SOLAR ENERGY is radiant light and heat from the
It is an important 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 .
The large magnitude of solar energy available makes it a highly
appealing source of electricity. The United Nations Development
Programme in its 2000 World
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 , 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".
* 1 Potential
* 2 Thermal energy
* 4 Architecture and urban planning
Further information: Solar radiation 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 . 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 . 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.
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 anti-cyclones . Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 °C. By photosynthesis , green plants convert solar energy into chemically stored energy, which produces food, wood and the biomass from which fossil fuels are derived.
The total solar energy absorbed by Earth's atmosphere, oceans and
land masses is approximately 3,850,000 exajoules (EJ) per year. In
2002, this was more energy in one hour than the world used in one
YEARLY SOLAR FLUXES font-weight: normal; text-align: left; padding:
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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.
Geography affects solar energy potential because areas that are
closer to the equator have a greater amount of solar radiation.
However, the use of photovoltaics that can follow the position of the
sun can significantly increase the solar energy potential in areas
that are farther from the equator. Time variation effects the
potential of solar energy because during the nighttime there is little
solar radiation on the surface of the Earth for solar panels to
absorb. This limits the amount of energy that solar panels can absorb
in one day.
In addition, land availability has a large effect on the available solar energy because solar panels can only be set up on land that is otherwise unused and suitable for solar panels. Roofs have been found to be a suitable place for solar cells, as many people have discovered that they can collect energy directly from their homes this way. Other areas that are suitable for solar cells are lands that are not being used for businesses where solar plants can be established.
Solar technologies are characterized 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 distance from the equator. Although solar
energy refers primarily to the use of solar radiation for practical
ends, all renewable energies, other than
Active solar techniques use photovoltaics, concentrated solar power , solar thermal collectors , 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.
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 that took into account factors such as insolation, cloud cover, and the land that is usable by humans. The estimate found that solar energy has a global potential of 1,575–49,837 EJ per year (see table below).
Annual solar energy potential by region (Exajoules) 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
* 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)
Main article: Solar thermal energy
Solar thermal technologies can be used for water heating, space heating, space cooling and process heat generation.
EARLY COMMERCIAL ADAPTATION
1917 Patent drawing of Shuman's solar collector
Frank Shuman , a U.S. 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
Shuman built the world's first solar thermal power station in
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, New York Times, 2 July 1916
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. 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.
As of 2007, the total installed capacity of solar hot water systems
was approximately 154 thermal gigawatt (GWth). China is the world
leader in their deployment with 70
GWth installed as of 2006 and a
long-term goal of 210
GWth by 2020.
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. Solar heating, cooling and ventilation technologies can be used to offset a portion of this energy. MIT 's Solar House #1, built in 1939 in the U.S., used seasonal thermal energy storage for year-round heating.
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.
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. 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. 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.
Solar cookers use sunlight for cooking, drying and pasteurization . They can be grouped into three broad categories: box cookers, panel cookers and reflector cookers. The simplest solar cooker is the box cooker first built by Horace de Saussure in 1767. 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). 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.
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
Solar distillation can be used to make saline or brackish water potable. The first recorded instance of this was by 16th-century Arab alchemists. A large-scale solar distillation project was first constructed in 1872 in the Chilean mining town of Las Salinas. 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. 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. Exposure times vary depending on weather and climate from a minimum of six hours to two days during fully overcast conditions. It is recommended by the World Health Organization as a viable method for household water treatment and safe storage. Over two million people in developing countries use this method for their daily drinking water.
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 1995–1999. 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. The molten salt mixtures vary. The most extended mixture contains sodium nitrate , potassium nitrate and calcium nitrate . It is non-flammable and nontoxic, and has already been used in the chemical and metals industries as a heat-transport fluid, so 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 sun 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.
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 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. The María Elena plant is a 400 MW thermo-solar complex in the northern Chilean region of Antofagasta employing molten salt technology.
Main article: Solar power Some of the world's largest solar power stations: Ivanpah (CSP) and Topaz (PV)
Solar power is the conversion of sunlight into electricity , either directly using photovoltaics (PV), or indirectly using concentrated solar power (CSP). CSP systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. PV converts light into electric current using the photoelectric effect .
Solar power is anticipated to become the world's largest source of electricity by 2050, with solar photovoltaics and concentrated solar power contributing 16 and 11 percent to the global overall consumption, respectively.
Commercial CSP plants were first developed in the 1980s. Since 1985 the eventually 354 MW SEGS CSP installation, in the Mojave Desert of California, is the largest solar power plant in the world. Other large CSP plants include the 150 MW Solnova Solar Power Station and the 100 MW Andasol solar power station , both in Spain. The 250 MW Agua Caliente Solar Project , in the United States, and the 221 MW Charanka Solar Park in India, are the world\'s largest photovoltaic plants . Solar projects exceeding 1 GW are being developed, but most of the deployed photovoltaics are in small rooftop arrays of less than 5 kW, which are connected to the grid using net metering and/or a feed-in tariff. In 2013 solar generated less than 1% of the world's total grid electricity.
Worldwide growth of PV capacity grouped by region in MW (2006–2014)
In the last two decades, photovoltaics (PV), also known as solar PV,
has evolved from a pure niche market of small scale applications
towards becoming a mainstream electricity source. A solar cell is a
device that converts light directly into electricity using the
photoelectric effect. The first solar cell was constructed by Charles
Fritts in the 1880s. In 1931 a German engineer, Dr Bruno Lange,
developed a photo cell using silver selenide in place of copper oxide
. Although the prototype selenium cells converted less than 1% of
incident light into electricity, both
Ernst Werner von Siemens and
James Clerk Maxwell
CONCENTRATED SOLAR POWER
See also: 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 concentrating linear
fresnel reflector, the Stirling dish and the solar power tower.
Various techniques are used to track the
ARCHITECTURE AND URBAN PLANNING
Passive solar building design and
Urban heat island
Darmstadt University of Technology
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.
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.
AGRICULTURE AND HORTICULTURE
Greenhouses like these in the Westland municipality of the Netherlands grow vegetables, fruits and flowers.
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
Solar vehicle ,
Solar-charged vehicle , Electric boat
Development of a solar-powered car has been an engineering goal since
the 1980s. The
World Solar Challenge
Some vehicles use solar panels for auxiliary power, such as for air conditioning, to keep the interior cool, thus reducing fuel consumption.
In 1975, the first practical solar boat was constructed in England. By 1995, passenger boats incorporating PV panels began appearing and are now used extensively. 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. There were plans to circumnavigate the globe in 2010.
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. 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. The Zephyr , developed by
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.
Concentrated solar panels are getting a power boost. Pacific Northwest National Laboratory (PNNL) will be testing a new concentrated solar power system -- one that can help natural gas power plants reduce their fuel usage by up to 20 percent. Main articles: Solar chemical , Solar fuel , and Artificial photosynthesis
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 . A variety of fuels can be produced by artificial photosynthesis . 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. Some have envisaged working solar fuel plants in coastal metropolitan areas by 2050 – the splitting of sea water 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. Another vision involves all human structures covering the earth's surface (i.e., roads, vehicles and buildings) doing photosynthesis more efficiently than plants.
ENERGY STORAGE METHODS
Energy storage ,
Seasonal thermal energy storage ,
Phase change material ,
Grid energy storage
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,
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 a 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. 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.
DEVELOPMENT, DEPLOYMENT AND ECONOMICS
Participants in a workshop on sustainable development inspect solar panels at Monterrey Institute of Technology and Higher Education, Mexico City on top of a building on campus. Main article: Deployment of solar power to energy grids See also: Cost of electricity by source and Renewable energy by country
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 .
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. Deployment strategies focused on
incentive programs such as the Federal Photovoltaic Utilization
Program in the U.S. and the Sunshine Program in Japan. Other efforts
included the formation of research facilities in the U.S. (SERI, now
NREL ), Japan (NEDO ), and Germany (Fraunhofer Institute for Solar
Commercial solar water heaters began appearing in the United States in the 1890s. These systems saw increasing use until the 1920s but were gradually replaced by cheaper and more reliable heating fuels. 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. 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.
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:
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.
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 . 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".
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 , New York Times, 2 July 1916
The International Organization for Standardization has established several standards relating to solar energy equipment. For example, ISO 9050 relates to glass in building while ISO 10217 relates to the materials used in solar water heaters.
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