less cloud cover and a better solar angle, one can obtain closer to 8.3 kWh/mē/day. The unpopulated area of the Sahara desert is over 9 million kmē, which if covered with solar panels would provide 630 terawatts total power. The Earth's current energy consumption rate is around 13.5 TW at any given moment (including oil, gas, coal, nuclear, and hydroelectric).
Other factors affect PV performance. 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 dramatically due to internal 'short-circuiting' (the electrons reversing course through the shaded portion of the p-n junction). Therefore it is extremely important that a PV installation is not shaded at all by trees, architectural features, flag poles, or other obstructions. Sunlight can be absorbed by dust, fallout, or other impurities at the surface of the module. This can cut down the amount of light that actually strikes the cells by as much as half. Maintaining a clean module surface will increase output performance over the life of the module. 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 25 years or more, so replacement costs should be considered as well.
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 optimize 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.