'Solar power' (also known as 'solar energy') is a source of energy that uses radiation|radiation emitted by the Sun. It is a energy source that has been used in many traditional technologies for centuries. It is also in widespread use where other power supplies are absent, such as in remote locations and in space|space.

Solar energy is currently used in a number of applications:
* Heat (heating|hot water, heat, cooking)
* Electricity generation (photovoltaics, engines)
* Transportation (car)
* Desalination of seawater
* Photosynthesis by plants

Energy from the Sun
[[Image:Insolation.png|thumb|right|Theoretical annual mean insolation, at the top of Earth's atmosphere (top) and at the surface on a plane|horizontal square meter.]]

land area.png|right|thumb|Map of global solar energy resources. The colours show the average available solar energy on the surface (as measured from 1991 to 1993). For comparison, the dark disks represent the land area required to supply the total primary energy demand using PVs with a conversion efficiency of 8%.<!-- the dots are so tiny that making them 10 to 50 % bigger is moot -->

radiation reaches the Earth's upper atmosphere|atmosphere at a rate of constant|1366 watts per square meter (W/m<sup>2</sup>).<ref>Unknown action "cite"</ref> The first map shows how the solar energy varies in different latitudes.

While traveling through the atmosphere, 6% of the incoming solar radiation (insolation) is (physics)|reflected and 16% is (electromagnetic radiation)|absorbed resulting in a peak irradiance at the equator of 1,020 W/m².<ref>Unknown action "cite"</ref> Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation by 20% through reflection and 3% through absorption.<ref>[http://marine.rutgers.edu/mrs/education/class/yuri/erb.html Earth Radiation Budget]</ref> Atmospheric conditions not only reduce the quantity of insolation reaching the Earth's surface but also affect the quality of insolation by insolation|diffusing incoming light and altering its spectrum.

The second map shows the average global irradiance calculated from satellite data collected from 1991 to 1993. For example, in America the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day).<ref>[http://www.nrel.gov/gis/solar.html NREL: Dynamic Maps, GIS Data, and Analysis Tools - Solar Maps]</ref> This represents the available power, and not the delivered (physics)|power. At present, photovoltaic panels typically convert about 15% of incident sunlight into electricity; therefore, a solar panel in the contiguous United States on average delivers 19 to 56 W/m² or 0.45 - 1.35 kWh/m²/day.<ref>Unknown action "cite"</ref>

The dark disks in the third map on the right are an example of the land areas that, if covered with 8% efficient solar panels, would produce slightly more energy in the form of electricity than the total world primary energy supply in 2003.<ref>[http://www.iea.org/ International Energy Agency - Homepage]</ref> While average insolation and power offer insight into solar power's potential on a regional scale, locally relevant conditions are of primary importance to the potential of a specific site.

After passing through the Earth's atmosphere, most of the sun's energy is in the form of light|visible and infrared radiation. Plants use solar energy to create chemical energy through photosynthesis. Humans regularly use this energy burning wood or fuels, or when simply eating the plants.

A recent concern is dimming, an effect of pollution that is allowing less sunlight to reach the Earth's surface. It is intricately linked with pollution particles and warming, and it is mostly of concern for issues of climate change, but is also of concern to proponents of solar power because of the existing and potential future decreases in available solar energy. The order of magnitude is about 4% less solar energy available at sea level over the timeframe of 1961–90, mostly from increased reflection from clouds back into space.<ref>Unknown action "cite"</ref>

Types of technologies

Many types of technology have been developed to make use of solar radiation. Some of these technologies
make direct use of the solar energy (e.g. to provide light, heat, etc.), while others produce electricity.

Solar design in architecture

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Solar design in architecture involves the use of appropriate solar technologies to maintain a building’s environment at a comfortable temperature through the sun's daily and annual cycles. It may do this by storing solar energy as heat in the walls of a building, which then acts to heat the building at night. Another approach is to keep the interior cool during a hot day by designing in natural convection through the building’s interior.

Solar heating systems

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[[Image:Solarboiler.jpg|thumb|Solar water heaters, on a rooftop in Jerusalem, Israel]]
hot water systems use sunlight to heat water. Solar hot water systems were used extensively in the United States up to the 1920s until replaced by relatively cheap and more reliable conventional heating fuels. The economic advantage of conventional heating fuels has varied over time resulting in periodic interest in solar hot water; however, solar hot water and heating technologies have yet to show the sustained momentum they lost in the 1920s. That being said, the recent spikes and erratic availability of conventional fuels have resulted in a renewed interest in solar heating technologies.

On a technical level, solar water heating is particularly appropriate for low temperature applications (100-150F). This advantage has been successfully applied to heating swimming pools where solar water heating can economically increase pool use. Solar water heating is also used in stand alone or hybrid domestic water heating systems.

Solar water heating systems are composed of solar thermal collectors, a storage tank and a circulation loop.<ref>[http://www.nrel.gov/learning/re_solar_hot_water.html NREL - Solar Hot Water]</ref> The three basic classifications of solar water heaters are:

* Batch systems which consist of a tank that is directly heated by sunlight. These are the oldest and simplest solar water heater designs, however; the exposed tank can be vulnerable to cooldown.<ref>[http://www.californiasolarcenter.org/history_solarthermal.html Solar Hot Water Heating History]</ref>
* Active systems which use pumps to circulate water or a heat transfer fluid.
* Passive systems which circulate water or a heat transfer fluid by circulation. These are also called thermosiphon systems.

A wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between a window and a sun-facing wall. Sunlight heats the air space during the day causing natural circulation through vents at the top and bottom of the wall and storing heat in the mass. During the evening the Trombe wall radiates stored heat.<ref>[http://www.eere.energy.gov/consumer/your_home/designing_remodeling/index.cfm/mytopic=10300 EERE - Indirect Gain (Trombe Walls)]</ref>

A transpired collector is a perforated sun-facing wall. The wall absorbs sunlight and pre-heats air up 40F as it is drawn into the building's ventilation system. These systems are inexpensive and pay for themselves within 3-12 years in offset heating costs.<ref>[http://www.nrel.gov/docs/fy06osti/29913.pdf NREL - Transpired Air Collectors (Ventilation Preheating)]</ref>

Solar cooking

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Cookers use sunshine as a source of heat for cooking as an alternative to fire.

A box cooker traps the sun's energy in an insulated box; such boxes have been successfully used for cooking, pasteurization and fruit canning. Solar cooking is helping many developing countries, both reducing the demands for local firewood and maintaining a cleaner breathing environment for the community.

The first known western solar oven is attributed to de Saussure in 1767, which impressed Sir John Herschel enough to build one for cooking meals on his astronomical expedition to the Cape of Good Hope in Africa in 1830.<ref>Unknown action "cite"</ref> Today, there are many different designs in use around the world.<ref>Unknown action "cite"</ref>

Solar lighting

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Daylighting is a passive solar method of using natural light to provide illumination. Daylighting directly offsets energy use in electric lighting systems and indirectly offsets energy use through a reduction in cooling load.<ref>[http://www.iea-shc.org/task22/reports/IEA%20Daylighting%20Report%20Final.pdf IEA - Daylighting HVAC Interaction (pg 85)]</ref> Although difficult to quantify, the use of natural light also offers physiological and psychological benefits compared to conventional lighting.

Daylight features include building orientation, incident angle|window orientation, exterior shading, sawtooth roofs, windows, light shelves, skylights and tubes.<ref>[http://www.eere.energy.gov/buildings/info/design/integratedbuilding/passivedaylighting.html DOE - Daylighting]</ref> These features may be incorporated in existing structures but are most effective when integrated in a solar building design|solar design package which accounts for factors such as pollution#Glare|glare, heat gain, heat loss and time-of-use. Architectural trends increasingly recognize daylighting as a cornerstone of design.

Hybrid solar lighting (HSL) is an active solar method of using natural light to provide illumination. Hybrid solar lighting systems collect sunlight using focusing mirrors that track the sun. The collected light is transmitted via fibers into a building's interior to supplement conventional lighting.<ref>[http://www.ornl.gov/sci/solar/ ORNL - Solar Technologies Program]</ref>

saving time (DST) can be seen as a method of utilising solar energy by matching available sunlight to the hours of the day in which it is most useful. In 2001 this was estimated to reduce peak demand in California by 35–70 MW (0.08%–0.16%) in June through August, though total electricity use was unaffected.<ref>Unknown action "cite"</ref> However, there is some question whether these estimates are valid. In 2000 when parts of Australia began DST in late winter, overall electricity consumption did not decrease, but the peak load increased.<ref>Unknown action "cite"</ref>

Solar electricity

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[[Image:Kyocera-hq-01.jpg|right|thumb|150px|[[Kyocera]] headquarters. PV cells on the side of the building generate electricity from sunlight.]]
panels on yacht at sea.jpg|thumb|right|The solar panels (photovoltaic arrays) on this small yacht at sea can charge the 12 V batteries at up to 9 A in full, direct sunlight

cells, also referred to as photovoltaic cells, are devices or banks of devices that use the cell|photovoltaic effect of semiconductors to generate electricity directly from sunlight. 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 crystal display|LCDs. Another use has been in remote applications such as roadside emergency telephones, remote sensing, 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 tie inverter. However, these solar panels may also be used to charge batteries or other energy storage|energy storage device. Solar panels produce more power during summer months because they receive more sunlight. The cost payback time may take over 10 years depending on the cost of grid electricity and tax rebates.

Total power of installed PV is around 3,700 MW as of the end of 2005.<ref>[http://www.iea-pvps.org/isr/01.htm Installed PV power]</ref> This is only one part of solar-generated electric power.

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 array declined from $7.50 to $4 per watt between 1990 and of 2005|2005.<ref>[http://www.regional-renewables.org/cms/front_content.php?idcatart=50 Regional Renewables.org] Retrieved November 2006</ref> 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 (electrical)|inverter to connect to the distribution|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%.<ref>[http://www.earth-policy.org/Indicators/2004/indicator12.htm World Sales of Solar Cells Jump 32 Percent]Viviana Jiménez, 2004 Earth Policy Institute. Retrieved September 2006. </ref> Between 2000 and 2004, the increase in worldwide solar energy capacity was an annualized 60%.<ref>[http://www.forbes.com/free_forbes/2006/0327/062.html Sun King] Russell Flannery March 2006. Retrieved September 2006. </ref> 2005 was expected to see large growth again, but shortages of refined silicon have been hampering production worldwide since late 2004.<ref>[http://www.wired.com/news/planet/0,2782,67013,00.html Silicon Shortage Stalls Solar] John Gartner, Wired News, March 2005. Retrieved September 2006. </ref> Analysts have predicted similar supply problems for 2006 and 2007.<ref>[http://www.renewableenergyaccess.com/rea/news/story?id=41508 2005 Solar Year-end Review & 2006 Solar Industry Forecast] Jesse W. Pichel and Ming Yang, Research Analysts, Piper Jaffray, January 2006. Retrieved September 2006. </ref>

Solar thermal electric power plants

[[Image:Solar Two 2003.jpg|right|thumb|Solar Two, a concentrating solar power tower|power tower (an example of solar thermal energy applied to electrical power production).]]

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thermal energy can be focused on a exchanger, and converted in a engine to produce power or applied to other industrial processes.

Power towers

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Parabolic troughs

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A long row of parabolic mirrors concentrates sunlight on a tube filled with a transfer fluid (usually oil). As with the power tower, this heated oil is used to power a conventional steam turbine, or stored for nighttime use. The largest operating solar power plant, as of 2007, is one of the SEGS parabolic trough systems in the Desert in California, USA (see power plants in the Mojave Desert).

Concentrating collector with steam engine

Solar energy converted to heat in a concentrating collector can be used to boil water into steam (as is done in nuclear and coal power plants) to drive a engine or turbine. The concentrating collector can be a trough collector, parabolic collector, or power tower.

Concentrating collector with Stirling engine

[[Image:SolarStirlingEngine.jpg|right|thumb|A parabolic solar collector concentrating the sun's rays on the heating element of a engine. The entire unit acts as a tracker.]]

Solar energy converted to heat in a concentrating (dish or trough parabolic) collector can be used to drive a engine, a type of heat engine which uses a sealed working gas (i.e. a closed cycle) and does not require a water supply.

Until recently, a solar Stirling system held the efficiency record for converting solar energy into electricity (30% at 1,000 watts per square meter).<ref>Unknown action "cite"</ref> Such concentrating systems produce little or no power in overcast conditions and incorporate a solar tracker to point the device directly at the sun. That record has been broken by a high-performance crystalline silicon solar cell platform developed by a consortium led by the University of Delaware which has achieved a conversion efficiency of 42.8 percent.<ref name="New World Record">Unknown action; the action name must not contain special characters.</ref>

Solar updraft tower

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A updraft tower (also known as a solar chimney, but this term is avoided by many proponents due to its association with fossil fuels) is a relatively low-tech solar thermal power plant where air passes under a very large agricultural glass house (between 2 and 8 km in diameter), is heated by the sun and channeled upwards towards a convection tower. It then rises naturally and is used to drive turbines, which generate electricity.

Energy tower

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An tower (downdraft)|energy tower is an alternative proposal to the solar updraft tower. It is driven by spraying water at the top of the tower, evaporation of water causes a downdraft by cooling the air thereby increasing its density, driving wind turbines at the bottom of the tower. It requires a hot arid climate and large quantities of water (seawater may be used) but does not require the large glass house of the solar updraft tower.

Solar pond

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A pond is simply a pool of water which collects and stores solar energy. It contains layers of salt solutions with increasing concentration (and therefore density) to a certain depth, below which the solution has a uniform high salt concentration. It is a relatively low-tech, low-cost approach to harvesting solar energy. The principle is to fill a pond with 3 layers of water:

# A top layer with a low salt content.
# An intermediate insulating layer with a salt gradient, which sets up a Gradient|density gradient that prevents heat exchange by natural convection in the water.
# A bottom layer with a salt content which reaches a temperature approaching 90 degrees Celsius.

The layers have different densities due to their different salt content, and this prevents the development of currents which would otherwise transfer the heat to the surface and then to the air above. The heat trapped in the salty bottom layer can be used for heating of buildings, industrial processes, generating electricity or other purposes. One such system is in use at Bhuj, Gujarat, India<ref>[http://edugreen.teri.res.in/explore/renew/pond.htm Solar pond in Gujarat]</ref> and another at the of Texas El Paso.<ref>[http://www.solarpond.utep.edu/ Solar pond at University of Texas El Paso]</ref>

Solar chemical

chemical is any process that harnesses solar energy by absorbing sunlight and using it to drive an endothermic or cell|photoelectrochemical reaction. Prototypes, but no large-scale systems, have been constructed.

One approach has been to use conventional solar thermal collectors to drive chemical dissociation reactions. Ammonia can be separated into nitrogen and hydrogen at high temperature and with the aid of a catalyst, stored indefinitely, then recombined later to release the heat stored. A prototype system was constructed at the Australian National University<ref>K. Lovegrove, A. Luzzi, I. Soldiani and H. Kreetz "Developing Ammonia Based Thermochemical Energy Storage for Dish Power Plants." Solar Energy, 2003. http://engnet.anu.edu.au/DEresearch/solarthermal/pages/pubs/SolarEAmmonia4.pdf or http://dx.doi.org/10.1016/j.solener.2003.07.020</ref>.

Another approach is to use focused sunlight to provide the energy needed to split water via photoelectrolysis into its constituent hydrogen and oxygen in the presence of a metallic catalyst such as zinc.<ref>[http://www.isracast.com/tech_news/090905_tech.htm IsraCast: ZINC POWDER WILL DRIVE YOUR HYDROGEN CAR], [http://www.wired.com/news/technology/0,1282,65936,00.html Wired News: Sunlight to Fuel Hydrogen Future] and [http://solar.web.psi.ch//data/research/synmet/ Solar Technology Laboratory: SynMet]</ref>. Other research in this area has focused on semiconductors, and on the use of examined metal compounds, in particular oxide|titanium, niobium and tantalum oxides <ref>[http://www.newscientisttech.com/channel/tech/mg19225776.700-take-a-leaf-out-of-natures-book-to-tap-solar-power.html New Scientist issue 2577, 13 November 2006] Take a leaf out of nature's book to tap solar power by Duncan Graham-Rowe Accessed Nov 2006</ref>. Unfortunately, these materials exhibit very low efficiencies, because they require light to drive the photoelectrolysis of water.<!-- need to insert references --> Current materials also require an electrical voltage bias for the hydrogen and oxygen gas to evolve from the surface, another disadvantage.<!-- need to insert references --> Current research is focusing on the development of materials capable of the same water splitting reaction using lower energy visible light.<!-- need to insert references -->

Solar thermal energy also has the potential to be used directly to drive engineering|chemical processes that require significant amounts of process heat, including at high temperatures that can be otherwise quite hard to attain<ref>J Murray. Investigation of Opportunities for High-Temperature Solar Energy in the Aluminum Industry, National Renewable Energy Laboratory report NREL/SR-550-39819 (USA).</ref>.

Solar desalination

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This technique uses solar energy to evaporate sea water. The humidity|humid air is then condensed and desalination|desalinated water is collected.

Classifications of solar power technology
Solar power technologies can be classified in a number of ways.

cell.png|right|thumb|Photovoltaic cells produce electricity directly from sunlight

Direct or Indirect

In general, 'direct' solar power involves a single transformation of sunlight which results in a usable form of energy.
* Sunlight hits a cell|photovoltaic cell creating electricity.
* Sunlight warms a mass.
* Sunlight strikes a sail on a space craft and is converted directly into a force on the sail which causes motion of the craft.
* Sunlight strikes a mill and causes the vanes to rotate as mechanical energy (little practical application has yet been found for this effect).
* In a direct solar water heater the water heated in the collector is used in the domestic water system.
<!-- Unsourced image removed: power stations produce indirect solar power. The Itaipu Dam, Brazil / Paraguay -->

In general, 'indirect' solar power involves multiple transformations of sunlight which result in a usable form of energy.

*Vegetation uses photosynthesis to convert solar energy to energy. The resulting biomass may be burned directly to produce heat and electricity or processed into ethanol, methane, hydrogen and other biofuels.
*Hydroelectric dams and turbines are powered by solar energy through its interaction with the Earth's atmosphere and the resulting of meteorological phenomena|weather phenomena.
*thermal energy conversion|Ocean thermal energy production uses the thermal gradients present across ocean depths to generate power. These temperature differences are produced by sunlight.<ref>[http://www.nrel.gov/learning/re_ocean.html NREL - Ocean Energy Basics]</ref>
*fuels are ultimately derived from solar energy captured by vegetation in the timescale|geological past, but they would not normally be classed as solar energy.

Passive or active

This distinction is made in the context of building construction and building services engineering.

solar systems use non-mechanical techniques of capturing, converting and distributing sunlight into usable outputs such as heating, lighting or ventilation. These techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air and referencing the position of a building to the sun.
*Passive solar water heaters use a thermosiphon to circulate fluid.
*A wall circulates air by natural circulation and acts as a thermal mass which absorbs heat during the day and radiates heat at night.
*Clerestory windows, light shelves, skylights and light tubes can be used among other daylighting techniques to illuminate a building's interior.
*Passive solar water distillers may use action to pump water.

solar systems use electrical and mechanical components such as photovoltaic panels, pumps and fans to process sunlight into usable outputs.

Concentrating or non-concentrating

[[Image:Font Romeu France.jpg|thumb|right|A large parabolic reflector furnace is located in the Pyrenees at Odeillo, Cerdagne. It is used for various research purposes.<ref>[http://www.imp.cnrs.fr/foursol/index.shtml Les Fours solaires]</ref>]]

Concentrating solar power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam capable of producing high temperatures and correspondingly high efficiency|thermodynamic efficiencies. Concentrating solar is generally associated with thermal energy|solar thermal applications but cells#Concentrating Photovoltaics (CPV)|concentrating photovoltaic (CPV) applications exist as well and these technologies also exhibit improved efficiencies. CSP systems require insolation to operate properly.<ref>[http://www1.eere.energy.gov/solar/pv_cell_light.html DOE - Solar Basics]</ref>

Concentrating solar power systems vary in the way they track the sun and focus light.
*Line focus/Single-axis
A trough consists of a linear parabolic reflector which concentrates light on a receiver positioned along the reflector's focal line. These systems use single-axis tracking to follow the sun. A working fluid (oil, water) flows through the receiver and is heated up to 400 °C before transferring its heat to a distillation or power generation system.<ref>[http://www.psa.es/webeng/instalaciones/parabolicos.html Plataforma Solar de Almería Concentrator Facilities] </ref><ref>[http://www.energylan.sandia.gov/sunlab/overview.htm#trough Sandia - Concentrating Solar Power Overview]</ref> Trough systems are the most developed CSP technology. The Solar Electric Generating System (SEGS) plants in California and Plataforma Solar de Almería's SSPS-DCS plant in Spain are representatives of this technology.<ref>[http://www.psa.es/webeng/instalaciones/parabolicos.html Plataforma Solar de Almería - Linear-focusing Concentrator Facilities]</ref>
*Point focus/Dual-axis
A power tower|power tower consists of an array of flat reflectors (heliostats) which concentrate light on a central receiver located on a tower. These systems use dual-axis tracking to follow the sun. A working fluid (air, water, molten salt) flows through the receiver where it is heated up to 1000 °C before transferring its heat to a power generation or energy storage system. Power towers are less advanced than trough systems but they offer higher efficiency and energy storage capability.<ref name="Quaschning">Unknown action "cite"</ref> The Two in Daggett, California and the Planta Solar 10 (PS10) in Sanlucar la Mayor, Spain are representatives of this technology.
A dish or dish/engine system consists of a stand-alone parabolic reflector which concentrates light on a receiver positioned at the reflector's focal point. These systems use dual-axis tracking to follow the sun. A working fluid (hydrogen, helium, air, water) flows through the receiver where it is heated up to 1500 °C before transferring its heat to a sterling engine for power generation.<ref>[http://www.energylan.sandia.gov/sunlab/overview.htm#trough Sandia - Concentrating Solar Power Overview]</ref><ref name="Quaschning" /> Parabolic dish systems display the highest solar-to-electric efficiency among CSP technologies and their modular nature offers scalability. The Stirling Energy Systems (SES) and Science Applications International Corporation (SAIC) dishes at UNLV and the Big Dish in Canberra, Australia are representatives of this technology.

Non-concentrating photovoltaic and solar thermal systems do not concentrate sunlight. While the maximum attainable temperatures (200 °C) and thermodynamic efficiencies are lower, these systems offer simplicity of design and have the ability to effectively utilize diffuse insolation.<ref name="Quaschning" /> Flat-plate thermal and photovoltaic panels are representatives of this technology.

Advantages and disadvantages of solar power
pv annual may2004.jpg|right|thumb|US annual average solar energy received by a latitude tilt photovoltaic cell.


*The 89 of magnitude (power)#Petawatt (1015 watt)|petawatts of sunlight reaching the earth's surface is plentiful compared to the 15 of magnitude (power)#Terawatt (1012 watt)|terawatts of average power consumed by humans.<ref name="Smil">[http://www.oecd.org/dataoecd/52/25/36760950.pdf#search=%22worldwide%20consumption%20of%20energy%2013%20TW%20smil%22 Vaclav Smil - Energy at the Crossroads]</ref> Additionally, solar electric generation has the highest power density (global mean of 170 W/m<sup>2</sup>) among renewable energies.<ref name="Smil" />
*Solar power is pollution free during use. Production end wastes and emissions are manageable using existing pollution controls. End-of-use recycling technologies are under development.<ref>[http://www.nrel.gov/ncpv/thin_film/docs/environmental_aspects_of_pv_power_systems_iea_workshop.pdf Environmental Aspects of PV Power Systems]</ref>
*Facilities can operate with little maintenance or intervention after initial setup.
*Solar electric generation is economically competitive where grid connection or fuel transport is difficult, costly or impossible. Examples include satellites, island communities, remote locations and ocean vessels.
*When grid-connected, solar electric generation can displace the highest cost electricity during times of peak demand (in most climatic regions), can reduce grid loading, and can eliminate the need for local battery power for use in times of darkness and high local demand; such application is encouraged by metering. Time-of-use net metering can be highly favorable to small photovoltaic systems.
*Grid-connected solar electricity can be used locally thus minimizing transmission/distribution losses (approximately 7.2%).<ref>[http://www.climatetechnology.gov/library/2003/tech-options/tech-options-1-3-2.pdf U.S. Climate Change Technology Program - Transmission and Distribution Technologies]</ref>
*Once the initial cost of building a solar power plant has been spent, costs are low compared to existing power technologies.


*Solar electricity can currently be more expensive than electricity generated by other sources.
*Solar heat and electricity are not available at night and may be unavailable due to weather conditions; therefore, a power source#Intermittency: solar energy|storage or complementary power system is required for most applications.
*Limited power density: Average daily insolation in the contiguous U.S. is 3-7 kW·h/m<sup>2</sup><ref>[http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/serve.cgi NREL Map of Flat Plate Collector at Latitude Tilt Yearly Average Solar Radiation]</ref><ref>http://www.eere.energy.gov/solar/cfm/faqs/third_level.cfm/name=Photovoltaics/cat=The%20Basics#Q43 DOE's Energy Efficiency and Renewable Energy Solar FAQ]</ref><ref>[http://www.engineeringtalk.com/news/spo/spo103.html]</ref> (see [http://en.wikipedia.org/wiki/Image:Us_pv_annual_may2004.jpg picture])
*Solar cells produce Current|DC which must be converted to Current|AC (using a tie inverter) when used in currently existing distribution grids. This incurs an energy loss of 4-12%.<ref>[http://rredc.nrel.gov/solar/codes_algs/PVWATTS/system.html Renewable Resource Data Center - PV Correction Factors]</ref>

Availability of solar energy
sun1.jpg|thumb|right|The Sun.

There is no shortage of solar-derived energy on Earth. Indeed the storages and flows of energy on the planet are very large relative to human needs.

* The amount of solar energy intercepted by the Earth every minute is greater than the amount of energy the world uses in fossil fuels each year.<ref>Unknown action "cite"</ref>
* Tropical oceans absorb 560 trillion gigajoules (GJ) of solar energy each year, equivalent to 1,600 times the world’s annual energy use.<ref>Unknown action "cite"</ref>
* The energy in the winds that blow across the United States each year could produce more than 16 billion GJ of electricity—more than one and one-half times the electricity consumed in the United States in 2000.<ref>Unknown action "cite"</ref>
* Annual photosynthesis by the vegetation in the United States is 50 billion GJ, equivalent to nearly 60% of the nation’s annual fossil fuel use.<ref>Unknown action "cite"</ref>

Plants, on average, capture 0.1% of the solar energy reaching the Earth. The land area of the lower 48 United States intercepts 50 trillion GJ per year, equivalent to 500 times the nation’s annual energy use.<ref>Unknown action "cite"</ref> This energy is spread over 8 million square kilometers of land area, so that each square meter is exposed to 6.1 GJ per year. This results in potential biomass production of 6,100 GJ per square kilometer per year. Compared to the 0.1% efficiency of vegetation, roof installable amorphous silicon solar panels capture 8%-14% of the solar energy, while more expensive crystalline panels capture 14%-20%, and large scale desert mirror-concentrator heat engine based systems may capture up to 30-50%.

Energy storage
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For a stand-alone system, some means must be employed to store the collected energy for use during hours of darkness or cloud cover. The following list includes both mature and immature techniques:

A Solar powered garden lamp

*Using traditional (electricity)|batteries
*battery|Flow batteries
*Molten salt<ref>[http://www.solarpaces.org/SOLARTRES.HTM Solar Tres Project]</ref>
*nitrogen economy|Cryogenic liquid air or nitrogen
*Compressed air in Pneumatics|cylinders and in air energy storage|caverns
*energy storage
*Hydrogen produced by electrolysis
*magnetic energy storages
*oil economy

Storage always has an extra stage of energy conversion, with consequent energy losses, increasing the total capital costs. One way around this is to export excess power to the power grid, drawing it back when needed. This appears to use the power grid as a battery but in fact is relying on conventional energy production through the grid during the night. However, since the grid always has a positive outflow, the result is exactly the same.

Electric power costs are highly dependent on the consumption per time of day, since plants must be built for peak power (not average power). Expensive gas-fired "peaking generators" must be used when base capacity is insufficient. Fortunately for solar, solar capacity parallels energy demand; since much of the electricity is for removing heat produced by too much solar energy (using conditioners). This is less true in the winter when the peak energy use is in the early evening when food is being prepared and lighting, heating, and entertainment equipment loads are higher. Winter heating loads can be time shifted by storing thermal energy in bulk materials such as rock, water, or thermal phase transistion materials such as sulfate|glauber's salt] or wax, provided solar ilumination is sufficient. power complements solar power since it can produce energy when there is no sunlight but this advantage is highly dependant upon local and seasonal wind availability.

Deployment of solar power
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"The Stone Age did not end for a lack of stones, and the oil age will end not for a lack of oil." — Sheik Yamani, Saudi oil minister, 1973

"We stopped using stone because bronze and iron were superior materials, and likewise we will stop using oil when other energy technologies provide superior benefits." — Bjørn Lomborg, The Skeptical Environmentalist
(New York: Cambridge University Press, 2001), p. 120<ref>[http://strategis.ic.gc.ca/epic/site/trm-crt.nsf/en/rm00114e.html Technology Roadmaps]</ref>

Deployment of solar power depends largely upon local conditions and requirements. All industrialised nations share a need for electricity and it is believed that solar power will increasingly be used as an option for electricity supply. The Very Large Scale Photovoltaic Power Generation (VLS-PV) proposal argues that "PV systems could generate many times the current primary global energy supply".<ref>[http://www.iea-pvps.org/products/rep8_02s.htm Summary Energy from the Desert]</ref> To compensate for night time energy demands they would need to be complemented with storage.

Solar power by country
:See the articles for individual countries listed at power by country

Solar powered car
[[Image:Nuna3atZandvoort1.JPG|thumb|The solar powered car The Nuna 3 built by the Netherlands|Dutch Nuna team]]
Development of a practical car|solar powered car has been an engineering goal for 20 years. The center of this development is the Solar Challenge, a biannual solar powered car race over 3021 km (1877mi) through central Australia from Northern Territory|Darwin to Adelaide. The race's stated objective is to promote research into solar-powered cars. Teams from universities and enterprises participate.
In 1987 when it was founded, the winner's average speed was 67 per hour|km/h (42 mph).<ref>[http://www.wsc.org.au/history/ History of World Solar Challenge] The World Solar Challenge. Retrieved September 2006. </ref> By the 2005 race this had increased to an average speed of greater than 100 km/h (62 mph), even though the cars were faced with the 110 km/h (68 mph) Australia speed limit.<ref>[http://www.wsc.org.au/2007/ Panasonic World Solar Challenge 21-28 October 2007] The World Solar Challenge. Retrieved September 2006. </ref>

See also
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*energy crisis
*(electromagnetic radiation)
*COMES, French Solar Energy Authority
*of solar power to energy grids
*world resources and consumption
*Union Climate Change Programme
*energy development
*of conservation topics

| width=450 align=left |
*air conditioning
*power plants in the Mojave Desert
*power satellite
*power tower
*updraft tower
*of solar energy
*Renewable Energy Cooperation (TREC)

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External links
* [http://www.carbonsolutionsgroup.com/CESwhitepaper.pdf White Paper Discussing the use of Carbon Finance to Develop Solar Power].
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