Solar energy
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226167995
2008-07-17T03:26:00Z
Mrshaba
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[[WP:UNDO|Undid]] revision 226167905 by [[Special:Contributions/Mrshaba|Mrshaba]] ([[User talk:Mrshaba|talk]])test complete
[[Image:Sunburst over Earth.JPG|thumb|right|Heat and light from the Sun fuels life on Earth.]]
[[Image:Giant photovoltaic array.jpg|thumb|right|[[Nellis Solar Power Plant]], the largest [[Photovoltaics|photovoltaic]] power plant in [[North America]]]]
[[Image:Available Energy-3.png|thumb|right|Solar energy reaching the earth's surface (left) greatly exceeds both total wind energy (center) and global energy consumption (right), although only a small portion of each is recoverable.<ref>The volume of each cube represents the amount of energy available and consumed. The amount of solar energy available to the earth in one hour exceeds global energy demand for a year.[http://web.mit.edu/hockfield/speeches/2008-aaas.html Energy and Inspiration: Inventing the Future in Time]<!--There are references that say in one minute and references that say in one hour.--></ref>]]
{{renewable energy sources}}
'''Solar energy''' is the [[heat]] and [[light]] [[radiant energy|radiated]] from the [[Sun]] that powers [[Earth]]'s [[climate]] and supports [[life]]. Solar [[Technology|technologies]] allow controlled use of this energy resource. '''Solar power''' is used interchangeably with solar energy but refers more specifically to the conversion of sunlight into electricity. The primary solar power technologies are [[photovoltaics]] and concentrating solar thermal devices but several experimental technologies such as thermoelectric converters, solar chimneys and solar ponds have also been used.
The controlled use of solar energy is an important consideration in building design. [[Thermal mass]] is used to conserve the heat that sunshine delivers to all buildings. [[Daylighting]] techniques optimize the use of light in buildings. [[Solar hot water|Solar water heaters]] heat swimming pools and provide domestic hot water. In agriculture, [[greenhouse]]s grow specialty crops and photovoltaic-powered pumps provide water for grazing animals. [[Evaporation pond]]s find applications in the commercial and industrial sectors where they are used to harvest salt and clean waste streams of contaminants.
Solar [[distillation]] and [[disinfection]] techniques produce [[potable water]] for millions of people worldwide. Family-scale [[solar cookers]] and larger solar kitchens concentrate sunlight for cooking, drying and [[pasteurization]]. More sophisticated concentrating technologies magnify the rays of the Sun for high-temperature material testing, metal [[smelting]] and industrial [[chemical]] production. A range of [[prototype]] solar vehicles provide ground, air and sea transportation.
== Energy from the Sun ==
{{main|Insolation|Solar radiation}}
[[Image:Breakdown of the incoming solar energy.svg|thumb|left|About half the incoming solar energy is absorbed by water and land; the rest is reradiated back into space.]]
[[Image:Solar land area.png|thumb|right|Average [[insolation]] showing land area (small black dots) required to replace the total world energy supply with solar electricity]]
Earth receives 174 [[Orders of magnitude (power)#petawatt (1015 watts)|PW]] of incoming solar radiation ([[insolation]]) at the upper [[Earth's atmosphere|atmosphere]].<ref name="Smil 1991">Smil (1991), p. 240</ref> Approximately 30% is reflected back to space while the rest is absorbed by the atmosphere, oceans and land masses. After passing through the atmosphere, the insolation [[electromagnetic spectrum|spectrum]] is mostly split between the [[visible light|visible]] and [[infrared]] ranges with a small part in the ultraviolet.<ref>{{cite web
| title=Natural Forcing of the Climate System
| publisher=Intergovernmental Panel on Climate Change
| url=http://www.grida.no/climate/ipcc_tar/wg1/041.htm#121
| accessdate=2007-09-29}}</ref>
The absorption of solar energy by atmospheric convection ([[sensible heat]] transport) and evaporation and condensation of water vapor ([[latent heat]] transport) powers the [[water cycle]] and drives the [[wind]]s.<ref>{{cite web
| title=Radiation Budget
| date=2006-10-17
| publisher=NASA Langley Research Center
| url=http://marine.rutgers.edu/mrs/education/class/yuri/erb.html
| accessdate=2007-09-29}}</ref> Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 [[Celsius|°C]].<ref>{{cite web
| author=Somerville, Richard
| title=Historical Overview of Climate Change Science
| url=http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_Ch01.pdf
| publisher=Intergovernmental Panel on Climate Change
| accessdate=2007-09-29}}</ref> The conversion of solar energy into chemical energy via [[photosynthesis]] produces food, wood and the [[biomass]] from which fossil fuels are derived.<ref>{{cite web
| author=Vermass, Wim
| title=An Introduction to Photosynthesis and Its Applications
| publisher=Arizona State University
| url=http://photoscience.la.asu.edu/photosyn/education/photointro.html
| accessdate=2007-09-29}}</ref>
[[Solar radiation]] along with secondary solar resources such as [[wind power|wind]] and [[wave power]], [[hydroelectricity]] and [[biomass]] account for over [[Earth's energy budget|99.9%]] of the available flow of [[renewable energy]] on Earth.<ref>Scheer (2002), p. 8</ref><ref>{{cite web
| author=Plambeck, James
| title=Energy on a Planetary Basis
| publisher=University of Alberta
| url=http://www.ualberta.ca/~jplambec/che/p101/p01264.htm
| accessdate=2008-05-21}}</ref> The flows and stores of solar energy in the environment are vast in comparison to current human energy needs.
<!-- See Image:Breakdown of the incoming solar energy.jpg, and [[Talk:Solar energy#Energy from the Sun]] -->
* The total solar energy absorbed by Earth's atmosphere, oceans and land masses is approximately 3,850 [[joule#SI multiples|zettajoules]] (ZJ) per year.<ref>Smil (2006), p. 12</ref><!-- Smil quotes an absorbed solar flux of 122 PW. Multiplying this number by the number of seconds in a year yields 3,850 ZJ. -->
* Global wind energy at 80 m <!--at locations with mean annual wind speeds ≥ 6.9 m/s-->is estimated at 2.25 ZJ per year.<ref>{{cite web
| author=Archer, Cristina
| coauthor=Jacobson, Mark
| title=Evaluation of Global Wind Power
| publisher=Stanford
| url=http://www.stanford.edu/group/efmh/winds/global_winds.html
| accessdate=2008-06-03}}</ref>
* Photosynthesis captures approximately 3 ZJ per year in biomass.<ref>{{cite web
| publisher=Food and Agriculture Organization of the United Nations
| url=http://www.fao.org/docrep/w7241e/w7241e06.htm#TopOfPage
| title=Energy conversion by photosynthetic organisms
| accessdate=2008-05-25}}</ref>
* Worldwide electricity consumption was approximately 0.0567 ZJ<!-- converted from 15,746.54 Billion Kilowatthours --> in 2005.<ref>{{cite web
| publisher=[[Energy Information Administration]]
| url=http://www.eia.doe.gov/iea/elec.html
| title=World Total Net Electricity Consumption, 1980-2005
| accessdate=2008-05-25}}</ref>
* Worldwide [[primary energy]] consumption was 0.487 ZJ in 2005.<ref>{{cite web
| publisher=[[Energy Information Administration]]
| url=http://www.eia.doe.gov/pub/international/iealf/table18.xls
| title=World Consumption of Primary Energy by Energy Type and Selected Country Groups, 1980-2004
| accessdate=2008-05-17}}</ref><!-- converted from 462 quadrillion BTUs -->
== Applications of solar energy technology ==
[[Image:Solar Spectrum.png|thumb|left|Solar radiation spectrum]]
Solar energy technologies use [[solar radiation]] for practical ends. Technologies that use secondary solar resources such as biomass, wind, waves and ocean thermal gradients can be included in a broader description of solar energy but only primary resource applications are discussed here. Because the performance of solar technologies varies widely between regions, solar technologies should be deployed in a way that carefully considers these variations.<ref>Butti and Perlin (1981), p. 15</ref>
Solar technologies such as photovoltaics and water heaters increase the supply of energy and may be characterized as supply side technologies.{{Fact|date=June 2008}} Technologies such as passive design and shading devices reduce the need for alternate resources and may be characterized as demand side. Optimizing the performance of solar technologies is often a matter of controlling the resource rather than simply maximizing its collection.{{Fact|date=June 2008}}
=== Architecture and urban planning ===
{{main|Passive solar building design|Urban heat island}}
[[Image:Technische Universität Darmstadt - Solar Decathlon 2007.jpg|thumb|left|[[Darmstadt University of Technology]] won the 2007 [[Solar Decathlon]] with this passive house designed specifically for the humid and hot subtropical climate in Washington, D.C.<ref>{{cite web
| title=Darmstadt University of Technology solar decathlon home design
| publisher=Darmstadt University of Technology
| url=http://www.solardecathlon.de/index.php/our-house/the-design
| accessdate=2008-04-25}}</ref>]]
Sunlight has influenced building design since the beginning of architectural history.<ref name="Schittich 2003">Schittich (2003), p. 14</ref> Fully developed solar architecture and urban planning methods were first employed by the [[ancient Greece|Greeks]] and [[Feng shui#Archaeology|Chinese]] who oriented their buildings toward the south to provide light and warmth.<ref>Butti and Perlin (1981), p. 4, 159</ref>
The elemental features of [[passive solar]] architecture are Sun orientation, compact proportion (a low surface area to volume ratio), selective shading (overhangs) and [[Thermal mass (Building)|thermal mass]].<ref name="Schittich 2003"/> When these features are tailored to the local climate and environment they can produce well-lit spaces that stay in a comfortable temperature range. [[Socrates|Socrates']] Megaron House is a classic example of passive solar design.<ref name="Schittich 2003"/> The most recent approaches to solar design use computer modeling to tie together [[daylighting|solar lighting]], [[solar heating|heating]] and [[solar air conditioning|ventilation]] systems in an integrated [[solar design]] package. [[Active solar]] equipment such as pumps, fans and switchable windows can also complement passive design and improve system performance.
Urban heat islands (UHI) are metropolitan areas with higher temperatures than the surrounding environment. These higher temperatures are the result of urban materials such as asphalt and concrete that have lower [[albedo]]s and higher [[heat capacity|heat capacities]] than the natural environment. A straightforward method of counteracting the UHI effect is to paint buildings and roads white and plant trees. Using these methods, a hypothetical "cool communities" program in [[Los Angeles]] has projected that urban temperatures could be reduced by approximately 3 °C at an estimated cost of US$1 billion, giving estimated total annual benefits of US$530 million from reduced air-conditioning costs and healthcare savings.<ref name="Heat Islands">{{cite web
| author=Rosenfeld, Arthur
| coauthors=Romm, Joseph
| coauthors=Akbari, Hashem
| coauthors=Lloyd, Alan
| title=Painting the Town White -- and Green
| publisher=Heat Island Group
| url=http://eetd.lbl.gov/HeatIsland/PUBS/PAINTING/
| accessdate=2007-09-29}}</ref>
=== Agriculture and horticulture ===
{{main|Agriculture|Horticulture|Greenhouse}}
[[Image:Westland kassen.jpg|thumb|right|250px|Greenhouses like these in the Netherland's Westland municipality grow a wide variety of vegetables, fruits and flowers.]]
Agriculture inherently seeks to optimize the capture of solar energy, and thereby plant productivity. Techniques such as timed planting cycles, tailored row orientation, staggered heights between rows and the mixing of plant varieties can improve crop yields.<ref>{{cite web
| title=Row Spacing, Plant Population, and Yield Relationships
| author=Jeffrey C. Silvertooth
| publisher=University of Arizona
| url=http://ag.arizona.edu/crop/cotton/comments/april1999cc.html
| accessdate=2008-06-24}}</ref><ref>Kaul (2005), p. 169–174</ref> While sunlight is generally considered a plentiful resource, there are exceptions which highlight the importance of solar energy to agriculture. During the short growing seasons of the [[Little Ice Age]], French and [[Solar power in the United Kingdom|English]] farmers employed fruit walls to maximize the collection of solar energy. These walls acted as thermal masses and accelerated ripening by keeping plants warm. Early fruit walls were built perpendicular to the ground with a south facing orientation but over time sloping walls were developed to make better use of sunlight. In 1699, [[Nicolas Fatio de Duillier]] even suggested using a [[Solar tracker|tracking mechanism]] which could pivot to follow the Sun.<ref>Butti and Perlin (1981), p. 42–46</ref> Solar energy is also used in many areas of agriculture aside from growing crops. Applications include pumping water, drying crops, brooding chicks and drying chicken manure.<ref>Bénard (1981), p. 347</ref><ref name="Leon 2006">Leon (2006), p. 62</ref>
[[Greenhouse]]s control the use of solar heat and light to grow plants in enclosed environments, enabling year-round production and the growth of specialty crops and other plants not naturally suited to the local climate. Primitive greenhouses were first used during Roman times to grow cucumbers year-round for the Roman emperor [[Tiberius]].<ref>Butti and Perlin (1981), p. 19</ref> The first modern greenhouses were built in [[Europe]] in the 16th century to conserve exotic plants brought back from explorations abroad.<ref>Butti and Perlin (1981), p. 41</ref> Greenhouses remain an important part of horticulture today, while plastic transparent materials have also been used to similar effect in [[polytunnel]]s and [[row cover]]s.
=== Solar lighting ===
[[Image:PantheonOculus.01.jpg|thumb|left|Daylighting features such as this [[oculus]] at the top of the Pantheon in Rome have been in use since antiquity.]]
The history of lighting is dominated by the use of natural light. The Romans recognized the [[Right to Light]] as early as the [[Corpus Juris Civilis|6th century]] and English law echoed these judgments with the Prescription Act of 1832.<ref>{{cite web
| title=Prescription Act (1872 Chapter 71 2 and 3 Will 4)
| publisher=Office of the Public Sector Information
| url=http://www.opsi.gov.uk/RevisedStatutes/Acts/ukpga/1832/cukpga_18320071_en_1
| accessdate=2008-05-18}}</ref><ref>{{cite news
| author=Noyes, WM
| title=The Law of Light
| work = The New York Times
| date=1860-03-31
| url=http://query.nytimes.com/mem/archive-free/pdf?_r=1&res=9503E1D81E30EE34BC4950DFB566838B679FDE&oref=slogin
| accessdate=2008-05-18}}</ref> In the 20th century artificial [[lighting]] became the main source of interior illumination.
[[Daylighting]] systems collect and distribute sunlight to provide interior illumination. These systems directly offset energy use by replacing artificial lighting, and indirectly offset non-solar energy use by reducing the need for [[HVAC#Air-conditioning|air-conditioning]].<ref name="Tzempelikos 2007">Tzempelikos (2007), p. 369</ref> Although difficult to quantify, the use of [[Sunlight#Effects on health|natural lighting]] also offers physiological and psychological benefits compared to [[lighting#Health effects|artificial lighting]].<ref name="Tzempelikos 2007"/> Daylighting design carefully selects window type, size and orientation and may also consider exterior shading devices. Individual features include sawtooth roofs, [[Clerestory|clerestory windows]], light shelves, [[skylight]]s and [[light tube]]s. These features may be incorporated into existing structures but are most effective when integrated in a [[Passive solar building design|solar design]] package that accounts for factors such as [[Light pollution#Glare|glare]], heat flux and [[Electricity meter#Time of use metering|time-of-use]]. When daylighting features are properly implemented they can reduce commercial lighting-related energy requirements by 25%.<ref name="ASHRAE windows">{{cite web
| author=Apte, J. et al.
| title=Future Advanced Windows for Zero-Energy Homes
| publisher=American Society of Heating, Refrigerating and Air-Conditioning Engineers
| url=http://windows.lbl.gov/adv_Sys/ASHRAE%20Final%20Dynamic%20Windows.pdf
| accessdate=2008-04-09}}</ref>
Hybrid solar lighting (HSL) is an [[active solar]] method of using sunlight to provide illumination. HSL systems collect sunlight using focusing mirrors that [[Solar tracker|track the Sun]] and use [[optical fiber]]s to transmit the light into a building's interior to supplement conventional lighting. In single-story applications, these systems are able to transmit 50% of the direct sunlight received.<ref name="hybrid lighting">{{cite web
| author=Muhs, Jeff
| title=Design and Analysis of Hybrid Solar Lighting and Full-Spectrum Solar Energy Systems
| publisher=Oak Ridge National Laboratory
| url=http://www.ornl.gov/sci/solar/pdfs/Muhs_ASME_Paper.pdf
| accessdate=2007-09-29}}</ref>
Although [[daylight saving time]] is promoted as a way to use sunlight to save energy, recent research is limited and reports contradictory results: several studies report savings, but just as many suggest no effect or even a net loss, particularly when [[gasoline]] consumption is taken into account. Electricity use is greatly affected by geography, climate and economics, making it hard to generalize from single studies.<ref>{{cite journal |journal= Energy Policy |date=2008 |volume=36 |issue=6 |pages=1858–1866 |title= Effect of daylight saving time on lighting energy use: a literature review |author= Myriam B.C. Aries; Guy R. Newsham |doi=10.1016/j.enpol.2007.05.021}}</ref>
=== Solar thermal ===
{{Main|Solar thermal energy}}
Solar thermal technologies can be used for water heating, space heating, space cooling and process heat generation.<ref>{{cite web
| title=Solar Energy Technologies and Applications
| publisher=Canadian Renewable Energy Network
| url=http://www.canren.gc.ca/tech_appl/index.asp?CaId=5&PgId=121
| accessdate=2007-10-22}}</ref>
==== Water heating ====
{{main|Solar hot water|Solar combisystem}}
[[Image:Twice Cropped Zonnecollectoren.JPG|thumb|right|Solar water heaters face the equator and are angled according to latitude to maximize solar gain.]]
Solar hot water systems use sunlight to heat water. When sited in low latitudes (below 40 degrees), solar heating system can provide around 60 to 70% of domestic hot water use with temperatures up to 60 °C.<ref>{{cite web
| title=Renewables for Heating and Cooling
| publisher=International Energy Agency
| url=http://www.iea.org/textbase/nppdf/free/2007/Renewable_Heating_Cooling.pdf
| accessdate=2008-05-26}}</ref> The most common types of solar water heaters are evacuated tube collectors (44%) and glazed flat plate collectors (34%) generally used for domestic hot water; and unglazed plastic collectors (21%) used mainly to heat swimming pools.<ref>{{cite web
| title=Solar Heat Worldwide (Markets and Contributions to the Energy Supply 2005)
| publisher=International Energy Agency
| author=Weiss, Werner
| coauthor=Bergmann, Irene
| coauthor=Faninger, Gerhard
| url=http://www.iea-shc.org/publications/statistics/IEA-SHC_Solar_Heat_Worldwide-2007.pdf
| accessdate=2008-05-30}}</ref>
As of 2007, the total installed capacity of solar hot water systems is approximately 154 [[watt#SI multiples|GW]].<ref name="SWH 2008"/> [[China]] is the world leader in the deployment of solar hot water with 70 GW installed as of 2006 and a long term goal of 210 GW by 2020.<ref name="Renewables 2007">{{cite web
| title=Renewables 2007 Global Status Report
| publisher=Worldwatch Institute
| url=http://www.ren21.net/pdf/RE2007_Global_Status_Report.pdf
| accessdate=2008-04-30}}</ref> Israel is the per capita leader in the use of solar hot water with 90% of homes using this technology.<ref name="Environment California SWH">{{cite web
| author=Del Chiaro, Bernadette
| coauthor= Telleen-Lawton, Timothy
| title=Solar Water Heating (How California Can Reduce Its Dependence on Natural Gas)
| publisher=Environment California Research and Policy Center
| url=http://www.environmentcalifornia.org/uploads/at/56/at563bKwmfrtJI6fKl9U_w/Solar-Water-Heating.pdf
| accessdate=2007-09-29}}</ref> In the United States, [[Canada]] and [[Australia]], heating swimming pools is the dominant application of solar hot water, with an installed capacity of 18 GW as of 2005.<ref name="IEA Solar Thermal">{{cite web
| author=Philibert, Cédric
| title=The Present and Future use of Solar Thermal Energy as a Primary Source of Energy
| publisher=International Energy Agency
| url=http://www.iea.org/textbase/papers/2005/solarthermal.pdf
| accessdate=2008-05-05}}</ref>
==== Heating, cooling and ventilation ====
{{main|Solar heating|Thermal mass|Solar chimney|Solar air conditioning}}
[[Image:Flipped MIT Solar One house.png|left|thumb|MIT's Solar House #1, built in 1939, used [[seasonal thermal storage]] for year-round heating.]]
In the United States, [[HVAC|heating, ventilation and air conditioning]] (HVAC) systems account for 30% (4.65 EJ) <!--converted from 30% of 14.7 quads: 1.055 EJ/quad x 14.7 quad x 30%-->of the energy used in commercial buildings and nearly 50% (10.1 EJ) <!--source quotes residential HVAC energy usage of 10.1 EJ and total energy use of 20.3 EJ-->of the energy used in residential buildings.<ref>{{cite web
| title=Energy Consumption Characteristics of Commercial Building HVAC Systems Volume III: Energy Savings Potential
| publisher=United States Department of Energy
| url=http://www.doas-radiant.psu.edu/DOE_report.pdf
| accessdate=2008-06-24
| pages=2-2}}</ref><ref name="ASHRAE windows"/> Solar heating, cooling and ventilation technologies can be used to offset a portion of this energy.
Thermal mass, in the most general sense, is any material that has the capacity to store heat. In the context of solar energy, thermal mass materials are used to store heat from the Sun. Common thermal mass materials include stone, cement and water. These materials have historically been used in arid climates or warm temperate regions to keep buildings cool by absorbing solar energy during the day and radiating stored heat to the cooler atmosphere at night, but they can also be used in cold temperate areas to maintain warmth. The size and placement of thermal mass should consider several factors such as climate, daylighting and shading conditions. When properly incorporated, thermal mass maintains space temperatures in a comfortable range and reduces the need for auxiliary heating and cooling equipment.<ref>Mazria(1979), p. 29–35</ref>
A solar chimney (or thermal chimney) is a passive solar ventilation system composed of a vertical shaft connecting the interior and exterior of a building. As the chimney warms, the air inside is heated causing an [[updraft]] that pulls air through the building. Performance can be improved by using glazing and thermal mass materials in a way that mimics greenhouses. These systems have been in use since Roman times and remain common in the Middle East.{{Fact|date=June 2008}}
[[Deciduous]] trees and plants have often been promoted as a means of controlling solar heating and cooling. When planted on the southern side of a building, the leaves provide shade during the summer while the bare limbs allow light and warmth to pass during the winter.<ref>Mazria(1979), p. 255</ref> It should be noted that even bare, leafless trees shade 1/3 to 1/2 of the incident solar radiation; therefore, the benefits of summer shading will compete against a loss of winter heating.<ref>Balcomb(1992), p. 56</ref> In climates with significant heating loads, deciduous trees should not be planted on the southern side of a building because they will interfere with winter solar availability but they can be used on the east and west sides to provide a degree of summer shading without appreciably affecting winter solar gain.<ref>Balcomb(1992), p. 57</ref>
==== Desalination and disinfection ====
{{main|Solar still|Solar water disinfection|Desalination}}
[[Image:Indonesia-sodis-gross.jpg|thumb|right|A SODIS application in Indonesia demonstrates the simplicity of this approach to water disinfection.]]
Solar [[distillation]] is the production of potable water from [[saline water|saline]] or [[brackish water]] using solar energy. The first recorded use was by 16th century Arab alchemists.<ref name="Tiwari 2003">Tiwari (2003), p. 368–371</ref> The first large-scale solar distillation project was constructed in 1872 in the [[Chile]]an mining town of Las Salinas.<ref name ="Daniels 1964">Daniels (1964), p. 6</ref> This 4,700 m² [[still]] could produce up to 22,700 [[liters|L]] per day and operated for 40 years.<ref name ="Daniels 1964"/> Individual still designs include single-slope, double-slope (or greenhouse type), vertical, conical, inverted absorber, multi-wick and multiple effect.<ref name="Tiwari 2003"/> These stills can operate in passive, active or hybrid modes. Double slope stills are the most economical for decentralized domestic purposes while active multiple effect units are more suitable to large-scale applications.<ref name="Tiwari 2003"/>
Solar water disinfection (SODIS) is a method of [[disinfection|disinfecting]] water by exposing water-filled plastic [[polyethylene terephthalate|PET]] bottles to several hours of sunlight.<ref>{{cite web
| title=SODIS solar water disinfection
| publisher=EAWAG (The Swiss Federal Institute for Environmental Science and Technology)
| url=http://www.sodis.ch
| accessdate=2008-05-02}}</ref> Exposure times vary according weather and climate from a minimum of six hours to two days during fully overcast conditions.<ref name="SODIS CDC">{{cite web
| title=Household Water Treatment Options in Developing Countries: Solar Disinfection (SODIS)
| publisher=Centers for Disease Control and Prevention
| url=http://www.ehproject.org/PDF/ehkm/cdc-options_sodis.pdf
| accessdate=2008-05-13}}</ref> SODIS is recommended by the [[World Health Organization]] as a viable method for household water treatment and safe storage.<ref>{{cite web
| title=Household Water Treatment and Safe Storage
| publisher=World Health Organization
| url=http://www.who.int/household_water/en/
| accessdate=2008-05-02}}</ref> Over two million people in developing countries use SODIS for their daily drinking water needs.<ref name="SODIS CDC"/>
==== Cooking ====
{{main|Solar cooker}}
[[Image:Auroville Solar Bowl.JPG|left|thumb|The Solar Bowl in Auroville, India, concentrates sunlight on a movable receiver to produce steam for cooking.]]
Solar cookers use sunlight for cooking, drying and [[pasteurization]]. These devices can be grouped into three broad categories: box cookers, panel cookers and reflector cookers.<ref>Anderson and Palkovic (1994), p. xi</ref> The simplest type of solar cooker is the box cooker first built by [[Horace de Saussure]] in 1767.<ref>Butti and Perlin (1981), p. 54–59</ref> A basic box cooker consists of an insulated container with a transparent lid. These cookers can be used effectively with partially overcast skies and will typically reach temperatures of 90–150 °C.<ref>Anderson and Palkovic (1994), p. xii</ref> Panel cookers use a reflective panel to direct sunlight onto an insulated container and reach temperatures comparable to box cookers. Reflector cookers use various concentrating geometries (dish, trough, Fresnel mirrors) to focus light on a cooking container. These cookers reach temperatures of 315 °C and above but require direct light to function properly and must be repositioned to track the Sun.<ref>Anderson and Palkovic (1994), p. xiii</ref>
The solar bowl is a unique concentrating technology employed by the Solar Kitchen in [[Auroville]], India. The solar bowl is a stationary spherical reflector that focuses light along a line perpendicular to the sphere's interior surface and a computer control system moves the receiver to intersect this line. Steam is produced in the receiver at temperatures reaching 150 °C and then used for process heat in the kitchen.<ref>{{cite web
| title=The Solar Bowl
| publisher=Auroville Universal Township
| url=http://www.auroville.org/research/ren_energy/solar_bowl.htm
| accessdate=2008-04-25}}</ref>
A reflector developed by [[Wolfgang Scheffler]] in 1986 is used in many solar kitchens. Scheffler reflectors are flexible parabolic dishes that combine aspects of trough and power tower concentrators. [[Solar tracker#Polar|Polar tracking]] is used to follow the Sun's daily course and the curvature of the reflector is adjusted for seasonal variations in the incident angle of sunlight. These reflectors can reach temperatures of 450-650 °C and have a fixed focal point which improves the ease of cooking.<ref>{{cite web
| title=Scheffler-Reflector
| publisher=Solare Bruecke
| url=http://www.solare-bruecke.org/English/scheffler_e-Dateien/scheffler_e.htm
| accessdate=2008-04-25}}</ref> The world's largest Scheffler reflector system in Abu Road, [[Rajasthan]], India is capable of cooking up to 35,000 meals a day.<ref>{{cite web
| title=Solar Steam Cooking System
| publisher=Gadhia Solar
| url=http://gadhia-solar.com/products/steam.htm
| accessdate=2008-04-25}}</ref> [[As of 2008]], over 2,000 large Scheffler cookers had been built worldwide.<ref>{{cite web
| title=Scheffler Reflector
| publisher=Solare Bruecke
| url=http://www.solare-bruecke.org/infoartikel/info_vorstand.htm#english
| accessdate=2008-07-03}}</ref>
==== Process heat ====
{{main|Solar pond|Salt evaporation pond|Solar furnace}}
[[Image:7 Meter Sheet Metal Dishes (Flipped).png|right|thumb|200px|STEP parabolic dishes used for steam production and electrical generation]]
Concentrating solar technologies such as parabolic dish, trough and Scheffler reflectors can provide process heat for commercial and industrial applications. The first commercial system was the Solar Total Energy Project (STEP) in [[Shenandoah]], Georgia where a field of 114 parabolic dishes provided 50% of the process heating, air conditioning and electrical requirements for a clothing factory.<ref>{{cite web
| title=Solar total energy project at Shenandoah, Georgia system design
| author=Poche, A.
| publisher=SAO/NASA ADS Physics Abstract Service
| url=http://adsabs.harvard.edu/abs/1980fsdr.proc..251P
| accessdate=2008-05-20}}</ref> This cogeneration system generated 400 kW of electricity and 3 MW of thermal energy in the form of steam, and had a thermal storage system that allowed for peak-load shaving.{{Fact|date=June 2008}}
Evaporation ponds are shallow pools that concentrate dissolved solids through [[evaporation]]. The use of evaporation ponds to obtain salt from sea water is one of the oldest applications of solar energy. Modern uses include concentrating brine solutions used in leach mining and removing dissolved solids from waste streams.<ref>Bartlett (1998), p.393–394</ref>
[[Clothes line]]s, [[clotheshorse]]s, and clothes racks dry clothes through evaporation. These devices use wind and sunlight instead of electricity or natural gas. [[Florida]] legislation specifically protects the 'right to dry' and similar solar rights legislation has been passed in [[Utah]] and [[Hawaii]].<ref>{{cite web
| title=Right to Dry Legislation in New England and Other States
| publisher=Connecticut General Assembly
| author=Thomson-Philbrook, Julia
| url=http://www.cga.ct.gov/2008/rpt/2008-R-0042.htm
| accessdate=2008-05-27}}</ref>
Unglazed transpired collectors (UTC) are perforated sun-facing walls used for preheating ventilation air. UTCs can raise the incoming air temperature up to 22 °C and deliver outlet temperatures of 45-60 °C.<ref name="UTC">{{cite web
| title=Solar Buildings (Transpired Air Collectors - Ventilation Preheating)
| publisher=National Renewable Energy Laboratory
| url=http://www.nrel.gov/docs/fy06osti/29913.pdf
| accessdate=2007-09-29}}</ref> The short payback period of transpired collectors (3 to 12 years) makes them a more cost-effective alternative than glazed collection systems.<ref name="UTC"/> As of 2003, over 80 systems with a combined collector area of 35,000 [[Square metre|m²]] had been installed worldwide, including an 860 m² collector in [[Costa Rica]] used for drying coffee beans and a 1,300 m² collector in [[Coimbatore]], [[India]] used for drying marigolds.<ref name="Leon 2006"/>
=== Solar electricity ===
Sunlight can be converted into electricity using photovoltaics (PV), concentrating solar power (CSP), and various experimental technologies. PV has mainly been used to power small and medium-sized applications, from the [[calculator]] powered by a single solar cell to off-grid homes powered by a [[photovoltaic array]]. For large-scale generation, CSP plants like [[SEGS]] have been the norm but recently multi-megawatt PV plants are becoming common. Completed in 2007, the 14 MW power station in [[Clark County, Nevada|Clark County]], [[Nevada]] and the 20 MW site in Beneixama, Spain are characteristic of the trend toward larger [[photovoltaic power stations]] in the US and Europe.<ref>{{cite web
| title=Large-scale photovoltaic power plants
| publisher=pvresources
| url=http://www.pvresources.com/en/top50pv.php
| accessdate=2008-06-27}}</ref>
==== Photovoltaics ====
{{main|Photovoltaics}}
[[Image:ISS on 20 August 2001.jpg|thumb|right|Solar cells power the [[International Space Station]].]]
A [[solar cell]] (or photovoltaic cell) is a device that converts light into [[direct current]] using the [[photoelectric effect]]. The first solar cell was constructed by [[Charles Fritts]] in the 1880s.<ref> Perlin (1999), p. 147</ref> Although the prototype [[selenium]] cells converted less than 1% of incident light into electricity, both [[Ernst Werner von Siemens]] and [[James Clerk Maxwell]] recognized the importance of this discovery.<ref> Perlin (1999), p. 18–20</ref> 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.<ref> Perlin (1999), p. 29</ref> These early solar cells cost 286 USD/watt and reached efficiencies of 4.5-6%.<ref> Perlin (1999), p. 29–30, 38</ref>
The earliest significant application of solar cells was as a back-up power source to the [[Vanguard I]] satellite, which allowed the satellite to continue transmitting for over a year after its chemical battery was exhausted.<ref> Perlin (1999), p. 45–46</ref> The successful operation of solar cells on this mission was duplicated in many other [[Soviet Union|Soviet]] and [[USA|American]] satellites, and by the late 1960s PV had become the established source of power for satellites.<ref> Perlin (1999), p. 49–50</ref> Photovoltaics went on to play an essential part in the success of early commercial satellites such as [[Telstar]] and continue to remain vital to the telecommunications infrastructure today.<ref> Perlin (1999), p. 49–50, 190</ref>
The high cost of solar cells limited terrestrial uses throughout the 1960s. This changed in the early 1970s when prices reached levels that made PV generation competitive in remote areas without [[power grid|grid]] access. Early terrestrial uses included powering telecommunication stations, off-shore [[oil rig]]s, [[Buoy|navigational buoys]] and railroad crossings.<ref> Perlin (1999), p. 57–85</ref> These and other [[off-the-grid|off-grid]] applications have proven very successful and accounted for over half of worldwide installed capacity until 2004.<ref name="Renewables 2007"/>
[[Image:Tuebingen-friedenskirche.jpg||thumb|left|[[Building-integrated photovoltaics]] cover the roofs of an increasing number of homes.]]
The [[1973 oil crisis]] stimulated a rapid rise in the production of PV during the 1970s and early 1980s.<ref>{{cite web
| title=Photovoltaic Milestones
| publisher=Energy Information Agency - Department of Energy
| url=http://www.eia.doe.gov/cneaf/solar.renewables/renewable.energy.annual/backgrnd/chap11i.htm
| accessdate=2008-05-20}}</ref> [[Economies of scale]] which resulted from increasing production along with improvements in system performance brought the price of PV down from 100 USD/watt in 1971 to 7 USD/watt in 1985.<ref> Perlin (1999), p. 50, 118</ref> Steadily falling oil prices during the early 1980s led to a reduction in funding for photovoltaic R&D and a discontinuation of the tax credits associated with the [[Energy Tax Act]] of 1978. These factors moderated growth to approximately 15% per year from 1984 through 1996.<ref name="Earth Policy Institute">{{cite web
| title=World Photovoltaic Annual Production, 1971-2003
| publisher=Earth Policy Institute
| url=http://www.earth-policy.org/Indicators/2004/indicator12_data.htm
| accessdate=2008-05-29}}</ref>
Since the mid-1990s, leadership in the PV sector has shifted from the US to [[Japan]] and [[Germany]]. Between 1992 and 1994 Japan increased R&D funding, established [[net metering]] guidelines, and introduced a subsidy program to encourage the installation of residential PV systems.<ref name="EIA Non-hydro">{{cite web
| title=Policies to Promote Non-hydro Renewable Energy in the United States and Selected Countries
| publisher=Energy Information Agency - Department of Energy
| url=http://tonto.eia.doe.gov/ftproot/features/nonhydrorenewablespaper_final.pdf
| accessdate=2008-05-29}}</ref> As a result, PV installations in the country climbed from 31.2 MW in 1994 to 318 MW in 1999,<ref>{{cite web
| title=Japan Pholtovoltaics Market Overview
| author=Foster, Robert
| publisher=Department of Energy
| url=http://solar.nmsu.edu/publications/Japan%20Report.pdf
| accessdate=2008-06-05}}</ref> and worldwide production growth increased to 30% in the late 1990s.<ref>{{cite web
| title=An Experience Curve Based Model for the Projection of PV Module Costs and Its Policy Implications
| publisher=Heliotronic
| author=Handleman, Clayton
| url=http://www.heliotronics.com/papers/PV_Breakeven.pdf
| accessdate=2008-05-29}}</ref>
Germany has become the leading PV market worldwide since revising its [[Feed-in tariffs in Germany|Feed-in tariff]] system as part of the Renewable Energy Sources Act. Installed PV capacity has risen from 100 MW in 2000 to approximately 4,150 MW at the end of 2007.<ref>{{cite web
| title=Renewable energy sources in figures - national and international development
| publisher=Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (Germany)
| url=http://www.bmu.de/files/english/renewable_energy/downloads/application/pdf/broschuere_ee_zahlen_en.pdf
| accessdate=2008-05-29}}</ref><ref>{{cite web
| title=Marketbuzz 2008: Annual World Solar Pholtovoltaic Industry Report
| publisher=solarbuzz
| url=http://www.solarbuzz.com/Marketbuzz2008-intro.htm
| accessdate=2008-06-05}}</ref> Spain has become the third largest PV market after adopting a similar feed-in tariff structure in 2004, while France, Italy, South Korea and the US have also seen rapid growth recently due to various incentive programs and local market conditions.<ref>{{cite web
| title=Trends in Photovoltaic Applications - Survey report of selected IEA countries between 1992 and 2006
| publisher=International Energy Agency
| url=http://www.iea-pvps.org/products/download/rep1_16.pdf
| accessdate=2008-06-05}}</ref>
==== Concentrating solar power ====
{{main|Concentrating solar energy}}
[[Image:Dish Stirling Systems of SBP in Spain.JPG|right|thumb|Dish engine systems eliminate the need to transfer heat to a boiler by placing a [[Stirling engine]] at the focal point.]]
Concentrating Solar Power (CSP) systems is divided into [[Concentrating solar thermal]] (CST) and [[Concentrating PV]] (CPV). CSP use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated light is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exist; the most developed are the solar trough, parabolic dish and solar power tower. These methods vary in the way they track the Sun and focus light. In all these systems a [[working fluid]] is heated by the concentrated sunlight, and is then used for power generation or energy storage.<ref name="Martin 2005"> Martin and Goswami (2005), p. 45</ref>
[[Image:PS10 solar power tower.jpg|thumb|left|The [[PS10 solar power tower|PS10]] concentrates sunlight from a field of heliostats on a central tower.]]
A solar trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The reflector is made to follow the Sun during the daylight hours by tracking along a single axis. Trough systems are the most mature CSP technology. The [[Solar Energy Generating Systems|SEGS]] plants in California and Acciona's [[Nevada Solar One]] near [[Boulder City, Nevada]] are representatives of this technology.<ref name="SolarPaces 2001">{{cite web
| title=Concentrating Solar Power in 2001 - An IEA/SolarPACES Summary of Present Status and Future Prospects
| publisher=International Energy Agency - SolarPACES
| url=http://www.solarpaces.org/Library/docs/CSP_Brochure_2001.pdf
| accessdate=2008-07-02}}</ref><ref>{{cite web
| title=UNLV Solar Site
| publisher=University of Las Vegas
| url=http://www.solar.unlv.edu/projects/eldorado.php
| accessdate=2008-07-02}}</ref>
==== Experimental solar power ====
{{main|Solar updraft tower|Solar pond|Thermogenerator}}
[[Image:Suntower.jpg|thumb|An artist's depiction of a solar satellite, which could [[wireless energy transfer|send energy wirelessly]] to a space vessel or planetary surface.]]
A solar updraft tower (also known as a solar chimney or solar tower) consists of a large greenhouse that funnels into a central tower. As sunlight shines on the greenhouse, the air inside is heated and expands. The expanding air flows toward the central tower where a turbine converts the air flow into electricity. A 50 kW prototype was constructed in [[Ciudad Real]], Spain and operated for eight years before decommissioning in 1989.<ref>Mills (2004), p. 19–31</ref>
A [[solar pond]] is a pool of salt water (usually 1-2 [[Meter|m]] deep) that collects and stores solar energy. Solar ponds were first proposed by Dr. Rudolph Bloch in 1948 after he came across reports of a lake in [[Hungary]] in which the temperature increased with depth. This effect was due to salts in the lake's water, which created a "density gradient" that prevented [[convection current]]s. A prototype was constructed in 1958 on the shores of the Dead Sea near [[Jerusalem]].<ref>Halacy (1973), p. 181</ref> The pond consisted of layers of water that successively increased from a weak salt solution at the top to a [[brine|high salt]] solution at the bottom. This solar pond was capable of producing temperatures of 90 °C in its bottom layer and had an estimated solar-to-electric efficiency of two percent.
[[Thermogenerator|Thermoelectric]] devices convert a temperature difference between dissimilar materials into an electric current. First proposed as a method to store solar energy by solar pioneer Mouchout in the 1800s,<ref>Perlin and Butti (1981), p. 73</ref> thermoelectrics reemerged in the Soviet Union during the 1930s. Under the direction of Soviet scientist [[Abram Ioffe]] a concentrating system was used to thermoelectrically generate power for a 1 [[horsepower|hp]] engine.<ref>Halacy (1973), p. 76</ref> Thermogenerators were later used in the US space program as an energy conversion technology for powering deep space missions such as [[Cassini–Huygens|Cassini]], [[Galileo (spacecraft)|Galileo]] and [[Viking program|Viking]]. Research in this area is focused on raising the efficiency of these devices from 7–8% to 15–20%.<ref name="Tritt">Tritt (2008), p. 366–368</ref>
[[Space solar power]] systems use a large solar array in geosynchronous orbit to collect sunlight and beam this energy in the form of microwave radiation to receivers ([[rectenna]]s) on Earth for distribution. This concept was first proposed by [[Peter Glaser|Dr. Peter Glaser]] in 1968 and since then a wide variety of systems have been studied with both photovoltaic and concentrating solar thermal technologies being proposed. Although still in the concept stage, these systems offer the possibility of delivering power approximately 96% of the time.<ref>{{cite web
| title=Space Solar Power Satellite Technology Development at the Glenn Research Center — An Overview
| publisher=National Aeronautics and Space Administration
| url=http://www.ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000084157_2000118199.pdf
| accessdate=2008-06-27}}</ref>
=== Solar chemical ===
{{main|Solar chemical}}
Solar chemical processes use solar energy to drive chemical changes. These processes offset energy that would otherwise be required from an alternate source and can convert solar energy into a storable and transportable fuel. Solar chemical reactions are diverse but can generically be described as either thermochemical or [[photochemical]].
[[Hydrogen production]] technologies have been a significant area of solar chemical research since the 1970s. Aside from electrolysis driven by photovoltaic or photochemical cells, several thermochemical processes have also been explored. The seemingly most direct of these routes uses concentrators to split water at high temperatures (2300-2600 °C), but this process has been limited by complexity and low solar-to-hydrogen efficiency (1-2%).<ref>Agrafiotis (2005), p. 409</ref> A more conventional approach uses process heat from solar concentrators to drive the [[steam reforming|steam reformation]] of natural gas thereby increasing the overall hydrogen yield. Thermochemical cycles characterized by the decomposition and regeneration of reactants present another avenue of hydrogen production. The Solzinc process under development at the [[Weizmann Institute of Science|Weizmann Institute]] is one such method. This process uses a 1 MW solar furnace to decompose [[zinc oxide]] (ZnO) at temperatures above 1200 °C. This initial reaction produces pure zinc which can subsequently be reacted with water to produce hydrogen.<ref>{{cite web
| title=Solar Energy Project at the Weizmann Institute Promises to Advance the use of Hydrogen Fuel
| publisher=Weizmann Institute of Science
| url=http://wis-wander.weizmann.ac.il/site/en/weizman.asp?pi=371&doc_id=4210
| accessdate=2008-06-25}}</ref>
[[Sandia National Laboratories|Sandia's]] Sunshine to Petrol (S2P) technology uses the high temperatures generated by concentrating sunlight along with a [[zirconia]]/[[ferrite]] catalyst to break down atmospheric carbon dioxide into oxygen and [[carbon monoxide]] (CO). The CO may then be used to synthesize fuels such as methanol, gasoline and jet fuel.<ref>{{cite web
| title=Sandia’s Sunshine to Petrol project seeks fuel from thin air
| publisher=Sandia Corporation
| url=http://www.sandia.gov/news/resources/releases/2007/sunshine.html
| accessdate=2008-05-02}}</ref>
Photoelectrochemical cells or PECs consist of a semiconductor, typically titanium dioxide or related titanates, immersed in an electrolyte. When the semiconductor is illuminated an electrical potential develops. There are two types of photoelectrochemical cells: photoelectric cells that convert light into electricity and photochemical cells that use light to drive chemical reactions such as [[electrolysis]].<ref name="Bolton">Bolton (1977), p. 11</ref>
A photogalvanic device is a type of battery in which the cell solution (or equivalent) forms energy-rich chemical intermediates when illuminated. These chemical intermediates then react at the electrodes to produce an electric potential. The ferric-thionine chemical cell is an example of this technology.<ref>Bolton (1977), p. 16, 119</ref>
=== Solar vehicles ===
{{main|Solar vehicle|Electric boat|Solar balloon}}
[[Image:Nuna3Team.JPG|thumb|left|Australia hosts the [[World Solar Challenge]] where solar cars like the Nuna3 race through a {{convert|3021|km|mi|abbr=on}} course from Darwin to Adelaide.]]
Development of a solar powered car has been an engineering goal since the 1980s. The center of this development is the [[World Solar Challenge]], a biannual solar-powered car race in which teams from universities and enterprises compete over {{convert|3021|km|mi|lk=on}} across central Australia from [[Darwin]] to [[Adelaide]]. In 1987, when it was founded, the winner's average speed was {{convert|67|km/h|mph|lk=on}}.<ref>{{cite web
| title=World Solar Challenge History
| publisher=Electrick Publications and NJK
| url=http://www.speedace.info/solar_racing_events/world_solar_challenge_history.htm
| accessdate=2008-06-24}}</ref> The 2007 race included a new challenge class using cars which could be a practical proposition for sustainable transport with little modification. The winning car averaged {{convert|90.87|km/h|mph|2}}.{{Fact|date=June 2008}} The [[North American Solar Challenge]] and the planned [[South African Solar Challenge]] are comparable competitions that reflect an international interest in the engineering and development of solar powered vehicles.<ref>{{cite web
| title=North American Solar Challenge
| publisher=New Resources Group
| url=http://americansolarchallenge.org/
| accessdate=2008-07-03}}</ref><ref>{{cite web
| title=South African Solar Challenge
| publisher=Advanced Energy Foundation
| url=http://www.solarchallenge.org.za/Default.aspx?AspxAutoDetectCookieSupport=1
| accessdate=2008-07-03}}</ref>
In 1975, the first practical solar boat was constructed in [[England]].<ref>''Electrical Review'' Vol 201 No 7 [[12 August]] [[1977]]</ref> By 1995, passenger boats incorporating PV panels began appearing and are now used extensively.<ref>{{cite web
| author=Schmidt, Theodor
| title=Solar Ships for the new Millennium
| publisher=TO Engineering
| url=http://www.umwelteinsatz.ch/IBS/solship2.html
| accessdate=2007-09-30}}</ref> In 1996, [[Kenichi Horie]] made the first solar powered crossing of the Pacific Ocean, and the ''sun21'' catamaran made the first solar powered crossing of the Atlantic Ocean in the winter of 2006–2007.<ref>{{cite web
| title=The sun21 completes the first transatlantic crossing with a solar powered boat
| publisher=Transatlantic 21
| url=http://www.transatlantic21.org/
| accessdate=2007-09-30}}</ref> Plans to circumnavigate the globe in 2009 are indicative of the progress solar boats have made.
[[Image:Helios in flight.jpg|thumb|right|Helios UAV in solar powered flight]]
In 1974, the unmanned ''Sunrise II'' inaugurated the era of solar flight. In 1980, the ''[[Gossamer Albatross#Solar-powered variants|Gossamer Penguin]]'' made the first piloted flights powered solely by photovoltaics. This was quickly followed by the ''Solar Challenger'' which demonstrated a more airworthy design with its crossing of the English Channel in July 1981. Developments then turned back to unmanned aerial vehicles (UAV) with the ''[[NASA Pathfinder|Pathfinder]]'' (1997) and subsequent designs, culminating in the ''[[Helios Prototype|Helios]]'' which set the altitude record for a non-rocket-propelled aircraft at {{convert|29524|m|ft}} in 2001.<ref>{{cite web
| title=Solar-Power Research and Dryden
| publisher=NASA
| url=http://www.nasa.gov/centers/dryden/news/FactSheets/FS-054-DFRC.html
| accessdate=2008-04-30}}</ref> The ''[[QinetiQ Zephyr|Zephyr]]'', developed by [[BAE Systems]], is the latest in a line of record-breaking solar aircraft, making a 54-hour flight in 2007, and month-long flights are envisioned by 2010.<ref>{{cite web
| title=The NASA ERAST HALE UAV Program
| publisher=Greg Goebel
| url=http://www.vectorsite.net/twuav_15.html#m7
| accessdate=2008-04-30}}</ref>
A [[solar balloon]] is a black balloon that is filled with ordinary air. As sunlight shines on the balloon, the air inside is heated and expands, causing an upward [[buoyancy]] force, much like an artificially-heated [[hot air balloon]]. Some solar balloons are large enough for human flight, but usage is limited to the toy market as the surface-area to payload-weight ratio is relatively high.{{Fact|date=June 2008}}
[[Solar sail]]s are a proposed form of spacecraft propulsion using large membrane mirrors to exploit radiation pressure from the sun. Unlike rockets, solar sails require no fuel. Although the thrust is small compared to rockets, it continues as long as the Sun shines onto the deployed sail and in the frictionless vacuum of space significant speeds can eventually be achieved.<ref>{{cite web
| title=Solar Sails Could Send Spacecraft 'Sailing' Through Space
| publisher=National Aeronautics and Space Administration
| url=http://www.nasa.gov/vision/universe/roboticexplorers/solar_sails.html
| accessdate=2007-11-26}}</ref>
== Energy storage methods==
{{main|Thermal mass|Thermal energy storage|Phase change material|Grid energy storage}}
[[Image:Solar two.jpg|thumb|right|Solar Two's thermal storage system allowed it to generate electricity during cloudy weather and at night.]]
Storage is an important issue in the development of solar energy because modern energy systems usually assume continuous availability of energy. Solar energy is not available at night, and the performance of solar power systems is affected by unpredictable weather patterns; therefore, storage media or back-up power systems must be used.
Thermal mass systems can store solar energy in the form of heat at domestically useful temperatures for daily or [[seasonal thermal store|seasonal durations]]. Thermal storage systems generally use readily available materials with high [[specific heat]] capacities such as water, earth and stone. Well-designed systems can lower [[peak demand]], shift time-of-use to [[wikt:off-peak|off-peak]] hours and reduce overall heating and cooling requirements.{{Fact|date=June 2008}}
Phase change materials such as [[paraffin wax]] and [[Sodium sulfate#Thermal storage|Glauber's salt]] are another thermal storage media. These materials are inexpensive, readily available, and can deliver domestically useful temperatures (approximately 64 °C). The "Dover House" (in [[Dover, Massachusetts]]) was the first to use a Glauber's salt heating system, in 1948.<ref>Butti and Perlin (1981), p. 212–214</ref>
Solar energy can be stored at high temperatures using molten salts. Salts are an effective storage medium because they are low-cost, have a high specific heat capacity and can deliver heat at temperatures compatible with conventional power systems. The [[The Solar Project#Solar Two|Solar Two]] used this method of energy storage, allowing it to store 1.44 [[joule#SI multiples|TJ]] in its 68 [[Cubic metre|m³]] storage tank with an annual storage efficiency of about 99%.<ref>{{cite web
| title=Advantages of Using Molten Salt
| publisher=Sandia National Laboratory
| url=http://www.sandia.gov/Renewable_Energy/solarthermal/NSTTF/salt.htm
| accessdate=2007-09-29}}</ref>
Off-grid PV systems have traditionally used [[rechargeable batteries]] to store excess electricity. With grid-tied systems, excess electricity can be sent to the transmission [[Grid-tied electrical system|grid]]. [[Net metering]] programs give these systems a credit for the electricity they deliver to the grid. This credit offsets electricity provided from the grid when the system cannot meet demand, effectively using the grid as a storage mechanism.{{Fact|date=June 2008}}
[[Pumped-storage hydroelectricity]] stores energy in the form of water pumped when energy is available from a lower elevation reservoir to a higher elevation one. The energy is recovered when demand is high by releasing the water to run through a hydroelectric power generator.{{Fact|date=June 2008}}
== Development, deployment and economics ==
{{main|Deployment of solar power to energy grids}}
[[Image:SolarPowerPlantSerpa.jpg|thumb|right|11 MW Serpa solar power plant in Portugal]]
[[Image:Moody Sunburst.jpg|thumb|right|Solar troughs are the most widely deployed and cost-effective CSP technology.]]
Beginning with the surge in [[coal]] use which accompanied the [[Industrial Revolution]], energy consumption has steadily transitioned from wood and biomass to [[fossil fuel]]s. The early development of solar technologies starting in the 1860s was driven by an expectation that coal would soon become scarce, but solar development stagnated in the early 20th century in the face of the increasing availability, economy, and utility of fossil fuels such as coal and [[petroleum]].<ref>Butti and Perlin (1981), p. 63, 77, 101</ref>
The [[1973 oil crisis|1973 oil embargo]] and [[1979 energy crisis]] caused a reorganization of energy policies around the world and brought renewed attention to developing solar technologies. Deployment strategies focused on incentive programs such as the Federal Photovoltaic Utilization Program in the US and the Sunshine Program in Japan. Other efforts included the formation of research facilities in the US (SERI, now [[NREL]]), Japan ([[New Energy and Industrial Technology Development Organization|NEDO]]), and [[Solar power in Germany|Germany]] ([[Fraunhofer Society|Fraunhofer Institute for Solar Energy Systems ISE]]).<ref>{{cite web
| title=Chronicle of Fraunhofer-Gesellschaft
| publisher=Fraunhofer-Gesellschaft
| url=http://www.fraunhofer.de/EN/company/profile/chronicle/1972-1982.jsp
| accessdate=2007-11-04}}</ref>
Between 1970 and 1983, photovoltaic installations grew rapidly, but dropping oil prices in the early 1980s moderated the growth of PV from 1984 to 1996. Since 1997, PV development has accelerated due to supply issues with oil and natural gas, global warming concerns (see [[Kyoto Protocol]]), and the improving economic position of PV relative to other energy technologies. Photovoltaic production growth has averaged 40% per year since 2000 and installed capacity reached 10.6 GW at the end of 2007.<ref name="Renewables 2007"/> Since 2006 it has been economical for investors to install photovoltaics for free in return for a long term [[power purchase agreement]]. 50% of commercial systems were installed in this manner in 2007 and it is expected that 90% will by 2009.<ref>[http://www.greentechmedia.com/reports/research-report-solar-power-services.html Solar Power Services: How PPAs are Changing the PV Value Chain]</ref> [[Nellis Air Force Base]] is receiving photoelectric power for about 2.2 ¢/kWh and grid power for 9 ¢/kWh.<ref>[http://www.nellis.af.mil/news/nellissolarpowersystem.asp Nellis Solar Power System]</ref><ref>{{cite web
| title=Supporting Solar Photovoltaic Electricity - An Argument for Feed-in Tariffs
| publisher=European Photovoltaic Industry Association
| url=http://www.epia.org/fileadmin/EPIA_docs/documents/An_Argument_for_Feed-in_Tariffs.pdf
| accessdate=2008-06-09}}</ref>
Commercial solar water heaters began appearing in the United States in the 1890s.<ref>Butti and Perlin (1981), p. 117</ref> These systems saw increasing use until the 1920s but were gradually replaced by cheaper and more reliable heating fuels.<ref>Butti and Perlin (1981), p. 139</ref> As with photovoltaics, solar water heating attracted renewed attention as a result of the oil crises in the 1970s but interest subsided in the 1980s due to falling petroleum prices. Development in the solar water heating sector progressed steadily throughout the 1990s and growth rates have averaged 20% per year since 1999.<ref name="SWH 2008">{{cite web
| title=Solar Heat Worldwide - Markets and Contribution to the Energy Supply 2006
| author=Weiss, Werner
| coauthor=Bergmann, Irene
| coauthor=Faninger, Gerhard
| publisher=International Energy Agency
| url=http://www.iea-shc.org/publications/statistics/IEA-SHC_Solar_Heat_Worldwide-2008.pdf
| accessdate=2008-06-09}}</ref> Although generally underestimated, solar water heating is by far the most widely deployed solar technology with an estimated capacity of 154 GW as of 2007.<ref name="SWH 2008"/>
Commercial concentrating solar power (CSP) plants were first developed in the 1980s. CSP plants such as [[SEGS]] project in the United States have a LEC of 12-14 ¢/kWh.<ref>{{cite web
| title=DOE Concentrating Solar Power 2007 Funding Opportunity Project Prospectus
| publisher=Department of Energy
| url=http://www1.eere.energy.gov/solar/pdfs/csp_prospectus_112807.pdf
| accessdate=2008-06-12}}</ref> The 11 MW [[PS10]] power tower in Spain, completed in late 2005, is Europe's first commercial CSP system and a total capacity of 300 MW is expected to be installed in the same area by 2013.<ref>{{cite web
| title=PS10
| publisher=SolarPACES (Solar Power and Chemical Energy Systems)
| url=http://www.solarpaces.org/Tasks/Task1/PS10.HTM
| accessdate=2008-06-24}}</ref>
== See also ==
{{Portalpar|Sustainable development|Sustainable development.svg}}
{{EnergyPortal}}
{|
|- valign=top
|
* [[Carbon finance]]
* [[Carbon nanotubes in photovoltaics]]
* [[Crookes radiometer]]
* [[Desertec]]
* [[Drake Landing Solar Community]]
* [[Energy storage]]
* [[Global dimming]]
* [[Greasestock]]
* [[Green electricity]]
* [[List of conservation topics]]
* [[List of renewable energy organizations]]
* [[List of solar energy topics]]
* [[List of solar thermal power stations]]
|
* [[Low cost solar power]]
* [[Nanosolar|Nanosolar, Inc.]]
* [[Photovoltaic power stations]]
* [[Renewable heat]]
* [[Solar lamp]]
* [[Solar power satellite]]
* [[Soil solarization]]
* [[Thin-film|Thin-film cell]]
* [[Timeline of solar energy]]
* [[Trombe wall]]
* [[Wafer (electronics)]]
* [[World energy resources and consumption]]
|}
== Notes ==
{{reflist|colwidth=25em}}
== References ==
<div class="references-small">
* {{cite journal
| last = Agrafiotis
| first = C.
| last2 = Roeb
| first2 = M.
| last3 = Konstandopoulos
| first3 = A.G.
| last4 = Nalbandian
| first4 = L.
| last5 = Zaspalis
| first5 = V.T.
| last6 = Sattler
| first6 = C.
| last7 = Stobbe
| first7 = P.
| last8 = Steele
| first8 = A.M.
| title = Solar water splitting for hydrogen production with monolithic reactors
| journal = Solar Energy
| volume = 79
| issue = 4
| pages = 409–421
| year = 2005
| url =
| doi = 10.1016/j.solener.2005.02.026
| id =
}}
* {{cite book
| author=Anderson, Lorraine
| coauthor=Palkovic, Rick
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</div>
== External links ==
{{Commonscat|Solar energy}}
* [http://www.energyeducationfoundation.org Energy Education a2z from the Energy Education Foundation]
* [http://www.builditsolar.com Build It Solar, The Renewable Energy site for Do-It-Yourselfers]
* [http://science.nasa.gov/headlines/y2002/solarcells.htm NASA photovoltaic info]
* [http://www.findsolar.com/ US Solar calculator]
* [http://www.whitehousemuseum.org/images/white-house-west-1980.jpg Jimmy Carter solar panels] on the west wing of the White House in 1980 (removed by Ronald Reagan)
* [http://www.ecomall.com/greenshopping/solarwhitehouse.htm National Park Service installed 8.75 kW solar panels] on a maintenance building of the White House in 2002
{{Navbox/Solar energy}}
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[[Category:Energy conversion]]
[[Category:Energy]]
[[Category:Solar energy|*]]
[[Category:Sun|Power]]
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[[be:Сонечная энергія]]
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[[pt:Energia solar]]
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[[simple:Solar energy]]
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