Solar energy

Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy (including solar water heating), and solar architecture.<ref name=ie11/><ref>"Energy". Royal Society of Chemistry. 2 April 2014.</ref> It is an essential source of renewable energy, and its technologies are broadly characterized as either passive solar or active solar depending on how they capture and distribute solar energy or convert it into solar power. Active solar techniques include the use of photovoltaic systems, concentrated solar power, and solar water heating to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light-dispersing properties, and designing spaces that naturally circulate air.

In 2011, the International Energy Agency said that "the development of affordable, inexhaustible and clean solar energy technologies will have huge longer-term benefits. It will increase countries' energy security through reliance on an indigenous, inexhaustible, and mostly import-independent resource, enhance sustainability, reduce pollution, lower the costs of mitigating global warming .... these advantages are global".<ref name=ie11>"Solar Energy Perspectives: Executive Summary" (PDF). International Energy Agency. 2011. Archived from the original (PDF) on 13 January 2012.</ref>

Potential

About half the incoming solar energy reaches the Earth's surface.

Average insolation. The theoretical area of the small black dots is sufficient to supply the world's total energy needs of 18 TW with solar power.

The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere.<ref>Smil (1991), p. 240</ref> Approximately 30% is reflected back to space while the rest, 122 PW, 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.<ref>"Natural Forcing of the Climate System". Intergovernmental Panel on Climate Change. Archived from the original on 29 September 2007. Retrieved 29 September 2007.</ref> Most of the world's population live in areas with insolation levels of 150–300 watts/m², or 3.5–7.0 kWh/m² per day.<ref>Karuppu, Karthik; Sitaraman, Venk; NVICO (2019). Solar Assessment Guidance: A Guide for Solar Trainee, Trainer & Assessor Examination. Notion Press. ISBN 978-1646505227.</ref>

Solar radiation is absorbed by the Earth's land surface, oceans – which cover about 71% of the globe – and atmosphere. Warm air containing evaporated water from the oceans rises, causing atmospheric circulation or convection. When the 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 wind, cyclones and anticyclones.<ref>"Radiation Budget". NASA Langley Research Center. 17 October 2006. Retrieved 29 September 2007.</ref> Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 °C.<ref>Somerville, Richard. "Historical Overview of Climate Change Science" (PDF). Intergovernmental Panel on Climate Change. Retrieved 29 September 2007.</ref> By photosynthesis, green plants convert solar energy into chemically stored energy, which produces food, wood and the biomass from which fossil fuels are derived.<ref>Vermass, Wim. "An Introduction to Photosynthesis and Its Applications". Arizona State University. Archived from the original on 3 December 1998. Retrieved 29 September 2007.</ref>

The total solar energy absorbed by Earth's atmosphere, oceans and land masses is approximately 122 PW·year = 3,850,000 exajoules (EJ) per year.<ref name="Smil 2006, p. 12"/> In 2002 (2019), this was more energy in one hour (one hour and 25 minutes) than the world used in one year.<ref>Morton, Oliver (6 September 2006). "Solar energy: A new day dawning?: Silicon Valley sunrise". Nature. 443 (7107): 19–22. Bibcode:2006Natur.443...19M. doi:10.1038/443019a. PMID 16957705. S2CID 13266273.</ref><ref>Lewis, N. S.; Nocera, D. G. (2006). "Powering the Planet: Chemical challenges in solar energy utilization" (PDF). Proceedings of the National Academy of Sciences. 103 (43): 15729–35. Bibcode:2006PNAS..10315729L. doi:10.1073/pnas.0603395103. PMC 1635072. PMID 17043226. Retrieved 7 August 2008.</ref> Photosynthesis captures approximately 3,000 EJ per year in biomass.<ref>"Energy conversion by photosynthetic organisms". Food and Agriculture Organization of the United Nations. Retrieved 25 May 2008.</ref>

Yearly solar fluxes & human consumption¹
¹ Energy given in Exajoule (EJ) = 10¹⁸ J = 278 TWh ² Consumption as of year 2019
Solar	3,850,000	<ref name="Smil 2006, p. 12">Smil (2006), p. 12</ref>
Wind	2,250	<ref>Archer, Cristina; Jacobson, Mark. "Evaluation of Global Wind Power". Stanford. Retrieved 3 June 2008.</ref>
Biomass potential	~200	<ref>"Renewable Energy Sources" (PDF). Renewable and Appropriate Energy Laboratory. p. 12. Archived from the original (PDF) on 19 November 2012. Retrieved 6 December 2012.</ref>
Primary energy use²	633	<ref>"Total Primary Energy Consumption". Energy Information Administration. Retrieved 28 June 2022.</ref>
Electricity²	~86	<ref>"Total Electricity Net Consumption". Energy Information Administration. Retrieved 28 June 2022.</ref>

The potential solar energy that could be used by humans differs from the amount of solar energy present near the surface of the planet because factors such as geography, time variation, cloud cover, and the land available to humans limit the amount of solar energy that we can acquire. In 2021, Carbon Tracker Initiative estimated the land area needed to generate all our energy from solar alone was 450,000 km² — or about the same as the area of Sweden, or the area of Morocco, or the area of California (0.3% of the Earth's total land area).<ref>Bond, Kingsmill (April 2021). "The sky's the limit" (PDF). epbr. Carbon Tracker Initiative. p. 6. Archived (PDF) from the original on April 30, 2021. Retrieved October 22, 2021.</ref>

Solar technologies are categorized as either passive or active depending on the way they capture, convert and distribute sunlight and enable solar energy to be harnessed at different levels around the world, mostly depending on the distance from the Equator. Although solar energy refers primarily to the use of solar radiation for practical ends, all types of renewable energy, other than geothermal power and tidal power, are derived either directly or indirectly from the Sun.

Active solar techniques use photovoltaics, concentrated solar power, solar thermal collectors, pumps, and fans to convert sunlight into useful output. Passive solar techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air, and referencing^{[clarification needed]} 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 alternative resources and are generally considered demand-side technologies.<ref name="IEA Solar Thermal"/>

In 2000, the United Nations Development Programme, UN Department of Economic and Social Affairs, and World Energy Council published an estimate of the potential solar energy that could be used by humans each year. This took into account factors such as insolation, cloud cover, and the land that is usable by humans. It was stated that solar energy has a global potential of 1,600 to 49,800 exajoules (4.4×10¹⁴ to 1.4×10¹⁶ kWh) per year (see table below).<ref name="World Energy Assessment">"Energy and the challenge of sustainability" (PDF). United Nations Development Programme and World Energy Council. September 2000. Retrieved 17 January 2017.</ref>

Annual solar energy potential by region (Exajoules) <ref name="World Energy Assessment" />
Region	North America	Latin America and Caribbean	Western Europe	Central and Eastern Europe	Former Soviet Union	Middle East and North Africa	Sub-Saharan Africa	Pacific Asia	South Asia	Centrally planned Asia	Pacific OECD
Minimum	181.1	112.6	25.1	4.5	199.3	412.4	371.9	41.0	38.8	115.5	72.6
Maximum	7,410	3,385	914	154	8,655	11,060	9,528	994	1,339	4,135	2,263
Notes: Total global annual solar energy potential amounts to 1,575 EJ (minimum) to 49,837 EJ (maximum) Data reflects assumptions of annual clear sky irradiance, annual average sky clearance, and available land area. All figures given in Exajoules. Quantitative relation of global solar potential vs. the world's primary energy consumption: Ratio of potential vs. current consumption (402 EJ) as of year: 3.9 (minimum) to 124 (maximum) Ratio of potential vs. projected consumption by 2050 (590–1,050 EJ): 1.5–2.7 (minimum) to 47–84 (maximum) Ratio of potential vs. projected consumption by 2100 (880–1,900 EJ): 0.8–1.8 (minimum) to 26–57 (maximum) Source: United Nations Development Programme – World Energy Assessment (2000)<ref name="World Energy Assessment" />

Thermal energy

Solar thermal technologies can be used for water heating, space heating, space cooling and process heat generation.<ref>"Solar Energy Technologies and Applications". Canadian Renewable Energy Network. Archived from the original on 25 June 2002. Retrieved 22 October 2007.</ref>

Early commercial adaptation

In 1878, at the Universal Exposition in Paris, Augustin Mouchot successfully demonstrated a solar steam engine but could not continue development because of cheap coal and other factors.

In 1897, Frank Shuman, a US inventor, engineer and solar energy pioneer built a small demonstration solar engine that worked by reflecting solar energy onto square boxes filled with ether, which has a lower boiling point than water and were fitted internally with black pipes which in turn powered a steam engine. In 1908 Shuman formed the Sun Power Company with the intent of building larger solar power plants. He, along with his technical advisor A.S.E. Ackermann and British physicist Sir Charles Vernon Boys,<ref name="Kryza2003">Frank Kryza (2003). The Power of Light. McGraw Hill Professional. pp. 64, 135. ISBN 978-0-07-140021-3. Retrieved 30 August 2022.</ref> developed an improved system using mirrors to reflect solar energy upon collector boxes, increasing heating capacity to the extent that water could now be used instead of ether. Shuman then constructed a full-scale steam engine powered by low-pressure water, enabling him to patent the entire solar engine system by 1912.

Shuman built the world's first solar thermal power station in Maadi, Egypt, between 1912 and 1913. His plant used parabolic troughs to power a 45–52 kilowatts (60–70 hp) engine that pumped more than 22,000 litres (4,800 imp gal; 5,800 US gal) of water per minute from the Nile River to adjacent cotton fields. Although the outbreak of World War I and the discovery of cheap oil in the 1930s discouraged the advancement of solar energy, Shuman's vision, and basic design were resurrected in the 1970s with a new wave of interest in solar thermal energy.<ref>Smith, Zachary Alden; Taylor, Katrina D. (2008). Renewable And Alternative Energy Resources: A Reference Handbook. ABC-CLIO. p. 174. ISBN 978-1-59884-089-6.</ref> In 1916 Shuman was quoted in the media advocating solar energy's utilization, saying:

We have proved the commercial profit of sun power in the tropics and have more particularly proved that after our stores of oil and coal are exhausted the human race can receive unlimited power from the rays of the Sun.
— Frank Shuman

Water heating

Solar hot water systems use sunlight to heat water. In middle geographical latitudes (between 40 degrees north and 40 degrees south), 60 to 70% of the domestic hot water use, with water temperatures up to 60 °C (140 °F), can be provided by solar heating systems.<ref>"Renewables for Heating and Cooling" (PDF). International Energy Agency. Archived from the original (PDF) on 24 September 2015. Retrieved 13 August 2015.</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>Weiss, Werner; Bergmann, Irene; Faninger, Gerhard. "Solar Heat Worldwide (Markets and Contributions to the Energy Supply 2005)" (PDF). International Energy Agency. Retrieved 30 May 2008.</ref>

As of 2015, the total installed capacity of solar hot water systems was approximately 436 thermal gigawatt (GW_th), and China is the world leader in their deployment with 309 GW_th installed, taken up 71% of the market.<ref name = "ADB-China2019">"Solar District Heating In The People's Republic of China" (PDF). Status and Development Potential. Asian Development Bank: 23. 1 July 2019. Retrieved 6 July 2021.</ref> Israel and Cyprus are the per capita leaders in the use of solar hot water systems with over 90% of homes using them.<ref>Del Chiaro, Bernadette; Telleen-Lawton, Timothy. "Solar Water Heating (How California Can Reduce Its Dependence on Natural Gas)" (PDF). Environment California Research and Policy Center. Archived from the original (PDF) on 27 September 2007. Retrieved 29 September 2007.</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_th as of 2005.<ref name="IEA Solar Thermal">Philibert, Cédric (2005). "The Present and Future use of Solar Thermal Energy as a Primary Source of Energy" (PDF). IEA. Archived (PDF) from the original on 26 April 2012.</ref>

Heating, cooling and ventilation

In the United States, heating, ventilation and air conditioning (HVAC) systems account for 30% (4.65 EJ/yr) of the energy used in commercial buildings and nearly 50% (10.1 EJ/yr) of the energy used in residential buildings.<ref>Apte, J.; et al. "Future Advanced Windows for Zero-Energy Homes" (PDF). American Society of Heating, Refrigerating and Air-Conditioning Engineers. Archived from the original (PDF) on 10 April 2008. Retrieved 9 April 2008.</ref><ref>"Energy Consumption Characteristics of Commercial Building HVAC Systems Volume III: Energy Savings Potential" (PDF). United States Department of Energy. Retrieved 24 June 2008.</ref> Solar heating, cooling and ventilation technologies can be used to offset a portion of this energy. Use of solar for heating can roughly be divided into passive solar concepts and active solar concepts, depending on whether active elements such as sun tracking and solar concentrator optics are used.

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 duly incorporated, thermal mass maintains space temperatures in a comfortable range and reduces the need for auxiliary heating and cooling equipment.<ref>Mazria (1979), pp. 29–35</ref>

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<ref>Bright, David (18 February 1977). "Passive solar heating simpler for the average owner". Bangor Daily News. Retrieved 3 July 2011.</ref> in a way that mimics greenhouses.

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 in the northern hemisphere or the northern side in the southern hemisphere, their leaves provide shade during the summer, while the bare limbs allow light to pass during the winter.<ref>Mazria (1979), p. 255</ref> 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.<ref>Balcomb (1992), p. 56</ref> In climates with significant heating loads, deciduous trees should not be planted on the Equator-facing 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.<ref>Balcomb (1992), p. 57</ref>

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.<ref>Anderson and Palkovic (1994), p. xi</ref> The simplest solar cooker is the box cooker first built by Horace de Saussure in 1767.<ref>Butti and Perlin (1981), pp. 54–59</ref> 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 (194–302 °F).<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 (599 °F) 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>

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, US 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.<ref>Stine, W.B. & Harrigan, R.W. (1982). "Shenandoah Solar Total Energy Project". NASA Sti/Recon Technical Report N. John Wiley. 83: 25168. Bibcode:1982STIN...8325168L. Retrieved 20 July 2008.</ref> Evaporation ponds are shallow pools that concentrate dissolved solids through evaporation. The use of evaporation ponds to obtain salt from seawater 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), pp. 393–94</ref>

Clothes lines, clotheshorses, and clothes racks dry clothes through evaporation by wind and sunlight without consuming electricity or gas. In some states of the United States legislation protects the "right to dry" clothes.<ref>Thomson-Philbrook, Julia. "Right to Dry Legislation in New England and Other States". Connecticut General Assembly. Retrieved 27 May 2008.</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 (40 °F) and deliver outlet temperatures of 45–60 °C (113–140 °F).<ref name="UTC">"Solar Buildings (Transpired Air Collectors – Ventilation Preheating)" (PDF). National Renewable Energy Laboratory. Retrieved 29 September 2007.</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 metres (380,000 sq ft) had been installed worldwide, including an 860 m² (9,300 sq ft) collector in Costa Rica used for drying coffee beans and a 1,300 m² (14,000 sq ft) collector in Coimbatore, India, used for drying marigolds.<ref name="Leon 2006"/>Template:Needs update

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.<ref name="Tiwari 2003">Tiwari (2003), pp. 368–71</ref> A large-scale solar distillation project was first constructed in 1872 in the Chilean mining town of Las Salinas.<ref name="Daniels 1964">Daniels (1964), p. 6</ref> The plant, which had solar collection area of 4,700 m² (51,000 sq ft), could produce up to 22,700 L (5,000 imp gal; 6,000 US gal) per day and operate 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. 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.<ref name="Tiwari 2003"/>

Solar water disinfection (SODIS) involves exposing water-filled plastic polyethylene terephthalate (PET) bottles to sunlight for several hours.<ref>"SODIS solar water disinfection". EAWAG (The Swiss Federal Institute for Environmental Science and Technology). Retrieved 2 May 2008.</ref> Exposure times vary depending on weather and climate from a minimum of six hours to two days during fully overcast conditions.<ref name="SODIS CDC">"Household Water Treatment Options in Developing Countries: Solar Disinfection (SODIS)" (PDF). Centers for Disease Control and Prevention. Archived from the original (PDF) on 29 May 2008. Retrieved 13 May 2008.</ref> It is recommended by the World Health Organization as a viable method for household water treatment and safe storage.<ref>"Household Water Treatment and Safe Storage". World Health Organization. Archived from the original on October 25, 2004. Retrieved 2 May 2008.</ref> Over two million people in developing countries use this method for their daily drinking water.<ref name="SODIS CDC"/>

Solar energy may be used in a water stabilization pond to treat waste water without chemicals or electricity. A further environmental advantage is that algae grow in such ponds and consume carbon dioxide in photosynthesis, although algae may produce toxic chemicals that make the water unusable.<ref>Shilton A.N.; Powell N.; Mara D.D.; Craggs R. (2008). "Solar-powered aeration and disinfection, anaerobic co-digestion, biological CO(2) scrubbing and biofuel production: the energy and carbon management opportunities of waste stabilization ponds". Water Sci. Technol. 58 (1): 253–58. doi:10.2166/wst.2008.666. PMID 18653962.</ref><ref>Tadesse I.; Isoaho S.A.; Green F.B.; Puhakka J.A. (2003). "Removal of organics and nutrients from tannery effluent by advanced integrated Wastewater Pond Systems technology". Water Sci. Technol. 48 (2): 307–14. doi:10.2166/wst.2003.0135. PMID 14510225.</ref>

Molten salt technology

Molten salt can be employed as a thermal energy storage method to retain thermal energy collected by a solar tower or solar trough of a concentrated solar power plant so that it can be used to generate electricity in bad weather or at night. It was demonstrated in the Solar Two project from 1995 to 1999. The system is predicted to have an annual efficiency of 99%, a reference to the energy retained by storing heat before turning it into electricity, versus converting heat directly into electricity.<ref>Mancini, Tom (10 January 2006). "Advantages of Using Molten Salt". Sandia National Laboratories. Archived from the original on 5 June 2011. Retrieved 14 July 2011.</ref><ref>Molten salt energy storage system – A feasibility study Jones, B.G.; Roy, R.P.; Bohl, R.W. (1977) – Smithsonian/NASA ADS Physics Abstract Service. Abstract accessed December 2007</ref><ref>Biello, David. "How to Use Solar Energy at Night". Scientific American. Retrieved 19 June 2011.</ref> The molten salt mixtures vary. The most extended mixture contains sodium nitrate, potassium nitrate and calcium nitrate. It is non-flammable and non-toxic, and has already been used in the chemical and metals industries as a heat-transport fluid. Hence, experience with such systems exists in non-solar applications.

The salt melts at 131 °C (268 °F). It is kept liquid at 288 °C (550 °F) in an insulated "cold" storage tank. The liquid salt is pumped through panels in a solar collector where the focused irradiance heats it to 566 °C (1,051 °F). It is then sent to a hot storage tank. This is so well insulated that the thermal energy can be usefully stored for up to a week.<ref>Ehrlich, Robert, 2013, "Renewable Energy: A First Course," CRC Press, Chap. 13.1.22 Thermal storage p. 375 ISBN 978-1-4398-6115-8</ref>

When electricity is needed, the hot salt is pumped to a conventional steam-generator to produce superheated steam for a turbine/generator as used in any conventional coal, oil, or nuclear power plant. A 100-megawatt turbine would need a tank about 9.1 metres (30 ft) tall and 24 metres (79 ft) in diameter to drive it for four hours by this design.

Several parabolic trough power plants in Spain<ref>Parabolic Trough Thermal Energy Storage Technology Archived 1 September 2013 at the Wayback Machine Parabolic Trough Solar Power Network. 4 April 2007. Accessed December 2007</ref> and solar power tower developer SolarReserve use this thermal energy storage concept. The Solana Generating Station in the U.S. has six hours of storage by molten salt. In Chile, The Cerro Dominador power plant has a 110 MW solar-thermal tower, the heat is transferred to molten salts.<ref>Chile to welcome largest solar concentration plant in Lat Am www.thisischile.cl Thursday, January 16, 2014 retrieved January 27, 2014</ref> The molten salts then transfer their heat in a heat exchanger to water, generating superheated steam, which feeds a turbine that transforms the kinetic energy of the steam into electric energy using the Rankine cycle.<ref>"Concentrating Solar Power Projects - Atacama-1". National Renewable Energy Laboratory. 1 July 2015. Retrieved 10 September 2016.</ref> In this way, the Cerro Dominador plant is capable of generating around 110 MW of power.<ref>Abengoa to build 110MW solar tower storage plant in Chile reneweconomy.com.au/ By Giles Parkinson on 13 January 2014</ref> The plant has an advanced storage system enabling it to generate electricity for up to 17.5 hours without direct solar radiation, which allows it to provide a stable electricity supply without interruptions if required. The Project secured up to 950 GW·h per year sale. Another project is the María Elena plant<ref>Here comes the sun Chile greenlights enormous 400-megawatt solar project www.thisischile.cl Friday, 23 August 2013 retrieved 30 August 2013</ref> is a 400 MW thermo-solar complex in the northern Chilean region of Antofagasta employing molten salt technology.

Electricity production

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Concentrated solar power

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 heat is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exists; the most developed are the parabolic trough, the solar tower collectors, the concentrating linear Fresnel reflector, and the Stirling dish. Various techniques are used to track the Sun and focus light. In all of these systems, a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage.<ref>Martin and Goswami (2005), p. 45</ref> Designs need to account for the risk of a dust storm, hail, or another extreme weather event that can damage the fine glass surfaces of solar power plants. Metal grills would allow a high percentage of sunlight to enter the mirrors and solar panels while also preventing most damage.

Architecture and urban planning

Sunlight has influenced building design since the beginning of architectural history.<ref name= "Schittich 2003">Schittich (2003), p. 14</ref> 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.<ref>Butti and Perlin (1981), pp. 4, 159</ref>

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.<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' Megaron House is a classic example of passive solar design.<ref name="Schittich 2003"/> The most recent approaches to solar design use computer modeling tying together solar lighting, heating and ventilation systems in an integrated solar design package.<ref>Balcomb (1992)</ref> 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 result from increased absorption of solar energy 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 to plant trees in the area. 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>Rosenfeld, Arthur; et al. "Painting the Town White – and Green". Heat Island Group. Archived from the original on 14 July 2007. Retrieved 29 September 2007.</ref>

Agriculture and horticulture

Agriculture and horticulture seek to optimize the capture of solar energy 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.<ref>Jeffrey C. Silvertooth. "Row Spacing, Plant Population, and Yield Relationships". University of Arizona. Retrieved 24 June 2008.</ref><ref>Kaul (2005), pp. 169–74</ref> 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.<ref>Butti and Perlin (1981), pp. 42–46</ref> Applications of solar energy in agriculture aside from growing crops include pumping water, drying crops, brooding chicks and drying chicken manure.<ref name="Leon 2006">Leon (2006), p. 62</ref><ref>Bénard (1981), p. 347</ref> More recently the technology has been embraced by vintners, who use the energy generated by solar panels to power grape presses.<ref>"A Powerhouse Winery". News Update. Novus Vinum. 27 October 2008. Retrieved 5 November 2008.</ref>

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.<ref>Butti and Perlin (1981), p. 19</ref> The first modern greenhouses were built in Europe in the 16th century to keep exotic plants brought back from explorations abroad.<ref>Butti and Perlin (1981), p. 41</ref> Greenhouses remain an important part of horticulture today. Plastic transparent materials have also been used to similar effect in polytunnels and row covers.

Transport

Winner of the 2013 World Solar Challenge in Australia

Solar electric aircraft circumnavigating the globe in 2015

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 3,021 kilometres (1,877 mi) across central Australia from Darwin to Adelaide. In 1987, when it was founded, the winner's average speed was 67 kilometres per hour (42 mph) and by 2007 the winner's average speed had improved to 90.87 kilometres per hour (56.46 mph).<ref>"The World Solar Challenge – The Background" (PDF). Australian and New Zealand Solar Energy Society. Archived from the original (PDF) on 19 July 2008. Retrieved 5 August 2008.</ref> 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>"North American Solar Challenge". New Resources Group. Retrieved 3 July 2008.</ref><ref>"South African Solar Challenge". Advanced Energy Foundation. Archived from the original on 12 June 2008. Retrieved 3 July 2008.</ref>

Some vehicles use solar panels for auxiliary power, such as for air conditioning, to keep the interior cool, thus reducing fuel consumption.<ref>Vehicle auxiliary power applications for solar cells. Institution of Electrical Engineers. 1991. pp. 187–191. ISBN 0-85296-525-7. Retrieved 11 October 2008.</ref><ref>"Systaic AG: Demand for Car Solar Roofs Skyrockets". Archived from the original on 5 May 2009. Retrieved 29 March 2011.</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>Schmidt, Theodor. "Solar Ships for the new Millennium". TO Engineering. Archived from the original on 9 October 2007. Retrieved 30 September 2007.</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>"The sun21 completes the first transatlantic crossing with a solar powered boat". Transatlantic 21. Retrieved 30 September 2007.</ref> There were plans to circumnavigate the globe in 2010.<ref>"PlanetSolar, the first solar-powered round-the-world voyage". PlanetSolar. 14 August 2015. Retrieved 20 November 2016.</ref>

In 1974, the unmanned AstroFlight Sunrise airplane 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 40 ft (12 m). 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 Scott Raymond in 21 hops flew from California to North Carolina using solar power.<ref>"EVWORLD FEATURE: Sunseeker Seeks New Records:SUNSEEKER | SAILPLANE | SOARING | GLIDER | ELECTRIC | RAYMOND | PV | PHOTOVOLTAIC | SOLAR | SUN | PLANE | AIRCRAFT | KITTYHAWK | AC PROPULSION". Archived from the original on 8 February 2008. Retrieved 8 February 2008.</ref> 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 29,524 metres (96,864 ft) in 2001.<ref>"Solar-Power Research and Dryden". NASA. Retrieved 30 April 2008.</ref> 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 were envisioned by 2010.<ref>"The NASA ERAST HALE UAV Program". Greg Goebel. Archived from the original on 10 February 2008. Retrieved 30 April 2008.</ref> From March 2015 to July 2016, Solar Impulse, an electric aircraft, successfully circumnavigated the globe. It is a single-seat plane powered by solar cells and capable of taking off under its own power. The design allows the aircraft to remain airborne for several days.<ref>Solar Impulse Project. "HB-SIA Mission". Archived from the original on 26 July 2011. Retrieved 5 December 2009.</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 generally limited to the toy market as the surface-area to payload-weight ratio is relatively high.<ref>"Phenomena which affect a solar balloon". pagesperso-orange.fr. Retrieved 19 August 2008.</ref>

Squad Solar vehical

The Squad Solar is a Neighborhood Electric Vehicle that has a solar roof and can be plugged into a normal 120 volt outlet to be charged.<ref>"Squad solar electric city car is coming to the US for $6,250". 6 December 2022.</ref>

Fuel production

Solar chemical processes use solar energy to drive chemical reactions. These processes offset energy that would otherwise come from a fossil fuel source and can also convert solar energy into storable and transportable fuels. Solar induced chemical reactions can be divided into thermochemical or photochemical.<ref>Bolton (1977), p. 1</ref> A variety of fuels can be produced by artificial photosynthesis.<ref>Wasielewski MR Photoinduced electron transfer in supramolecular systems for artificial photosynthesis. Chem. Rev. 1992; 92: 435–61.</ref> The multielectron catalytic chemistry involved in making carbon-based fuels (such as methanol) from reduction of carbon dioxide is challenging; a feasible alternative is hydrogen production from protons, though use of water as the source of electrons (as plants do) requires mastering the multielectron oxidation of two water molecules to molecular oxygen.<ref>Hammarstrom L. and Hammes-Schiffer S. Artificial Photosynthesis and Solar Fuels. Accounts of Chemical Research 2009; 42 (12): 1859–60.</ref> Some have envisaged working solar fuel plants in coastal metropolitan areas by 2050 – the splitting of seawater providing hydrogen to be run through adjacent fuel-cell electric power plants and the pure water by-product going directly into the municipal water system.<ref>Gray H.B. Powering the planet with solar fuel. Nature Chemistry 2009; 1: 7.</ref> In addition, chemical energy storage is another solution to solar energy storage.<ref>Feng, Hao; Liu, Jian; Zhang, Ying; Liu, Dong (2022-06-20). "Solar Energy Storage in an All-Vanadium Photoelectrochemical Cell: Structural Effect of Titania Nanocatalyst in Photoanode". Energies. 15 (12): 4508. doi:10.3390/en15124508. ISSN 1996-1073.</ref>

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. One such route uses concentrators to split water into oxygen and hydrogen at high temperatures (2,300–2,600 °C or 4,200–4,700 °F).<ref>Agrafiotis (2005), p. 409</ref> 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.<ref>Zedtwitz (2006), p. 1333</ref> 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 of Science uses a 1 MW solar furnace to decompose zinc oxide (ZnO) at temperatures above 1,200 °C (2,200 °F). This initial reaction produces pure zinc, which can subsequently be reacted with water to produce hydrogen.<ref>"Solar Energy Project at the Weizmann Institute Promises to Advance the use of Hydrogen Fuel". Weizmann Institute of Science. Archived from the original on 6 April 2008. Retrieved 25 June 2008.</ref>

Energy storage methods

Thermal mass systems can store solar energy in the form of heat at domestically useful temperatures for daily or interseasonal 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 off-peak hours and reduce overall heating and cooling requirements.<ref>Balcomb(1992), p. 6</ref><ref>"Request for Participation Summer 2005 Demand Shifting with Thermal Mass" (PDF). Demand Response Research Center. Archived from the original (PDF) on 7 September 2008. Retrieved 26 November 2007.</ref>

Phase change materials such as paraffin wax and Glauber's salt are another thermal storage medium. These materials are inexpensive, readily available, and can deliver domestically useful temperatures (approximately 64 °C or 147 °F). The "Dover House" (in Dover, Massachusetts) was the first to use a Glauber's salt heating system, in 1948.<ref>Butti and Perlin (1981), pp. 212–14</ref> Solar energy can also 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 project used this method of energy storage, allowing it to store 1.44 terajoules (400,000 kWh) in its 68 m³ storage tank with an annual storage efficiency of about 99%.<ref>"Advantages of Using Molten Salt". Sandia National Laboratory. Retrieved 29 September 2007.</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, while standard grid electricity can be used to meet shortfalls. Net metering programs give household systems credit for any electricity they deliver to the grid. This is handled by 'rolling back' the meter whenever the home produces more electricity than it consumes. If the net electricity use is below zero, the utility then rolls over the kilowatt-hour credit to the next month.<ref>"PV Systems and Net Metering". Department of Energy. Archived from the original on 4 July 2008. Retrieved 31 July 2008.</ref> Other approaches involve the use of two meters, to measure electricity consumed vs. electricity produced. This is less common due to the increased installation cost of the second meter. Most standard meters accurately measure in both directions, making a second meter unnecessary.

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, with the pump becoming a hydroelectric power generator.<ref>"Pumped Hydro Storage". Electricity Storage Association. Archived from the original on 21 June 2008. Retrieved 31 July 2008.</ref>

Development, deployment and economics

Beginning with the surge in coal use, which accompanied the Industrial Revolution, energy consumption 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.<ref>Butti and Perlin (1981), pp. 63, 77, 101</ref>

The 1973 oil embargo and 1979 energy crisis caused a reorganization of energy policies around the world. It brought renewed attention to developing solar technologies.<ref>Butti and Perlin (1981), p. 249</ref><ref>Yergin (1991), pp. 634, 653–73</ref> 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).<ref>"Chronicle of Fraunhofer-Gesellschaft". Fraunhofer-Gesellschaft. Retrieved 4 November 2007.</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 annual growth rates have averaged 20% since 1999.<ref name="SWH 2008">Weiss, Werner; Bergmann, Irene; Faninger, Gerhard. "Solar Heat Worldwide – Markets and Contribution to the Energy Supply 2006" (PDF). International Energy Agency. Retrieved 9 June 2008.</ref> Although generally underestimated, solar water heating and cooling is by far the most widely deployed solar technology with an estimated capacity of 154 GW as of 2007.<ref name="SWH 2008"/>

The International Energy Agency has said that solar energy can make considerable contributions to solving some of the most urgent problems the world now faces:<ref name=ie11/>

The development of affordable, inexhaustible, and clean solar energy technologies will have huge longer-term benefits. It will increase countries' energy security through reliance on an indigenous, inexhaustible, and mostly import-independent resource, enhance sustainability, reduce pollution, lower the costs of mitigating climate change, and keep fossil fuel prices lower than otherwise. These advantages are global. Hence the additional costs of the incentives for early deployment should be considered learning investments; they must be wisely spent and need to be widely shared.<ref name=ie11/>

In 2011, a report by the International Energy Agency found that solar energy technologies such as photovoltaics, solar hot water, and concentrated solar power could provide a third of the world's energy by 2060 if politicians commit to limiting climate change and transitioning to renewable energy. The energy from the Sun could play a key role in de-carbonizing the global economy alongside improvements in energy efficiency and imposing costs on greenhouse gas emitters. "The strength of solar is the incredible variety and flexibility of applications, from small scale to big scale".<ref>"IEA Says Solar May Provide a Third of Global Energy by 2060". Bloomberg Businessweek. 1 December 2011. Archived from the original on July 21, 2012.</ref>

We have proved ... that after our stores of oil and coal are exhausted the human race can receive unlimited power from the rays of the Sun.
— Frank Shuman

In 2021 Lazard estimated the levelized cost of new build unsubsidized utility scale solar electricity at less than 37 dollars per MWh and existing coal-fired power above that amount.<ref name="auto">"'Renewables' power ahead to become the world's cheapest source of energy in 2020". World Economic Forum. 5 July 2021. Retrieved 2022-01-25.</ref><ref name=":0">"Levelized Cost Of Energy, Levelized Cost Of Storage, and Levelized Cost Of Hydrogen". Lazard.com. Retrieved 2022-01-25.</ref> The 2021 report also said that new solar was also cheaper than new gas-fired power, but not generally existing gas power.<ref name=":0" />

Emerging technologies

Experimental solar power

Concentrated photovoltaics (CPV) systems employ sunlight concentrated onto photovoltaic surfaces for the purpose of electricity generation. Thermoelectric, or "thermovoltaic" devices convert a temperature difference between dissimilar materials into an electric current.

Floating solar arrays

page-not-found

Solar-assisted heat pump

A heat pump is a device that provides heat energy from a source of heat to a destination called a "heat sink". Heat pumps are designed to move thermal energy opposite to the direction of spontaneous heat flow by absorbing heat from a cold space and releasing it to a warmer one. A solar-assisted heat pump represents the integration of a heat pump and thermal solar panels in a single integrated system. Typically these two technologies are used separately (or only placing them in parallel) to produce hot water.<ref name="ref1">"Solar-assisted heat pumps". Archived from the original on 28 February 2020. Retrieved 21 June 2016.</ref> In this system the solar thermal panel performs the function of the low temperature heat source and the heat produced is used to feed the heat pump's evaporator.<ref name="ref2">"Pompe di calore elio-assistite" (in italiano). Archived from the original on 7 January 2012. Retrieved 21 June 2016.</ref> The goal of this system is to get high COP and then produce energy in a more efficient and less expensive way.

It is possible to use any type of solar thermal panel (sheet and tubes, roll-bond, heat pipe, thermal plates) or hybrid (mono/polycrystalline, thin film) in combination with the heat pump. The use of a hybrid panel is preferable because it allows covering a part of the electricity demand of the heat pump and reduces the power consumption and consequently the variable costs of the system.

Solar aircraft

An electric aircraft is an aircraft that runs on electric motors rather than internal combustion engines, with electricity coming from fuel cells, solar cells, ultracapacitors, power beaming,<ref>"Power Beaming". Archived from the original on February 17, 2013.</ref> or batteries.

Currently, flying manned electric aircraft are mostly experimental demonstrators, though many small unmanned aerial vehicles are powered by batteries. Electrically powered model aircraft have been flown since the 1970s, with one report in 1957.<ref name="Noth">Noth, André (July 2008). "History of Solar Flight" (PDF). Autonomous Systems Lab. Zürich: Swiss Federal Institute of Technology. p. 3. Archived from the original (PDF) on 1 February 2012. Retrieved 8 July 2010. Günter Rochelt was the designer and builder of Solair I, a 16 m wingspan solar airplane ... 21st of August 1983 he flew in Solair I, mostly on solar energy and also thermals, during 5 hours 41 minutes.</ref><ref>"Infographic: A Timeline Of The Present And Future Of Electric Flight". Popular Science. 20 August 2015. Archived from the original on 14 January 2016. Retrieved 7 January 2016.</ref> The first man-carrying electrically powered flights were made in 1973.<ref name="JAWA74">Taylor, John W R (1974). Jane's All the World's Aircraft 1974-75. London: Jane's Yearbooks. p. 573. ISBN 0-354-00502-2.</ref> Between 2015 and 2016, a manned, solar-powered plane, Solar Impulse 2, completed a circumnavigation of the Earth.<ref name="Batrawy">Batrawy, Aya (9 March 2015). "Solar-powered plane takes off for flight around the world". Associated Press. Archived from the original on 6 March 2016. Retrieved 14 March 2015.</ref>

References

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