Solar energy

Solar energy is the light and radiant heat from the Sun that powers Earth's climate and weather and sustains life. Since ancient times it has been harnessed for human use through a range of technologies. Solar radiation along with secondary solar resources such as wind and wave power, hydroelectricity and biomass account for over 99.97% of the available flow of renewable energy on Earth.

Solar energy technologies can provide daylighting and thermal comfort in passive buildings, potable water via distillation and disinfection, hot water and space heating, space cooling by absorption or vapor-compression refrigeration, thermal energy for cooking, high temperature process heat for industrial purposes, and electrical generation by thermal or photovoltaic means.

Energy from the Sun

The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere.Smil (1991), p. 240] Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum of solar light at the Earth's surface is mostly spread across the visible and near-infrared ranges with a small part in the near-ultraviolet. [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]

The absorbed solar light heats the land surface, oceans and atmosphere. The warm air containing evaporated water from the oceans rises, driving atmospheric circulation or convection. When this air reaches a high altitude, where the temperature is low, water vapor condenses into clouds, which rain onto the earth's surface, completing the water cycle. The latent heat of water condensation amplifies convection, producing atmospheric phenomena such as cyclones and anti-cyclones. Wind is a manifestation of the atmospheric circulation driven by solar energy. [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] Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 °C. [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|format=PDF] The conversion of solar energy into chemical energy via photosynthesis produces food, wood and the biomass from which fossil fuels are derived. [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]

Solar radiation along with secondary solar resources such as wind and wave power, hydroelectricity and biomass account for over 99.9% of the available flow of renewable energy on Earth. [Scheer (2002), p. 8] [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] The total solar energy absorbed by Earth's atmosphere, oceans and land masses is approximately 3,850 zettajoules (ZJ) per year. [Smil (2006), p. 12] In 2002, this was more energy in one hour than the world used in one year. [ [http://www.nature.com/nature/journal/v443/n7107/full/443019a.html Solar energy: A new day dawning?] retrieved 7 August 2008] [ [http://web.mit.edu/mitpep/pdf/DGN_Powering_Planet.pdf Powering the Planet: Chemical challenges in solar energy utilization] retrieved 7 August 2008] Photosynthesis captures approximately 3 ZJ per year in biomass. [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] The amount of solar energy reaching the surface of the planet is so vast that in one year it is about twice as much as will ever be obtained from all of the Earth's non-renewable resources of coal, oil, natural gas, and mined uranium combined. [ [http://gcep.stanford.edu/research/exergycharts.html Exergy (available energy) Flow Charts] 2.7 YJ solar energy each year for two billion years vs. 1.4 YJ non-renewable resources available once.]

Applications of solar energy technology

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, they should be deployed in a way that carefully considers these variations.

Solar technologies are broadly characterized as either passive or active depending on the way they capture, convert and distribute sunlight. Active solar techniques use photovoltaic panels, pumps, and fans to convert sunlight into useful outputs. Passive solar techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air, and referencing the position of a building to the Sun. Active solar technologies increase the supply of energy and are considered supply side technologies, while passive solar technologies reduce the need for alternate resources and are generally considered demand side technologies.

Architecture and urban planning

in Washington, D.C. with this passive house designed specifically for the humid and hot subtropical climate [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] ]

Sunlight has influenced building design since the beginning of architectural history.Schittich (2003), p. 14] Advanced solar architecture and urban planning methods were first employed by the Greeks and Chinese, who oriented their buildings toward the south to provide light and warmth. [Butti and Perlin (1981), p. 4, 159]

The common features of passive solar architecture are orientation relative to the Sun, compact proportion (a low surface area to volume ratio), selective shading (overhangs) and thermal mass. 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' Megaron House is a classic example of passive solar design. The most recent approaches to solar design use computer modeling tying together solar lighting, heating and ventilation systems in an integrated solar design package. [Balcomb(1992)] Active solar equipment such as pumps, fans and switchable windows can complement passive design and improve system performance.

Urban heat islands (UHI) are metropolitan areas with higher temperatures than that of the surrounding environment. The higher temperatures are a result of increased absorption of the Solar light by urban materials such as asphalt and concrete, which have lower albedos and higher heat capacities than those in 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.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]

Agriculture and horticulture

Agriculture seeks to optimize the capture of solar energy in order to optimize the productivity of plants. Techniques such as timed planting cycles, tailored row orientation, staggered heights between rows and the mixing of plant varieties can improve crop yields. [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] [Kaul (2005), p. 169–174] While sunlight is generally considered a plentiful resource, the exceptions highlight the importance of solar energy to agriculture. During the short growing seasons of the Little Ice Age, French and 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 and facing south, but over time, sloping walls were developed to make better use of sunlight. In 1699, Nicolas Fatio de Duillier even suggested using a tracking mechanism which could pivot to follow the Sun. [Butti and Perlin (1981), p. 42–46] Applications of solar energy in agriculture aside from growing crops include pumping water, drying crops, brooding chicks and drying chicken manure. [Bénard (1981), p. 347] Leon (2006), p. 62]

Greenhouses convert solar light to heat, enabling year-round production and the growth (in enclosed environments) of specialty crops and other plants not naturally suited to the local climate. Primitive greenhouses were first used during Roman times to produce cucumbers year-round for the Roman emperor Tiberius. [Butti and Perlin (1981), p. 19] The first modern greenhouses were built in Europe in the 16th century to keep exotic plants brought back from explorations abroad. [Butti and Perlin (1981), p. 41] Greenhouses remain an important part of horticulture today, and plastic transparent materials have also been used to similar effect in polytunnels and row covers.

Solar lighting

The history of lighting is dominated by the use of natural light. The Romans recognized a right to light as early as the 6th century and English law echoed these judgments with the Prescription Act of 1832. [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] [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] In the 20th century artificial lighting became the main source of interior illumination but daylighting techniques and hybrid solar lighting solutions are gaining popularity.

Daylighting systems collect and distribute sunlight to provide interior illumination. This passive technology directly offsets energy use by replacing artificial lighting, and indirectly offsets non-solar energy use by reducing the need for air-conditioning.Tzempelikos (2007), p. 369] Although difficult to quantify, the use of natural lighting also offers physiological and psychological benefits compared to artificial lighting. Daylighting design implies careful selection of window types, sizes and orientation; exterior shading devices may be considered as well. Individual features include sawtooth roofs, clerestory windows, light shelves, skylights and light tubes. They may be incorporated into existing structures, but are most effective when integrated into a solar design package that accounts for factors such as glare, heat flux and time-of-use. When daylighting features are properly implemented they can reduce lighting-related energy requirements by 25%.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|format=PDF]

Hybrid solar lighting is an active solar method of providing interior illumination. HSL systems collect sunlight using focusing mirrors that track the Sun and use optical fibers to transmit it inside the building to supplement conventional lighting. In single-story applications these systems are able to transmit 50% of the direct sunlight received.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|format=PDF]

Although daylight saving time is promoted as a way to use sunlight to save energy, recent research has been 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. [cite journal |journal= Energy Policy |year=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]

Solar thermal

Solar thermal technologies can be used for water heating, space heating, space cooling and process heat generation. [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]

Water heating

Solar hot water systems use sunlight to heat water. In low geographical latitudes (below 40 degrees) from 60 to 70% of the domestic hot water use with temperatures up to 60 °C can be provided by solar heating systems. [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|format=PDF] 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. [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|format=PDF]

As of 2007, the total installed capacity of solar hot water systems is approximately 154 GW. China is the world leader in their deployment with 70 GW installed as of 2006 and a long term goal of 210 GW by 2020.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|format=PDF] Israel and Cypress are the per capita leaders in the use of solar hot water systems with over 90% of homes using them.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|format=PDF] 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.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|format=PDF]

Heating, cooling and ventilation

In the United States, heating, ventilation and air conditioning (HVAC) systems account for 30% (4.65 EJ) of the energy used in commercial buildings and nearly 50% (10.1 EJ) of the energy used in residential buildings. [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|format=PDF] Solar heating, cooling and ventilation technologies can be used to offset a portion of this energy.

Thermal mass is any material that can be used to store heat—heat from the Sun in the case of solar energy. Common thermal mass materials include stone, cement and water. Historically they have 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. However they can be used in cold temperate areas to maintain warmth as well. The size and placement of thermal mass depend on 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. [Mazria(1979), p. 29–35]

A solar chimney (or thermal chimney, in this context) 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.Fact|date=August 2008

Deciduous trees and plants have been promoted as a means of controlling solar heating and cooling. When planted on the southern side of a building, their leaves provide shade during the summer, while the bare limbs allow light to pass during the winter. [Mazria(1979), p. 255] Since bare, leafless trees shade 1/3 to 1/2 of incident solar radiation, there is a balance between the benefits of summer shading and the corresponding loss of winter heating. [Balcomb(1992), p. 56] 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. They can, however, be used on the east and west sides to provide a degree of summer shading without appreciably affecting winter solar gain. [Balcomb(1992), p. 57]

Desalination, water disinfection and waste water treatment

Solar distillation can be used to make saline or brackish water potable. The first recorded instance of this was by 16th century Arab alchemists.Tiwari (2003), p. 368–371] A large-scale solar distillation project was first constructed in 1872 in the Chilean mining town of Las Salinas.Daniels (1964), p. 6] The plant, which had solar collection area of 4,700 m², could produce up to 22,700 L per day and operated for 40 years. Individual still designs include single-slope, double-slope (or greenhouse type), vertical, conical, inverted absorber, multi-wick, and multiple effect. 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 for large-scale applications.

Solar water disinfection (SODIS) involves exposing water-filled plastic polyethylene terephthalate (PET) bottles to sunlight for several hours. [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] Exposure times vary depending on weather and climate from a minimum of six hours to two days during fully overcast conditions.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|format=PDF] SODIS is recommended by the World Health Organization as a viable method for household water treatment and safe storage. [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] Over two million people in developing countries use SODIS for their daily drinking water.

While comparing ocean water desalination to wastewater reclamation for drinking water shows desalination as the first option, using reclamation for irrigation and industrial use provides multiple benefits. [Cooley, Heather, Peter H. Gleick, and Gary Wolff. (June 2006.) [http://www.pacinst.org/reports/desalination/index.htm "Desalination, With a Grain of Salt – A California Perspective."] (Website). "Pacific Institute". Retrieved on 2007-09-20.] Urban runoff and storm water capture also provide multiple benefits in treating, restoring and recharging groundwater. [Gleick, Peter H., Heather Cooley, David Groves. (September 2005.) [http://pacinst.org/reports/california_water_2030/ca_water_2030.pdf "California water 2030: An efficient future."] (Website). "Pacific Institute". Retrieved on 2007-09-20.] Solar energy is being used in treating waste water for reuse.

Sunlight enables waste water stabilisation ponds to disinfect wastewaters very effectively without the need for any chemicals or electricity consumption and their associated CO₂ emissions. The energy and carbon emission savings gained over electromechanical treatment systems are immense. Furthermore, because algal photosynthesis consumes CO₂, WSP can be utilised as CO₂ scrubbers, and provide an energy source. cite journal |author=Shilton AN, Powell N, Mara DD, Craggs R |title=Solar-powered aeration and disinfection, anaerobic co-digestion, biological CO(2) scrubbing and biofuel production: the energy and carbon management opportunities of waste stabilisation ponds |journal=Water Sci. Technol. |volume=58 |issue=1 |pages=253–258 |year=2008 |pmid=18653962 |doi=10.2166/wst.2008.666 |url=] cite journal |author=Tadesse I, Isoaho SA, Green FB, Puhakka JA |title=Removal of organics and nutrients from tannery effluent by advanced integrated Wastewater Pond Systems technology |journal=Water Sci. Technol. |volume=48 |issue=2 |pages=307–14 |year=2003 |pmid=14510225 |doi= |url=]

Tests on two solar-driven advanced oxidation processes, namely heterogeneous semiconductor photocatalysis and homogeneous photo-Fenton, both coupled to biological treatment, were carried out in order to identify the environmentally preferable alternative to treat industrial wastewaters containing non-biodegradable priority hazardous substances. The experimental results obtained showed that solar photo-Fenton is able to obtain a biodegradable effluent much faster than solar heterogeneous photocatalysis. cite journal |author=Muñoz I, Peral J, Ayllón JA, Malato S, Passarinho P, Domènech X |title=Life cycle assessment of a coupled solar photocatalytic-biological process for wastewater treatment |journal=Water Res. |volume=40 |issue=19 |pages=3533–40 |year=2006 |month=November |pmid=16989886 |doi=10.1016/j.watres.2006.08.001 |url=] Combined solar photo-Fenton / biological treatment is being used or considered for removal of endocrine disrupting chemicals in sewage treatment waste water. cite journal |author=Zhang Y, Zhou JL |title=Occurrence and removal of endocrine disrupting chemicals in wastewater |journal=Chemosphere |volume= |issue= |pages= |year=2008 |month=July |pmid=18667225 |doi=10.1016/j.chemosphere.2008.06.001 |url=] for treating industrial effluents cite journal |author=Malato S, Blanco J, Maldonado MI, Oller I, Gernjak W, Pérez-Estrada L |title=Coupling solar photo-Fenton and biotreatment at industrial scale: main results of a demonstration plant |journal=J. Hazard. Mater. |volume=146 |issue=3 |pages=440–6 |year=2007 |month=July |pmid=17532127 |doi=10.1016/j.jhazmat.2007.04.084 |url=] including treatment of winery waste water cite journal |author=Mosteo R, Ormad MP, Ovelleiro JL |title=Photo-Fenton processes assisted by solar light used as preliminary step to biological treatment applied to winery wastewaters |journal=Water Sci. Technol. |volume=56 |issue=2 |pages=89–94 |year=2007 |pmid=17849982 |doi= |url=] for the bleaching wastewater effluent from a pulp and paper mill cite journal |author=Xu M, Wang Q, Hao Y |title=Removal of organic carbon from wastepaper pulp effluent by lab-scale solar photo-Fenton process |journal=J. Hazard. Mater. |volume=148 |issue=1-2 |pages=103–9 |year=2007 |month=September |pmid=17367923 |doi=10.1016/j.jhazmat.2007.02.015 |url=] tannery wastewater.cite journal |author=Schrank SG, José HJ, Moreira RF, Schröder HF |title=Comparison of different advanced oxidation process to reduce toxicity and mineralisation of tannery wastewater |journal=Water Sci. Technol. |volume=50 |issue=5 |pages=329–34 |year=2004 |pmid=15497865 |doi= |url=] and textile dyeing wastewater. cite journal |author=Sarayu G, Kanmani S |title=Treatment of textile dyeing wastewater using UV/solar photofenton oxidation processes |journal=Indian J Environ Health |volume=45 |issue=2 |pages=113–20 |year=2003 |month=April |pmid=15270343 |doi= |url=] cite journal |author=Pérez M, Torrades F, Domènech X, Peral J |title=Fenton and photo-Fenton oxidation of textile effluents |journal=Water Res. |volume=36 |issue=11 |pages=2703–10 |year=2002 |month=June |pmid=12146857 |doi= |url=http://linkinghub.elsevier.com/retrieve/pii/S0043-1354(01)00506-1] The technology is also being used for the treatment of groundwater contaminated with benzene or petroleum such as at garage sites. cite journal |author=Cho IH, Chang SW |title=The potential and realistic hazards after a solar-driven chemical treatment of benzene using a health risk assessment at a gas station site in Korea |journal=J Environ Sci Health A Tox Hazard Subst Environ Eng |volume=43 |issue=1 |pages=86–97 |year=2008 |month=January |pmid=18161562 |doi=10.1080/10934520701750090 |url=] cite journal |author=Cho IH, Kim YG, Yang JK, Lee NH, Lee SM |title=Solar-chemical treatment of groundwater contaminated with petroleum at gas station sites: ex situ remediation using solar/TiO(2) photocatalysis and Solar Photo-Fenton |journal=J Environ Sci Health A Tox Hazard Subst Environ Eng |volume=41 |issue=3 |pages=457–73 |year=2006 |pmid=16484076 |doi=10.1080/10934520500428336 |url=]

Cooking

Solar cookers use sunlight for cooking, drying and pasteurization. They can be grouped into three broad categories: box cookers, panel cookers and reflector cookers. [Anderson and Palkovic (1994), p. xi] The simplest solar cooker—the box cooker first built by Horace de Saussure in 1767. [Butti and Perlin (1981), p. 54–59] A basic box cooker consists of an insulated container with a transparent lid. It can be used effectively with partially overcast skies and will typically reach temperatures of 90–150 °C. [Anderson and Palkovic (1994), p. xii] 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. [Anderson and Palkovic (1994), p. xiii]

The solar bowl is a concentrating technology employed by the Solar Kitchen in Auroville, India, where a stationary spherical reflector 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. [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]

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. 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 simplifies cooking. [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] The world's largest Scheffler reflector system in Abu Road, Rajasthan, India is capable of cooking up to 35,000 meals a day. [cite web
title=Solar Steam Cooking System
publisher=Gadhia Solar
url=http://gadhia-solar.com/products/steam.htm
accessdate=2008-04-25] As of 2008, over 2,000 large Scheffler cookers had been built worldwide. [cite web
title=Scheffler Reflector
publisher=Solare Bruecke
url=http://www.solare-bruecke.org/infoartikel/info_vorstand.htm#english
accessdate=2008-07-03]

Process heat

Solar concentrating 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, USA where a field of 114 parabolic dishes provided 50% of the process heating, air conditioning and electrical requirements for a clothing factory. This grid-connected cogeneration system provided 400 kW of electricity plus thermal energy in the form of 401 kW steam and 468 kW chilled water, and had a one hour peak load thermal storage. [cite web
title=Shenandoah Solar Total Energy Project
author=Stine, W B and Harrigan, R W
publisher=John Wiley
url=http://www.powerfromthesun.net/chapter16/Chapter16Text.htm
accessdate=2008-07-20]

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. [Bartlett (1998), p.393–394]

Clothes lines, clotheshorses, 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. [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]

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.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|format=PDF] The short payback period of transpired collectors (3 to 12 years) makes them a more cost-effective alternative than glazed collection systems. As of 2003, over 80 systems with a combined collector area of 35,000 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.

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 and the 20 MW site in Beneixama, Spain are characteristic of the trend toward larger photovoltaic power stations in the US and Europe. [cite web
title=Large-scale photovoltaic power plants
publisher=pvresources
url=http://www.pvresources.com/en/top50pv.php
accessdate=2008-06-27]

Photovoltaics

A solar cell, or photovoltaic cell (PV), 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. [Perlin (1999), p. 147] 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. [ Perlin (1999), p. 18–20] Following the work of Russell Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and Daryl Chapin created the silicon solar cell in 1954. [Perlin (1999), p. 29] These early solar cells cost 286 USD/watt and reached efficiencies of 4.5–6%. [Perlin (1999), p. 29–30, 38]

The earliest significant application of solar cells was as a back-up power source to the Vanguard I satellite, which allowed it to continue transmitting for over a year after its chemical battery was exhausted. [Perlin (1999), p. 45–46] The successful operation of solar cells on this mission was duplicated in many other Soviet and American satellites, and by the late 1960s, PV had become the established source of power for them. [ Perlin (1999), p. 49–50] Photovoltaics went on to play an essential part in the success of early commercial satellites such as Telstar, and they remain vital to the telecommunications infrastructure today. [Perlin (1999), p. 49–50, 190]

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 grid access. Early terrestrial uses included powering telecommunication stations, off-shore oil rigs, navigational buoys and railroad crossings. [Perlin (1999), p. 57–85] These off-grid applications have proven very successful and accounted for over half of worldwide installed capacity until 2004.

The 1973 oil crisis stimulated a rapid rise in the production of PV during the 1970s and early 1980s. [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] 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. [ Perlin (1999), p. 50, 118] 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.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]

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.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] As a result, PV installations in the country climbed from 31.2 MW in 1994 to 318 MW in 1999, [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|format=PDF] and worldwide production growth increased to 30% in the late 1990s. [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|format=PDF]

Germany has become the leading PV market worldwide since revising its 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. [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|format=PDF] [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] 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 seen rapid growth recently due to various incentive programs and local market conditions. [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|format=PDF]

Concentrating solar power

Concentrated sunlight has been used to perform useful tasks since the time of ancient China. A legend claims that Archimedes used polished shields to concentrate sunlight on the invading Roman fleet and repel them from Syracuse. [Butti and Perlin (1981), p. 29] Auguste Mouchout used a parabolic trough to produce steam for the first solar steam engine in 1866, and subsequent developments led to the use of concentrating solar-powered devices for irrigation, refrigeration and locomotion. [Butti and Perlin (1981), p. 60–100]

Concentrating Solar Power (CSP) systems 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 exists; 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. Martin and Goswami (2005), p. 45] 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 provide the best land-use factor of any solar technology. [ [http://www.greenpeace.org/raw/content/international/press/reports/Concentrated-Solar-Thermal-Power.pdf Concentrated Solar Thermal Power - Now] Retrieved 19 August 2008] The SEGS plants in California and Acciona's Nevada Solar One near Boulder City, Nevada are representatives of this technology. [cite web
title=UNLV Solar Site
publisher=University of Las Vegas
url=http://www.solar.unlv.edu/projects/eldorado.php
accessdate=2008-07-02]

A parabolic dish system consists of a stand-alone parabolic reflector that concentrates light onto a receiver positioned at the reflector's focal point. The reflector tracks the Sun along two axes. Parabolic dish systems give the highest efficiency among CSP technologies. [cite web
title=An Assessment of Solar Energy Conversion Technologies and Research Opportunities
publisher=Stanford University - Global Climate Change & Energy Project
url=http://www.gcep.stanford.edu/pdfs/assessments/solar_assessment.pdf
accessdate=2008-07-02|format=PDF] The 50 kW Big Dish in Canberra, Australia is an example of this technology.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|format=PDF]

A solar power tower uses an array of tracking reflectors (heliostats) to concentrate light on a central receiver atop a tower. Power towers are less advanced than trough systems but offer higher efficiency and better energy storage capability. The Solar Two in Barstow, California and the Planta Solar 10 in Sanlucar la Mayor, Spain are representatives of this technology. [cite news
title=Power station harnesses Sun's rays
author=David Shukman
publisher=BBC News
url=http://news.bbc.co.uk/2/hi/science/nature/6616651.stm
accessdate=2008-07-02]

Experimental solar power

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. [Mills (2004), p. 19–31]

A solar pond is a pool of salt water (usually 1–2 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 currents. A prototype was constructed in 1958 on the shores of the Dead Sea near Jerusalem. [Halacy (1973), p. 181] The pond consisted of layers of water that successively increased from a weak salt solution at the top to a 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.

Thermoelectric, or "thermovoltaic" 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, [Perlin and Butti (1981), p. 73] 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 hp engine. [Halacy (1973), p. 76] Thermogenerators were later used in the US space program as an energy conversion technology for powering deep space missions such as Cassini, Galileo and Viking. Research in this area is focused on raising the efficiency of these devices from 7–8% to 15–20%.Tritt (2008), p. 366–368]

Space solar power systems would use a large solar array in geosynchronous orbit to collect sunlight and beam this energy in the form of microwave radiation to receivers (rectennas) on Earth for distribution. This concept was first proposed by 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. [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|format=PDF] In 2008, John C. Mankins, a former NASA scientist, successfully used radio waves to send solar power between two Hawaiian islands in an experiment funded by the Discovery Channel. Mankins claims that this "proves the technology exists to beam solar power from satellites back to Earth." [cite web
title=Experiment boosts hopes for space solar power
publisher=MSNBC
url=http://www.msnbc.msn.com/id/26678942/
accessdate=2008-09-12]

Solar chemical

Solar chemical processes use solar energy to drive chemical reactions. These processes offset energy that would otherwise come from an alternate source and can convert solar energy into storable and transportable fuels. Solar induced chemical reactions can be divided into thermochemical or photochemical. [Bolton (1977), p. 1]

Hydrogen production technologies 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. One such route uses concentrators to split water into oxygen and hydrogen at high temperatures (2300-2600 °C). [Agrafiotis (2005), p. 409] Another approach uses the heat from solar concentrators to drive the steam reformation of natural gas thereby increasing the overall hydrogen yield compared to conventional reforming methods. [Zedtwitz (2006), p. 1333] Thermochemical cycles characterized by the decomposition and regeneration of reactants present another avenue for hydrogen production. The Solzinc process under development at the Weizmann Institute 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. [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]

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 carbon monoxide can then be used to synthesize conventional fuels such as methanol, gasoline and jet fuel. [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]

A photogalvanic device is a type of battery in which the cell solution (or equivalent) forms energy-rich chemical intermediates when illuminated. These energy-rich intermediates can potentially be stored and subsequently reacted at the electrodes to produce an electric potential. The ferric-thionine chemical cell is an example of this technology. [Bolton (1977), p. 16, 119]

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.Bolton (1977), p. 11]

Solar vehicles

Development of a solar powered car has been an engineering goal since the 1980s. The World Solar Challenge is a biannual solar-powered car race, where teams from universities and enterprises compete over convert|3021|km|mi 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 and by 2007 the winner's average speed had improved to convert|90.87|km/h|mph|2. [cite web
title=The WORLD Solar Challenge - The Background
publisher=Australian and New Zealand Solar Energy Society
url=http://www.anzses.org/files/The%20WORLD%20Solar%20Challenge.pdf
accessdate=2008-08-05|format=PDF] 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. [cite web
title=North American Solar Challenge
publisher=New Resources Group
url=http://americansolarchallenge.org/
accessdate=2008-07-03] [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]

Some vehicles use solar panels for auxiliary power, such as for air conditioning, to keep the interior cool, thus reducing fuel consumption. [ [http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel3/1205/3985/00152037.pdf?arnumber=152037 Vehicle auxiliary power applications for solar cells] 1991 Retrieved 11 October 2008] [ [http://www.systaic.com/press/press-release/systaic-ag-demand-for-car-solar-roofs-skyrockets.html systaic AG: Demand for Car Solar Roofs Skyrockets] 26 June 2008 Retrieved 11 October 2008]

In 1975, the first practical solar boat was constructed in England. ["Electrical Review" Vol 201 No 7 12 August 1977] By 1995, passenger boats incorporating PV panels began appearing and are now used extensively. [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] 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. [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] There are plans to circumnavigate the globe in 2010. [cite web
title=PlanetSolar, the first solar-powered round-the-world voyage
publisher=PlanetSolar
url=http://www.planetsolar.org/objectifs.en.shtml
accessdate=2008-08-19]

In 1974, the unmanned "Sunrise II" plane made the first solar flight. On 29 April 1979, the "Solar Riser" made the first flight in a solar powered, fully controlled, man carrying flying machine, reaching an altitude of convert|40|ft|abbrv=on. In 1980, the "Gossamer Penguin" made the first piloted flights powered solely by photovoltaics. This was quickly followed by the "Solar Challenger" which crossed the English Channel in July 1981. In 1990 Eric Raymond in 21 hops flew from California to North Carolina using solar power. [ [http://www.evworld.com/article.cfm?storyid=709 Sunseeker Seeks New Records] ] Developments then turned back to unmanned aerial vehicles (UAV) with the "Pathfinder" (1997) and subsequent designs, culminating in the "Helios" which set the altitude record for a non-rocket-propelled aircraft at convert|29524|m|ft in 2001. [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] The "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. [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]

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 generally limited to the toy market as the surface-area to payload-weight ratio is relatively high. [cite web
title=Phenomena which affect a solar balloon
publisher=pagesperso-orange.fr
url=http://pagesperso-orange.fr/ballonsolaire/en-theorie1.htm
accessdate=2008-08-19]

Solar sails 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 vacuum of space significant speeds can eventually be achieved. [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]

The High-altitude airship (HAA) is an unmanned, long-duration, lighter-than-air vehicle using helium gas for lift, and thin-film solar cells for power. The United States Department of Defense Missile Defense Agency has contracted Lockheed Martin to construct it to enhance the Ballistic Missile Defense System (BMDS).cite web
url=http://www.lockheedmartin.com/products/HighAltitudeAirship/index.html
title=High Altitude Airship
publisher=Lockheed Martin
accessdate=2008-08-04
last=
first=] Airships have some advantages for solar-powered flight: they do not require power to remain aloft, and an airship's envelope presents a large area to the Sun.

Energy storage methods

Storage is an important issue in the development of solar energy because modern energy systems usually assume continuous availability of energy. [Carr (1976), p. 85] 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 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 hours and reduce overall heating and cooling requirements. [Balcomb(1992), p. 6] [cite web
title=Request for Participation Summer 2005 Demand Shifting with Thermal Mass
publisher=Demand Response Research Center
url=http://www.drrc.lbl.gov/pubs/RFP_071405.pdf
accessdate=2007-11-26|format=PDF]

Phase change materials such as paraffin wax and 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. [Butti and Perlin (1981), p. 212–214]

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 Solar Two used this method of energy storage, allowing it to store 1.44 TJ in its 68 m³ storage tank with an annual storage efficiency of about 99%. [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]

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. 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. [cite web
title=PV Systems and Net Metering
publisher=Department of Energy
url=http://www1.eere.energy.gov/solar/net_metering.html
accessdate=2008-07-31]

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. [cite web
title=Pumped Hydro Storage
publisher=Electricity Storage Association
url=http://www.electricitystorage.org/tech/technologies_technologies_pumpedhydro.htm
accessdate=2008-07-31]

Development, deployment and economics

Beginning with the surge in coal use which accompanied the Industrial Revolution, energy consumption has steadily transitioned from wood and biomass to fossil fuels. The early development of solar technologies starting in the 1860s was driven by an expectation that coal would soon become scarce. However development of solar technologies stagnated in the early 20th century in the face of the increasing availability, economy, and utility of coal and petroleum. [Butti and Perlin (1981), p. 63, 77, 101]

The 1973 oil embargo and 1979 energy crisis caused a reorganization of energy policies around the world and brought renewed attention to developing solar technologies. [Butti and Perlin (1981), p. 249] [Yergin (1991), p. 634, 653-673] 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 (NEDO), and Germany (Fraunhofer Institute for Solar Energy Systems ISE). [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]

Between 1970 and 1983 photovoltaic installations grew rapidly, but falling 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.Fact|date=August 2008 Photovoltaic production growth has averaged 40% per year since 2000 and installed capacity reached 10.6 GW at the end of 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. [ [http://www.greentechmedia.com/reports/research-report-solar-power-services.html Solar Power Services: How PPAs are Changing the PV Value Chain] ] Nellis Air Force Base is receiving photoelectric power for about 2.2 ¢/kWh and grid power for 9 ¢/kWh. [ [http://www.nellis.af.mil/news/nellissolarpowersystem.asp Nellis Solar Power System] ] [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|format=PDF]

Commercial solar water heaters began appearing in the United States in the 1890s. [Butti and Perlin (1981), p. 117] These systems saw increasing use until the 1920s but were gradually replaced by cheaper and more reliable heating fuels. [Butti and Perlin (1981), p. 139] 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.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|format=PDF] 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.

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. [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|format=PDF] 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. [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]

Solar installations in recent years have also largely begun to expand into residential areas, with governments offering incentive programs to make "green" energy a more economically viable option. In Canada the government offers the RESOP (Renewable Energy Standard Offer Program).Fact|date=September 2008 The program allows residential homeowners with solar panel installations to sell the energy they produce back to the grid (i.e., the government) at 41¢/kWh, while drawing power from the grid at an average rate of 20¢/kWh (see feed-in tariff). The program is designed to help promote the government's green agenda and lower the strain often placed on the energy grid at peak hours. With the incentives offered by the program the average payback period for a residential solar installation (sized between 1.3 kW and 5 kW) is estimated at 18 to 23 years, considering such cost factors as parts, installation and maintenance, as well as the average energy production of a system on an annual basis.Fact|date=September 2008

Daniel Lincot, the chairman of the 2008 European Photovoltaic Solar Energy Conference and the research director of the Paris-based Photovoltaic Energy Development and Research Institute, said that photovoltaics can cover all the world energy demand [http://www.physorg.com/news139844301.html] . Photovoltaics are 85 times as efficient as growing corn for ethanol. On a 300 feet by 300 feet (1 hectare) plot of land enough ethanol can be produced to drive a car convert|30000|mi|pl=on per year or convert|2500000|mi|pl=on by covering the same land with photo cells.

See also

Notes

References

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External links

*
*
*
* Renewable Energy Focus magazine
* [http://www.prometheus.org Prometheus Institute for sustainable development]
* cite web|url=http://www.geocities.com/daveclarkecb/Australia/SolarPower.html| title=Solar Power in Australia
* [http://www.earth-policy.org/Updates/2008/Update73.htm Online article by scientist Jonathan G. Dorn, 22 July-2008] The solar thermal power industry experienced a surge in 2007, with 100 megawatts of new capacity worldwide.
* [http://www.eurosolar.org Eurosolar]
* [http://solarcooking.wikia.com/wiki/Compendium_of_solar_cooker_designs Compendium of Solar Cooker Designs]