- Photovoltaic array
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 electricityvia the photovoltaic effect. The powerthat one module can produce is seldom enough to meet requirements of ahome or a business, so the modules are linked together to form an"array". Most PV arrays use an inverter toconvert the DC power produced by the modules into alternating currentthat can plug into the existing infrastructure to power lights, motors, and other loads. The modules in a PV array areusually first connected in series to obtain the desired voltage; the individualstrings are then connected in parallel to allow the system to produce more current. Solar arrays are typically measured by the electrical powerthey produce, in watts, kilowatts, or even megawatts.
In urban and suburban areas, photovoltaic arrays are commonly used onrooftops to supplement power use; often the building will have apreexisting connection to the
power grid, in which case the energyproduced by the PV array will be sold back to the utilityin somesort of net meteringagreement.In more rural areas, ground-mounted PV systems are more common. Thesystems may also be equipped with a battery backupsystem tocompensate for a potentially unreliable power grid.In agricultural settings, the array may be used todirectly power DC pumps, without the need for an inverter.In remote settings such as mountainous areas, islands, or other placeswhere a power grid is unavailable, solar arrays can be used as the solesource of electricity, usually by charging a storage battery. Satellites use solar arrays for their power. In particular the International Space Stationuses multiple solar arrays to power allthe equipment on board.Solar photovoltaic panels are frequently applied in satellitepower. However, costs of production have been reduced in recent yearsfor more widespread use through production and technological advances.For example, single crystal silicon solar cells have largely beenreplaced by less expensive multicrystalline silicon solar cells, andthin film silicon solar cells have also been developed recently atlower costs of production yet (see Solar cell). Although they arereduced in energy conversion efficiency from single crystalline Siwafers, they are also much easier to produce at comparably lower costs.Together with a storage battery, photovoltaicshave become commonplace for certain low-powerapplications, such as signal buoys or devices in remote areas orsimply where connection to the electricity mains would be impractical.In experimental form they have even been used to power automobilesin races such as the World solar challengeacross Australia.Many yachts and land vehicles use them to charge on-boardbatteries.
thumb|200px|right|A solar panel on topof 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 isabout 1 kW/m², on the Earth's surface, to a plane that isperpendicular to the sun's rays.Fact|date=April 2008 As such, PV arrays can track the sun through each day to greatly enhance energycollection. However, tracking devices add cost, and requiremaintenance, so it is more common for PV arrays to have fixed mountsthat tilt the array and face due South in the Northern Hemisphere (inthe Southern Hemisphere, they should point due North). The tilt angle,from horizontal, can be varied for season, but if fixed, should be setto give optimal array output during the peak electrical demand portionof a typical year.For large systems, the energy gained by using tracking systemsoutweighs the added complexity (trackers can increase efficiency by 30%or more). PV arrays that approach or exceed one megawatt often usesolar trackers.Accounting for clouds, and the fact that most of the world is not onthe equator, and that the sun sets in the evening, the correct measureof solar power is
insolation– the average number ofkilowatt-hours per square meter per day. For the weather and latitudesof the United States and Europe, typical insolation ranges from 4kWh/m²/day in northern climes to 6.5 kWh/m²/day in the sunniestregions. Typical solar panels have an average efficiency of 12%, withthe best commercially available panels at 20%Fact|date=May 2008. Thus, a photovoltaicinstallation in the southern latitudes of Europe or the United Statesmay expect to produce 1 kWh/m²/day. A typical "150 watt" solar panel isabout a square meter in size. Such a panel may be expected to produce 1kWh every day, on average, after taking into account the weather andthe latitude.In the Saharadesert, with less cloud cover and a better solarangle, one can obtain closer to 8.3 kWh/m²/day. The unpopulated area ofthe Sahara desert is over 9 million km², which if covered with solarpanels would provide 630 terawatts total power. The Earth's currentenergy 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' electricaloutput is extremely sensitive to shading. When even a small portion ofa cell, module, or array is shaded, while the remainder is in sunlight,the output falls dramatically due to internal 'short-circuiting' (theelectrons reversing course through the shaded portion of the
p-n junction). Therefore it is extremely important that a PV installationis not shaded at all by trees, architectural features, flag poles, orother obstructions like continuously parked cars. 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 modulesreduces this problem. However, effective module lives are typically 25years or more [http://www.agr.gc.ca/pfra/water/solardugout_e.htm] , soreplacement costs should be considered as well.
Solar photovoltaic panels on spacecraft
Solar panels can be used on
spacecraft, particularly when they arein the inner part of the solar system. They have been designed topivot on spacecraft, so that they will always be in the direct path ofsolar rays. In order to optimize the amount of energygenerated,solar panels on spacecraft can be equipped with a Fresnel lens,which concentrates sunlight. Because of theseefforts to maximize electric production, and the fact that the Sun ismostly the only source of energy, the constructionof solar cellson spacecraft could be one of the highest costs. When journeying toouter parts of the solar system (or beyond), nuclear reactors or radioisotope thermal generators are preferred, as the Sun's raysare too weak at such extreme distances to power a spacecraft.The ESAis researching the possibility of solar power satellites that would generate electricity in space and then beam itto Earth via laseror microwaves.In addition, solar power is being considered for use as a
propulsion mechanism in lieu of
* cite web | title=Solar Power Construction | url=http://www.solar-panels.cc/construction/
*cite web| title=History of Solar Photovoltaic Power and Solar Panels
*cite web | title=ENF.cn | url=http://www.enf.cn
* [http://howto.altenergystore.com/The-Basics/Solar-Electric-Panels-Overview/a36/ Solar Electric Panels: Overview]
* [http://www.californiasolarco.com/power-systems-photo-gallery.html Gallery of solar panels on residencies]
* [http://www.affordable-solar.com/pv.troubleshooting.htm Troubleshooting a Photovoltaic Array]
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