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Solar energy is energy from the Sun in the form of heat and light. This energy drives the climate and weather and supports virtually all life on Earth. Heat and light from the Sun, along with secondary solar resources such as wind and wave power, hydroelectricity and biomass, account for most of the available flow of renewable energy on Earth.

Solar energy technologies harness the Sun's heat and light for practical ends such as heating, lighting and electricity. These technologies date from the time of the early Greeks, Native Americans and Chinese, who warmed their buildings by orienting them toward the Sun.

Solar power is used synonymously with solar energy or more specifically to refer to the conversion of sunlight into electricity. This can be done with photovoltaics, concentrating solar thermal devices and various experimental technologies. Solar power provided 0.039% of the world's Total Primary Energy Supply (TPES) for the year 2004.

Energy from the Sun

Earth continuously receives 174 petawatts of incoming solar radiation (insolation) at the upper atmosphere. When it meets the atmosphere, 6 percent of the insolation is reflected and 16 percent is absorbed. Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation traveling through the atmosphere by 20 percent due to reflection and 3 percent via absorption. These atmospheric conditions not only reduce the quantity of energy reaching the earth's surface, but also diffuse approximately 20 percent of the incoming light and filter portions of its spectrum. After passing through the atmosphere, approximately half the insolation is in the visible electromagnetic spectrum with the other half mostly in the infrared spectrum (a small part is ultraviolet radiation).

The absorption of solar energy by atmospheric convection (sensible heat transport) and evaporation and condensation of water vapor (latent heat transport) powers the water cycle and drives the winds. Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 °C. The small portion of solar energy captured by plants and other phototrophs is converted to chemical energy via photosynthesis. All the food we eat, wood we build with, and fossil fuels we use are products of photosynthesis.

The flows and stores of solar energy in the environment are vast in comparison to human energy needs.

• The total solar energy absorbed by Earth's atmosphere, oceans, and land masses is approximately 3850zettajoules (ZJ) per year.
• Winds can potentially supply 6 ZJ of energy per year.
• Biomass captures approximately 1.8 ZJ of solar energy per year.
• Worldwide energy consumption was 0.471 ZJ in 2004.

The upper map on the right shows how solar radiation at the top of the earth's atmosphere varies with latitude, while the lower map shows annual average ground-level insolation. For example, in North America, the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day). At present, photovoltaic panels typically convert about 15 percent of incident sunlight into electricity; therefore, a solar panel in the contiguous United States, on average, delivers 19 to 56 W/m² or 0.45 - 1.35 kWh/m²/day.

Types of technologies

Solar energy technologies utilize heat and light from the Sun for practical ends. Technologies that utilize secondary solar resources such as biomass, wind, waves, and ocean thermal gradients are sometime included in a broader description of solar energy but only primary resource applications are discussed here. These applications span through the residential, commercial, industrial, agricultural and transportation sectors where solar energy is used to make clean water, produce food, dry clothes, heat and light buildings and generate electricity.

Architecture and urban planning

Solar design can provide practical lighting, comfortable temperatures, and improved air quality by tailoring building orientation, proportion, window placement, and material components to the local climate and environment. As climate varies by region so too will the features of solar-designed buildings. In the words of the first century Roman architect Vitruvius:

We must begin by taking note of the countries and climates in which homes are to be built if our designs for them are to be correct. One type of house seems appropriate for Egypt, another for Spain...one still different for Rome, and so on with lands and countries of varying characteristics. This is because one part of the Earth is directly under the Sun's course, another is far away from it, while another lies midway between these two....It is obvious that designs for homes ought to conform to diversities of climate.

Urban heat islands (UHI) are metropolitan areas with higher temperatures than the surrounding environment. These higher temperatures are the result of urban materials such as asphalt and concrete that have lower albedos and higher heat capacities than the natural environment. A straightforward method of counteracting the UHI effect is to paint buildings and roads white and plant trees. A hypothetical "cool communities" program in Los Angeles has projected that urban temperatures could be reduced by approximately 3 °C after planting ten million trees, reroofing five million homes, and painting one-quarter of the roads. The estimated cost of the cool communities program is approximately US$1 billion, with an annual benefit estimated at $170 million resulting from reduced air-conditioning costs alone. An additional $360 million in health costs could be saved annually by the associated reductions in smog.

Agriculture and horticulture

Agriculture inherently seeks to optimize the capture of solar energy, and thereby plant productivity. Techniques such as timed planting cycles, tailored row orientation, staggered heights between rows, and the mixing of plant varieties can improve crop yields. While sunlight is generally considered a plentiful resource, there are exceptions which highlight the importance of solar energy to agriculture. During the short growing seasons of the Little Ice Age, French and English farmers employed fruit walls to maximize the collection of solar energy. These walls acted as thermal masses and improved yields by keeping plants warm. Early fruit walls were built perpendicular to the ground with a south facing orientation but over time sloping walls were developed to make better use of sunlight. In 1699, Nicolas Fatio de Duillier even suggested using a tracking mechanism which could pivot to follow the Sun. Solar energy is also used in many areas of agriculture aside from growing crops. Applications include pumping water, drying crops, brooding chicks, and drying chicken manure.

Greenhouses control the use of solar heat and light to grow specialty crops. Primitive greenhouses were first used during Roman times to grow cucumbers for the Roman emperor Tiberius. In the 16th century the first modern greenhouses were built in Italy to conserve exotic plants brought back from explorations abroad. Greenhouses remain an important part of horticulture today where they are often used to grow fruits, vegetables, and flowers that are relatively exotic when considered against the local climate. The largest greenhouse complex in the world is in Willcox, Arizona where 106 hectares of tomatoes and cucumbers are entirely grown under glass. Plastic transparent materials have also been utilised to similar effect in Polytunnels.

Solar lighting

The history of lighting is dominated by the use of natural light. The Romans recognized the Right to Light as early as the 6th century and English law echoed these judgments with the Prescription Act of 1832. In the 20th century artificial lighting became the main source of interior illumination and today approximately 22 percent (8.6 EJ) of the electricity used in the United States is for lighting. When daylighting features are properly implemented they can reduce commercial lighting related energy requirements by 25 percent (1 EJ).

Daylighting systems collect and distribute sunlight to provide interior illumination. These systems directly offset energy use by replacing artificial lighting and indirectly offset energy use by reducing cooling loads. Although difficult to quantify, the use of natural lighting also offers physiological and psychological benefits compared to artificial lighting. Daylighting design carefully selects window type, size, and orientation and may consider exterior shading devices as well. Individual features include sawtooth roofs, clerestory windows, light shelves, skylights and light tubes. These features may be incorporated into existing structures but are most effective when integrated in a solar design package that accounts for factors such as glare, heat gain, heat loss and time-of-use. Architectural trends increasingly recognize daylighting as a cornerstone of sustainable design.

Hybrid solar lighting (HSL) is an active solar method of using sunlight to provide illumination. These systems collect sunlight using focusing mirrors that track the Sun and use optical fibers to transmit the light into a building's interior to supplement conventional lighting. In single-story applications, these systems are able to transmit 50 percent of the direct sunlight received.

Daylight saving time (DST) utilizes solar energy by matching available sunlight to the time of the day in which it is most useful. DST shifts electricity use from evening to morning hours thus lowering evening peak loads and the higher costs associated with peaking electricity. In California, winter season DST has been estimated to cut daily peak load by 3 percent and total electricity use by 3400 MWh. DST has been estimated to reduce early spring and late fall peak loads by 1.5 percent and total daily electricity use by 1000-2000 MWh. DST, like other solar energy technologies, has not proven successful in all regions.

Solar thermal

Solar thermal applications make up the most widely used and diverse category of solar energy technology. These technologies use heat from the Sun for water and space heating, ventilation, industrial process heat, cooking, water distillation and disinfection, and many other applications.

Water heating

Solar hot water systems use sunlight to heat water. 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 relatively cheap and more reliable conventional heating fuels. The economic advantage of conventional heating fuels has varied over time resulting in periodic interest in solar hot water; however, solar hot water technologies have yet to show the sustained momentum they had until the 1920s. Recent price spikes, erratic availability of conventional fuels, and other factors are renewing interest in solar heating technologies. Approximately 14 percent (15 EJ) of the total energy used in the United States is for water heating. In many climates, a solar heating system can provide 50 to 75 percent of domestic hot water use.

As of 2007, the total installed capacity of solar hot water systems is approximately 128 GWth and growth is 15-20 percent per year. China is the world leader in the deployment of solar hot water with 100 km² installed as of 2006 and a long term goal of 300 km² by 2020. Israel is the per capita leader in the use of solar hot water with 90 percent of homes using this technology. In the United States, Canada, and Australia, heating swimming pools is the dominant application of solar hot water with an installed capacity of 18 GWth as of 2005.

Solar water heating technologies have high efficiencies relative to other solar technologies. Performance will depend upon the site of deployment, but flat-plate and evacuated-tube collectors can be expected to have efficiencies above 60 percent during normal operating conditions. In addition, solar water heating is particularly appropriate for low-temperature (25-70 °C) applications such as swimming pools, domestic hot water, and space heating. The most common types of solar water heaters are batch systems, flat plate collectors and evacuated tube collectors.

Heating, cooling and ventilation

In the United States, heating, ventilation, and air conditioning (HVAC) systems account for over 25 percent (4.75 EJ) of the energy used in commercial buildings and nearly half (10.1 EJ) of the energy used in residential buildings. Solar heating, cooling, and ventilation technologies can be used to offset a portion of this energy.

Thermal mass materials store solar energy during the day and release this energy during cooler periods. Common thermal mass materials include stone, cement, and water. The proportion and placement of thermal mass should consider several factors such as climate, daylighting, and shading conditions. When properly incorporated, thermal mass can passively maintain comfortable temperatures while reducing energy consumption.

A solar chimney (or thermal chimney) is a passive solar ventilation system composed of a hollow thermal mass 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. These systems have been in use since Roman times and remain common in the Middle east.

A Trombe wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between a window and a sun-facing thermal mass. During the ventilation cycle, sunlight stores heat in the thermal mass and warms the air channel causing circulation through vents at the top and bottom of the wall. During the heating cycle the Trombe wall radiates stored heat.

Solar roof ponds are a unique solar heating and cooling technology developed by Harold Hay in the 1960s. A basic system consists of a roof mounted water bladder with a movable insulating cover. This system can control heat exchange between interior and exterior environments by covering and uncovering the bladder between night and day. When heating is a concern the bladder is uncovered during the day allowing sunlight to warm the water bladder and store heat for evening use. When cooling is a concern the covered bladder draws heat from the building's interior during the day and is uncovered at night to radiate heat to the cooler atmosphere. The Skytherm house in Atascadero, California uses a prototype roof pond for heating and cooling.

Active solar cooling can be achieved via absorption refrigeration cycles, desiccant cycles, and solar mechanical processes. In 1878, Auguste Mouchout pioneered solar cooling by making ice using a solar steam engine attached to a refrigeration device. Thermal mass, smart windows and shading methods can also be used to provide cooling. The leaves of deciduous trees provide natural shade during the summer while the bare limbs allow light and warmth into a building during the winter. The water content of trees will also help moderate local temperatures.

Process heat

Evaporation ponds are shallow ponds 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. Altogether, evaporation ponds represent one of the largest commercial applications of solar energy in use today.

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. The short payback period of transpired collectors (3 to 12 years) make them a more cost-effective alternative to glazed collection systems. As of 2003, over 80 systems with a combined collector area of 35,000 m² had been installed worldwide. Representatives include an 860 m² collector in Costa Rica used for drying coffee beans and a 1300 m² collector in Coimbatore, India used for drying marigolds.

A food processing facility in Modesto, California uses parabolic troughs to produce steam used in the manufacturing process. The 5,000 m² collector area is expected to provide 4.3 GJ per year.

Cooking

Solar cookers use sunlight for cooking, drying and pasteurization. Solar cooking offsets fuel costs, reduces demand for fuel or firewood, and improves air quality by reducing or removing a source of smoke.

The simplest type of 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. These cookers can be used effectively with partially overcast skies and will typically reach temperatures of 50-100 °C.

Concentrating solar cookers use reflectors to concentrate light on a cooking container. The most common reflector geometries are flat plate, disc and parabolic trough type. These designs cook faster and at higher temperatures (up to 350 °C) but require direct light to function properly.

The Solar Kitchen in Auroville, India uses a unique concentrating technology known as the solar bowl. Contrary to conventional tracking reflector/fixed receiver systems, the solar bowl uses a fixed spherical reflector with a receiver which tracks the focus of light as the Sun moves across the sky. The solar bowl's receiver reaches temperature of 150 °C that are used to produce steam that helps cook 2,000 daily meals.

Many other solar kitchen in India use another unique concentrating technology known as the Scheffler reflector. This technology was first developed by Wolfgang Scheffler in 1986. A Scheffler reflector is a parabolic dish that uses single axis tracking to follow the Sun's daily course. These reflectors have a flexible reflective surface that is able to change its curvature to adjust to seasonal variations in the incident angle of sunlight. Scheffler reflectors have the advantage of having a fixed focal point which improves the ease of cooking and are able to reach temperatures of 450-650 °C. Built in 1999, the world's largest Scheffler reflector system in Abu Road, Rajasthan India is capable of cooking up to 35,000 meals a day. By early 2008, over 2000 large cookers of the Scheffler design had been built worldwide.

Disinfection and desalination

Solar water disinfection, also known as SODIS, is a simple method of disinfecting water using only sunlight and plastic PET bottles. SODIS is a cheap and effective method for decentralized water treatment, usually applied at the household level and is recommended by the World Health Organization as a viable method for household water treatment and safe storage. SODIS has over two million users in developing countries such as Brazil, Cameroon and Uzbekistan.

A solar still uses solar energy to distill water. The main types are cone shaped, boxlike, and pit. The box shaped types are most sophisticated of these and the pit types the least sophisticated. In cone solar stills, impure water is inserted into the container, where it is evaporated by sunlight coming through clear plastic. Free of solids in suspension or solution, the water vapor condenses on top and drips down to the side, where it is collected and removed.

Solar electricity

Electricity can be generated from the Sun in several ways. Photovoltaics (PV) has been mainly developed for small and medium-sized applications, from the calculator powered by a single solar cell to the PV power plant. For large-scale generation, concentrating solar thermal power plants have been more common but new multi-megawatt PV plants have been built recently. Other solar electrical generation technologies are still at the experimental stage.

Photovoltaics

A solar cell or photovoltaic cell is a device that converts light into electricity using the photoelectric effect. The first working solar cells were constructed by Charles Fritts in 1883. These prototype cells were made of selenium and achieved efficiencies around one percent. Following the fundamental work of Russell Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and Daryl Chapin created the silicon solar cell in 1954.

In 1958, photovoltaics were used successfully as a power source for the Vanguard I satellite. This example was followed by many other Soviet and American satellites, so that by the late 1960s PV had become the established source of power for satellites. Photovoltaics went on to play an essential part in the success of early commercial satellites such as Telstar and Syncom.

The high cost of PV limited its application for terrestrial uses throughout the 1960s but this started to change in the early 1970s as module prices reached levels that made PV generation competitive in remote areas without grid access. Early terrestrial uses included powering remote telecommunication stations, cathodic protection of pipelines, off-shore oil rigs, railroad crossings and lighthouses.

The 1973 oil embargo initiated a reorganization of energy policies around the world and brought unprecedented attention to the development of photovoltaics. Incentive programs quickly followed in the USA (the Federal Photovoltaic Utilization Program) and Japan (the Sunshine Program). Other efforts included the formation of solar research facilities in the USA (NREL), Japan (NEDO), and Germany (Fraunhofer Institute for Solar Energy Systems ISE). These developments helped reduced the price of PV from $100 per watt in 1970 to $11 per watt in 1980 and lead to the installation of approximately 12.5 MW of PV during the 1970s.

When oil prices began to fall in the early 1980s the growth of PV slowed. In the context of historically-low oil prices from 1986-1999, funding for PV research was relatively low and the issue was not high in public consciousness. Nevertheless, the output of PV-generated electricity grew by 10 to 20 percent per year throughout the 1980s and 1990s and reached 1000 MW in 1999. During the same time period, PV cells steadily fell in price and reached $3.50 per watt in 1999.

Photovoltaics have grown at an average rate of 35 percent since 1997 with average growth rates in both 2006 and 2007 reaching 50 percent. Worldwide PV installations now total approximately 10.5 gigawatts as of year-end 2007.

With many jurisdictions now giving tax and rebate incentives, PV installations can pay for themselves in five to ten years in many places. "Grid-connected" systems - those systems that use an inverter to connect to the utility grid instead of relying on batteries - now make up the largest part of the market. While the deployment of PV power depends largely upon local conditions and requirements, most countries are taking an interest in developing PV as one of their options for renewable energy supply.

Concentrating solar

Concentrated sunlight has been used to perform useful tasks from the time of ancient China. A legend claims Archimedes used polished shields to concentrate sunlight on the invading Roman fleet and repel them from Syracuse in 212 BC. In 1866, Auguste Mouchout used a parabolic trough to produce steam for the first solar steam engine. Over the following 50 years, inventors such as John Ericsson and Frank Shuman developed concentrating solar-powered devices for irrigation, refrigeration and locomotion. The progeny of these early developments are the concentrating solar thermal power plants of today.

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. Although a wide range of concentrating technologies exist, the most developed are the solar trough, parabolic dish and solar power tower. Each concentration method is capable of producing high temperatures and correspondingly high thermodynamic efficiencies, but they vary in the way they track the Sun and focus light.

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. A working fluid is heated up to 500 °C as it flows through the receiver and is then used as a heat source for a power generation system. Trough systems are the most developed CSP technology. The Solar Energy Generating Systems (SEGS) plants in California, Acciona's Nevada Solar One near Boulder City, Nevada, and Plataforma Solar de Almería's SSPS-DCS plant in Spain are representatives of this technology.

A parabolic dish or dish/engine 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. The working fluid in the receiver is heated to 1000 °C and then used by a Stirling engine for power generation. Parabolic dish systems display the highest solar-to-electric efficiency among CSP technologies and their modular nature offers scalability. The Stirling Energy Systems (SES) and Science Applications International Corporation (SAIC) dishes at UNLV and the Big Dish in Canberra, Australia, are representatives of this technology.

A solar power tower consists of an array of dual axis tracking reflectors (heliostats) that concentrate light on a central receiver atop a tower. The working fluid in the receiver is heated up to 1500 °C and then used as a heat source for a power generation or energy storage system. Power towers are less advanced than trough systems but they offer higher efficiency and better energy storage capability. The Solar Two in Daggett, California and the Planta Solar 10 (PS10) in Sanlucar la Mayor, Spain are representatives of this technology.

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.

A solar pond is a pool of salt water (usually 1-2 meters 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. 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. Current representatives of this technology include a 150 kW pond in Ein Bokek, Israel, and another used for industrial process heat at the University of Texas El Paso.

Thermoelectric devices convert a temperature difference between dissimilar materials into an electric current. These devices are seen as a potential energy conversion technology for solar concentrating systems. The solar pioneer Mouchout envisioned using the thermoelectric effect to store solar energy; however, his experiments toward this end never progressed beyond primitive devices. Thermoelectrics reemerged in the Soviet Union during the 1930s. Under the direction of Soviet scientist Abram Ioffe a concentrating system was used to thermoelectricly generate power for a 1 hp engine. Thermogenerators were later used in US space program as an energy conversion technology for powering deep space missions such as Cassini, Galileo and Viking. Current research is focused on raising the efficiency of these devices from 7-8 percent up to 15-20 percent.

Solar vehicles

Development of a solar powered car has been an engineering goal since the 1980s. The center of this development is the World Solar Challenge, a biannual solar-powered car race in which teams from universities and enterprises compete over 3,021 km (1,877 mi) across central Australia from Darwin to Adelaide. In 1987, when it was founded, the winner's average speed was 67 km/h (42 mph). The 2007 race included a new challenge class using cars with an upright seating position and which, with little modification, could be a practical proposition for sustainable transport. The winning car averaged 90.87 km/h (56.46 mph). The North American Solar Challenge (formerly Sunrayce USA) and the planned South African Solar Challenge are comparable competitions that reflect an international interest in the engineering and development of solar powered vehicles.

The batteries of electric bicycles may be charged from solar-generated electricity; alternatively a PV panel may be located on the bicycle itself.

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. Plans to circumnavigate the globe in 2009 are indicative of the progress solar boats have made.

In 1974, the unmanned Sunrise II inaugurated the era of solar flight. In 1980, the Gossamer Penguin made the first piloted flights powered solely by photovoltaics. This was quickly followed by the Solar Challenger which demonstrated a more airworthy design with its crossing of the English Channel in July, 1981. Developments then turned back to unmanned aerial vehicles with the Pathfinder (1997), Pathfinder Plus (1998) and Centurion (1998) each building on one another. These designs culminated in the Helios which set the altitude record for a non-rocket-propelled aircraft of 29,524 m (96,864 ft) in 2001. The Zephyr, developed by BAE Systems, is the latest in a line of record breaking solar aircraft. This aircraft made a record setting 54 hours duration flight in 2007, and month long duration flights are envisioned by 2010.

A solar balloon is a black balloon that is filled with ordinary air. As sunlight shines on the balloon, the air inside is heated and expands, causing an upward buoyancy force, much like an artificially-heated hot air balloon. Some solar balloons are large enough for human flight, but usage is limited to the toy market as the surface-area to payload-weight ratio is rather high.

Solar sails are a proposed form of spacecraft propulsion using large membrane mirrors. Radiation pressure is small and decreases by the square of the distance from the Sun, but unlike rockets, solar sails require no fuel. Although the thrust is small compared to rockets, it continues as long as the Sun shines and the sail is deployed and in the frictionless vacuum of space significant speeds can eventually be achieved.

Solar chemical

Solar chemical processes utilize solar energy to drive chemical changes. These processes offset energy that would otherwise be required from an alternate source and can serve as a method of converting solar energy into a storable and transportable fuel. Solar chemical reactions are diverse but can generically be described as either thermochemical or photochemical.

Solar-to-hydrogen technologies have been a significant area of solar chemical research since the 1970s. Aside from electrolysis driven by photovoltaic or photochemical cells several thermochemical processes have also been explored. The seemingly most direct of these routes uses concentrators to split water at high temperatures (2300-2600 °C), but this process has been limited by complexity and low solar-to-hydrogen efficiency (1-2 percent). A more conventional approach uses process heat from solar concentrator to drive the steam reformation of natural gas thereby increasing the overall hydrogen yield. Thermochemical cycles characterized by the decomposition and regeneration of reactants present yet another avenue of hydrogen production. The Solzinc process under development at the Weitzman Institute is one such method. This process uses a 1 MW solar furnace to decomposed zinc oxide (ZnO) at temperatures above 1200 °C. This initial reaction produces pure zinc which can subsequently be reacted with water to produce hydrogen.

Concentrators can be used in the gasification of feedstocks such as coal, municipal solid waste and crop-grown biomass. The resulting hydrocarbons can be used to synthesize so-called "sunfuels".

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. The CO may then be used to synthesize fuels such as methanol, gasoline and jet fuel.

Concentrating solar technologies can also be used in the production of industrial chemicals. A prototype 10 kWth solar furnace at the Paul Scherrer Institute produced lime at 64.2 grams per minute with a solar energy to chemical energy efficiency of 34.8 percent.

Photoelectrochemical cells or PECs consists of a semiconductor, typically titanium dioxide or related titanates, immersed in an electrolyte. When the semiconductor is illuminated an electrical potential develops. As the name implies, 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.

A photogalvanic device is a type of battery in which the cell solution (or equivalent) forms energy rich chemical intermediates when illuminated. These chemical intermediates then react at the electrodes to produce an electric potential. The ferric-thionine chemical cell is an example of this technology.

Photodimerization is the photon induced formation of dimers. As early as 1909, the dimerization of anthracene into dianthracene was investigated as a means of storing solar energy. The photodimerization of the napthalene series has also been investigated.

Photoisomerization is the photon induced formation of isomers. Ketone, azepine and norbornadiene among other compounds have been investigated as potential energy storing isomers.

Solar mechanical

Solar mechanical technologies use sunlight to produce a mechanical effect. There are many such technologies covered within the solar thermal category but the devices listed here are notable for having both passive solar and mechanical characteristics.

• A light mill or Crookes radiometer is a simple solar mechanical device consisting of a glass bulb containing a set of vanes mounted on a spindle. Each vane has a dark side (which absorbs light energy and changes it to heat energy) and a reflective side (which stays relatively cool). Due to the motion of gases around the hot and cool sides of each vane, the vanes rotate with the dark side retracting, and the reflective side advancing towards the light. The rotation is proportional to the intensity of light. The power levels are low however and no practical application has been found for this device
• Passive solar tracking devices use imbalances caused by the movement of a low boiling point fluid to track the movement of the Sun. These systems can improve performance by 25 percent over fixed tilt PV systems.

• Passive solar shading systems also reposition with the Sun according to the movement of balancing fluids. They are used in buildings to maximize natural lighting during winter, and reduce summer glare and cooling loads.

Energy storage

Storage is an important issue in the development of solar energy because modern energy systems usually assume continuous availability of energy. Without storage, solar energy is not available at night.

Thermal storage

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 off-peak hours and reduce overall heating and cooling requirements.

Solar energy can be stored at high temperatures using molten salts. Salts are an effective storage medium because they are non-flammable, nontoxic, low-cost, have a high specific heat capacity, and can deliver heat at temperatures compatible with conventional power systems. A molten salt storage system consists of a salt loop connected to an insulated storage tank. During the heating cycle, the salt mixture is heated from 290 °C to 565 °C. During the power cycle, the salt is used to make steam for a thermal power station. The Solar Two used this method of energy storage, allowing it to store 1.44 TJ (400,000 kWh) in its 68 m³ storage tank with an annual storage efficiency of about 99 percent.

Solar energy can also be stored thermochemically with phase change materials. Suitable materials may be organic (paraffins, fatty acids) or inorganic (salts, metals, alloys).

• A Paraffin wax thermal storage system consists of a solar hot water loop connected to a paraffin wax tank. During the storage cycle, hot water flows through the storage tank melting the paraffin. The enthalpy of fusion for paraffin is 210-230 kJ/kg. During the heating cycle, stored heat is extracted from the tank as the wax resolidifies. These systems heat air and water to 64 °C and can reduce conventional energy use by 50 to 70 percent.

• Eutectic salts such as Glauber's salt also can be employed in thermal storage systems. Glauber's salt is relatively inexpensive and readily available. It can store 347 kJ/kg and deliver heat at 64 °C. The "Dover House" (in Dover, Massachusetts) was the first to use a Glauber's salt heating system in 1948.

Electrical storage

Rechargeable batteries can be used to store excess electricity from a photovoltaic system. Lead acid batteries are the most common type of battery associated with photovoltaic systems because they are relatively cheap and easily available. Batteries used in off-grid applications should be sized for three to five days of capacity and should limit depth of discharge to 50 percent to minimize cycling and prolong battery life.

Excess electricity from photovoltaic systems also can be sent to the transmission grid where it can be used to meet existing demand or temporarily stored for later use. Grid-tied electrical system policies often give photovoltaic system owners a credit for the electricity they deliver to the grid. This credit is used to offset electricity provided from the grid when the photovoltaic system cannot meet demand. Where there is net metering, the credit is equivalent to or greater than the cost of electricity to the consumer.

Development, deployment and economics

The following trends are a few examples by which the solar market is being helped to become competitive:

• Net metering laws which give credit for electricity fed into the grid. The Electricity Feed Law in Germany is currently the main driver of PV growth in the world.

• Incentives such as rebates and tax credits at the federal, state and local level to encourage consumers to consider solar power.

• Government grants for fundamental research in solar technology to make production cheaper and improve efficiency.

• Development of solar loan programs which lower deployment costs. The Indian Solar Loan Programme sponsored by UNEP has brought solar power to 18,000 homes in Southern India. Success in India's solar program has led to similar projects in other developing areas such as Tunisia, Morocco, Indonesia and Mexico.

Solar energy associations

Worldwide

• International Solar Energy Society (ISES) International NGO supporting renewable and sustainable technologies and member of the International Renewable Energy Alliance (IREA).

Europe

• ESTIF - European Solar Thermal Industry Federation

North America

• ASES: American Solar Energy Association US organization supporting solar energy, efficiency and sustainable technologies.
• SEIA: Solar Energy Industries Association US trade association of solar energy manufacturers, dealers, distributors, contractors
• Canadian Solar industry Association
• ANES: Mexican Solar Energy Association
• Prometheus Institute for Sustainable Development US non-profit which promotes PV, solar thermal and other sustainable technologies.

Solar energy research institutes

• See also: Photovoltaics research institutes

There are many research institutions and departments at universities around the world that research aspects of solar energy. Countries that are particularly active include Germany, Spain, Japan, Israel, India, Australia, China, and the USA.

• Solar Energy Laboratory at University of Southampton
• National Renewable Energy Laboratory NREL
• Centre for Renewable Energy Systems Technology, at Loughborough University
• Centre for Sustainable Energy Systems at the Australian National University
• Florida Solar Energy Center
• Solar Energy Laboratory at UW Madison
• Department of Energy Science and Engineering
• Plataforma Solar de Almería (PSA) Concentrating solar technology research, development and testing arm of the Center for Energy, Environment and Technological Research (CIEMAT).

See also

Template:EnergyPortal

• Carbon finance
• Desertec
• Drake Landing Solar Community
• Energy storage
• Global dimming
• Green electricity
• List of conservation topics
• List of solar thermal power stations
• Photovoltaic power stations
• Polysilicon
• Renewable heat
• Solar power satellite
• Solar tracker
• Timeline of solar energy
• Thin-film cell
• Wafer (electronics)
• World energy resources and consumption

Notes

1. Scheer (2002), p.8
2. Smil (2006), p. 12, 15
3. "The History of Solar" (PDF). United States Department of Energy. Retrieved 2007-09-29.
4. Butti and Perlin (1981), p.2-13
5. OECD, IEA (2007-Jan). "Renewables In Global Energy Supply - An IEA Fact Sheet" (PDF). IEA. Retrieved 2007-12-29. {{cite journal}}: Check date values in: |date= (help); External link in |journal= (help)
6. Smil (1991) p. 240
7. ^ Muhs, Jeff. "Design and Analysis of Hybrid Solar Lighting and Full-Spectrum Solar Energy Systems" (PDF). Oak Ridge National Laboratory. Retrieved 2007-09-29.
8. "Natural Forcing of the Climate System". Intergovernmental Panel on Climate Change. Retrieved 2007-09-29.
9. "Earth Radiation Budget". NASA Langley Research Center. 2006-10-17. Retrieved 2007-09-29.
10. Somerville, Richard. "Historical Overview of Climate Change Science" (PDF). Intergovernmental Panel on Climate Change. Retrieved 2007-09-29.
11. Vermass, Wim. "An Introduction to Photosynthesis and Its Applications". Arizona State University. Retrieved 2007-09-29.
12. Smil (2006), p.12
13. "Wind Energy Potential". American Wind Energy Association. Retrieved 2007-09-29.
14. Whittaker and Likens (1975), p. 305-328
15. Smil (2003), p. 15
16. "International Energy Outlook 2007". Energy Information Administration. 2007-05. Retrieved 2007-09-29. {{cite web}}: Check date values in: |date= (help)
17. "Dynamic Maps, GIS Data, and Analysis Tools - Solar Maps". National Renewable Energy Laboratory. Retrieved 2007-09-29.
18. "PV Solar Radiation (Flat Plate, Facing South, Latitude Tilt)". National Renewable Energy Laboratory. Retrieved 2007-09-29.
19. "Darmstadt University of Technology solar decathlon home design". Darmstadt University of Technology. Retrieved 2008-04-25.
20. Butti and Perlin (1981), p.15
21. Rosenfeld, Arthur. "Painting the Town White -- and Green". Heat Island Group. Retrieved 2007-09-29. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
22. Charles L. Deichman. "Plant arrangement for improving crop yields". Patent Storm. Retrieved 2007-11-22.
23. Kaul (2005), p.169-174
24. Butti and Perlin (1981), p.42-46
25. Bénard (1981), p.347
26. Leon (2006), p.62
27. Butti and Perlin (1981), p.19
28. "Lighting Research and Development". Department of Energy. Retrieved 2007-11-08.
29. ^ Apte, J.; et al. "Future Advanced Windows for Zero-Energy Homes" (PDF). ASHRAE. Retrieved 2008-04-09. {{cite web}}: Explicit use of et al. in: |author= (help)
30. Tzempelikos (2007), p.369
31. Tzempelikos (2007), p.369
32. "Daylighting". United States Department of Energy. Retrieved 2007-09-29.
33. Tzempelikos (2007), p.369-370
34. ^ Metz, Daryl. "Effects of Daylight Saving Time on California Electricity Use" (PDF). California Energy Commission. p. 6. Retrieved 2007-11-08.
35. "Solar Energy Technologies and Applications". Canadian Renewable Energy Network. Retrieved 2007-10-22.
36. Butti and Perlin (1981), p.112-155
37. Perlin, John. "Solar Hot Water Heating". California Solar Center. Retrieved 2007-09-29.
38. "R&D on Heating, Cooling, and Commercial Refrigeration". Department of Energy. Retrieved 2007-11-08.
39. ^ "Renewables 2007 Global Status Report" (PDF). Worldwatch Institute. Retrieved 2008-04-30.
40. Del Chiaro, Bernadette. "Solar Water Heating (How California Can Reduce Its Dependence on Natural Gas)" (PDF). Environment California Research and Policy Center. Retrieved 2007-09-29. {{cite web}}: Unknown parameter |coauthor= ignored (|author= suggested) (help)
41. Philibert, Cédric. "The Present and Future use of Solar Thermal Energy as a Primary Source of Energy" (PDF). International Energy Agency. Retrieved 2008-05-05.
42. Schittich (2003), p.166
43. "Energy Consumption Characteristics of Commercial Building HVAC Systems" (PDF). United States Department of Energy. pp. 1–6, 2–1. Retrieved 2008-04-09.
44. "Indirect Gain (Trombe Walls)". United States Department of Energy. Retrieved 2007-09-29.
45. "His passion for solar still burns". Los Angeles Times. Retrieved 2007-11-14.
46. Butti and Perlin (1981), p.72
47. Bartlett (1998), p.393-394
48. Leon (2006), p.62
49. "Solar Buildings (Transpired Air Collectors - Ventilation Preheating)" (PDF). National Renewable Energy Laboratory. Retrieved 2007-09-29.
50. "Frito-Lay solar system puts the sun in SunChips, takes advantage of renewable energy". The Modesto Bee. Retrieved 2008-04-25.
51. Butti and Perlin (1981), p.54-59
52. "Design of Solar Cookers". Arizona Solar Center. Retrieved 2007-09-30.
53. "The Solar Bowl". Auroville Universal Township. Retrieved 2008-04-25.
54. "Scheffler-Reflector". Solare Bruecke. Retrieved 2008-04-25.
55. "Solar Steam Cooking System". Gadhia Solar. Retrieved 2008-04-25.
56. "SODIS solar water disinfection". SANDEC. Retrieved 2008-05-02.
57. "Household Water Treatment and Safe Storage". World Health Organization. Retrieved 2008-05-02.
58. ^ Perlin, John. "Photovoltaics". California Solar Center. Retrieved 2007-09-29.
59. "Chronicle of Fraunhofer-Gesellschaft". Fraunhofer-Gesellschaft. Retrieved 2007-11-04.
60. Bellis, Mary. "History: Photovoltaics Timeline". About.com. Retrieved 2007-11-04.
61. Perlin (1999), p. 50, 118
62. "Annual Oil Market Chronology 1970-2006". Energy Information Administration. Retrieved 2007-11-01.
63. Hay, Harold. "Needle 5.0: Untold ISES History". Harold R. Hay. Retrieved 2008-05-02.
64. "Renewable Energy Annual - International Renewable Energy". Energy Information Administration. April 1997. Retrieved 2007-11-01.
65. Solar Cell Production Jumps 50 Percent in 2007
66. Butti and Perlin (1981), p.60-100
67. ^ "Linear-focusing Concentrator Facilities: DCS, DISS, EUROTROUGH and LS3". Plataforma Solar de Almería. Retrieved 2007-09-29.
68. Mills (2004), p.19-31
69. Halacy (1973), p.181
70. Tabor (1990), p.247
71. Perlin and Butti (1981), p.73
72. Halacy (1973), p.76
73. Tritt (2008), p.366-368
74. "History of World Solar Challenge". Panasonic World Solar Challenge. Retrieved 2007-09-30.
75. Electrical Review Vol 201 No 7 12 August 1977
76. Schmidt, Theodor. "Solar Ships for the new Millennium". TO Engineering. Retrieved 2007-09-30.
77. "The sun21 completes the first transatlantic crossing with a solar powered boat". Transatlantic 21. Retrieved 2007-09-30.
78. "Solar-Power Research and Dryden". NASA. Retrieved 2008-04-30.
79. "The NASA ERAST HALE UAV Program". Greg Goebel. Retrieved 2008-04-30.
80. "Breakthrough In Solar Sail Technology". Space.com. Retrieved 2007-11-26.
81. Agrafiotis (2005), p.409
82. "ZINC POWDER WILL DRIVE YOUR HYDROGEN CAR". Isracast. Retrieved 2008-04-30.
83. "Sandia's Sunshine to Petrol project seeks fuel from thin air". Sandia Corporation. Retrieved 2008-05-02.
84. "Sandia Applying Solar Thermochemical Hydrogen Technology to Recycling CO2 to Liquid Fuels". Green Car Congress. Retrieved 2008-05-02.
85. Meier (2005), p.1355-1358
86. Bolton (1977), p. 11
87. Bolton (1977), p. 16, 119
88. Bolton (1977), p. 235-237
89. Bolton (1977), p. 238-240
90. "Passive Solar Tracker for Photovoltaic Modules". e-Marine, Inc. Retrieved 2007-11-04.
91. "Large louvre blades (passive)". Schüco. Retrieved 2007-11-04.
92. "Advantages of Using Molten Salt". Sandia National Laboratory. Retrieved 2007-09-29.
93. Gok, Özgül. "Stabilization of Glauber's Salt for Latent Heat Storage" (PDF). Çukurova University. Retrieved 2007-09-30. {{cite web}}: Unknown parameter |coauthor= ignored (|author= suggested) (help)
94. Romanowicz, Goska (2006-05-19). "Heat 'batteries' dramatically cut energy use". edie newsroom. Retrieved 2007-09-29. {{cite news}}: Check |url= value (help)
95. Gok, Özgül. "Stabilization of Glauber's Salt for Latent Heat Storage" (PDF). Çukurova University. Retrieved 2007-09-30. {{cite web}}: Unknown parameter |coauthor= ignored (|author= suggested) (help)
96. Butti and Perlin (1981), p.212-214
97. "Batteries". DC Power Systems. Retrieved 2007-09-29.
98. "UNEPs India Solar Loan Programme Wins Prestigious Energy Globe". United Nations Environment Programme. 2007-04-12. Retrieved 2007-09-30.

References

Books and journals

• Agrafiotis, C.; Roeb, M.; Konstandopoulos, A.G.; Nalbandian, L.; Zaspalis, V.T.; Sattler, C.; Stobbe, P.; Steele, A.M. (2005). "Solar water splitting for hydrogen production with monolithic reactors". Solar Energy. 79 (4): 409–421. doi:10.1016/j.solener.2005.02.026.

• Bénard, C.; Gobin, D.; Gutierrez, M. (1981). "Experimental Results of a Latent-Heat Solar-Roof, Used for Breeding Chickens". Solar Energy. 26 (4): 347–359. doi:doi:10.1016/0038-092X(81)90181-X. {{cite journal}}: Check |doi= value (help)
• Bartlett, Robert (1998). Solution Mining: Leaching and Fluid Recovery of Materials. Routledge. ISBN 9056996339.

• Bolton, James (1977). Solar Power and Fuels. Academic Press, Inc. ISBN 0121123502.

• Butti, Ken (1981). A Golden Thread (2500 Years of Solar Architecture and Technology). Van Nostrand Reinhold. ISBN 0442240058. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

• Halacy, Daniel (1973). The Coming Age of Solar Energy. Harper and Row. ISBN 0380002337.

• Hunt, V. Daniel (1979). Energy Dictionary. Van Nostrand Reinhold Company. ISBN 0442273959.

• Karan, Kaul; Greer, Edith; Kasperbauer, Michael; Mahl, Catherine (2001). "Row Orientation Affects Fruit Yield in Field-Grown Okra". Journal of Sustainable Agriculture. 17 (2/3): 169–174. doi:10.1300/J064v17n02_14.

• Leon, M.; Kumar, S. (2007). "Mathematical modeling and thermal performance analysis of unglazed transpired solar collectors". Solar Energy. 81 (1): 62–75. doi:doi:10.1016/j.solener.2006.06.017. {{cite journal}}: Check |doi= value (help)

• Lieth, Helmut (1975). Primary Productivity of the Biosphere. Springer-Verlag1. ISBN 0387070834. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

• Meier, Anton; Bonaldi, Enrico; Cella, Gian Mario; Lipinski, Wojciech; Wuillemin, Daniel (2005). "Solar chemical reactor technology for industrial production of lime". Solar Energy. 80 (10): 1355–1362. doi:10.1016/j.solener.2005.05.017.

• Mills, David (2004). "Advances in solar thermal electricity technology". Solar Energy. 76 (1–3): 19–31. doi:doi=10.1016/S0038-092X(03)00102-6. {{cite journal}}: Check |doi= value (help); Missing pipe in: |doi= (help)

• Müller, Reto; Steinfeld, A. (2007). "Band-approximated radiative heat transfer analysis of a solar chemical reactor for the thermal dissociation of zinc oxide". Solar Energy. 81 (10): 1285–1294. doi:10.1016/j.solener.2006.12.006.

• Perlin, John (1999). From Space to Earth (The Story of Solar Electricity). Harvard University Press. ISBN 0674010132.

• Scheer, Hermann (2002). The Solar Economy (Renewable Energy for a Sustainable Global Future). Earthscan Publications Ltd. ISBN 1844070751.

• Schittich, Christian (2003). Solar Architecture (Strategies Visions Concepts). Architektur-Dokumentation GmbH & Co. KG. ISBN 3764307471.

• Smil, Vaclav (1991). General Energetics: Energy in the Biosphere and Civilization. Wiley. p. 369. ISBN 0471629057.

• Smil, Vaclav (2003). Energy at the Crossroads: Global Perspectives and Uncertainties. MIT Press. p. 443. ISBN 0262194929.

• Smil, Vaclav (2006-05-17). Energy at the Crossroads (PDF). Organisation for Economic Co-operation and Development. ISBN 0262194929. Retrieved 2007-09-29.

• Tabor, H. Z.; Doron, B. (1990). "The Beith Ha'Arava 5 MW(e) Solar Pond Power Plant (SPPP)--Progress Report". Solar Energy. 45 (4): 247–253. doi:10.1016/0038-092X(90)90093-R.

• Tritt, T.; Böttner, H.; Chen, L. (2008). "Thermoelectrics: Direct Solar Thermal Energy Conversion" (PDF). MRS Bulletin. 33 (4): 355–372.

• Tzempelikos, Athanassios; Athienitis, Andreas K. (2007). "The impact of shading design and control on building cooling and lighting demand". Solar Energy. 81 (3): 369–382. doi:10.1016/j.solener.2006.06.015.

• Vecchia, A.; Formisano, W.; Rosselli, V; Ruggi, D. (1981). "Possibilities for the Application of Solar Energy in the European Community Agriculture". Solar Energy. 26 (6): 479–489. doi:doi:10.1016/0038-092X(81)90158-4. {{cite journal}}: Check |doi= value (help)

External links

• Energy transitions past and future, Encyclopedia of Earth
• Energy Education a2z from the Energy Education Foundation
• Find solar/calculator (US DOE/ASES/SEPA)
• Build It Solar, The Renewable Energy site for Do-It-Yourselfers

NASA's Photovoltaic Info http://science.nasa.gov/headlines/y2002/solarcells.htm

• Energy conversion
• Solar energy
• Solar design
• Solar power by country
• Sun