Solar energy is energy from the Sun. This energy drives climate and the weather supports virtually all life on Earth. Heat and light from the sun, along with solar-based resources such as wind and wave power, hydroelectricity and biomass, account for over 99.9 percent of the available flow of renewable energy.^[1][2]

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Solar energy technologies harness the sun's energy for practical ends. These technologies date from the time of the early Greeks, Indian, Native Americans and Chinese, who warmed their buildings by orienting them toward the sun. Modern solar technologies provide heating, lighting, electricity and even flight.^[4][5]

Solar power is used synonymously with solar energy or more specifically to refer to the conversion of sunlight into electricity. This can be done either through the photovoltaic effect or by heating a transfer fluid to produce steam to run a generator.

Solar technologies range from traditional methods related to food, heat and light to large-scale electrical generation systems. Applications include:

• Biomass (wood, biofuel)
• Electricity generation (photovoltaics, heat engines)
• Evaporation (clothes drying, desalination, food preservation)
• Heat (hot water, building heat, cooking)
• Lighting (daylighting, hybrid lighting, daylight saving time)
• Transportation (solar car, solar plane, solar boat)

Energy from the Sun

Earth receives 174 petawatts of incoming solar radiation (insolation) at the upper atmosphere at any given time. 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.^[6] After passing through the Earth's 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).^[7]

The absorption of solar energy by atmospheric convection (sensible heat transport) and evaporation and condensation of water vapor (latent heat transport) affects the winds and the water cycle.^[8] Upon reaching the surface, sunlight is absorbed by the oceans, land masses and plants. The energy captured in the oceans drives the thermohaline cycle. As such, solar energy is ultimately responsible for temperature-driven ocean currents such as the thermohaline cycle and wind-driven currents such as the Gulf Stream. The energy absorbed by the earth, in conjunction with that recycled by the Greenhouse effect, warms the surface to an average temperature of approximately 14 °C.^[9] 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.^[10] The flows and stores of solar energy in the environment are vast in comparison to human energy needs.

• The total solar energy available to the earth is approximately 3850 zettajoules (ZJ) per year.^[2]
• Oceans absorb approximately 285 ZJ of solar energy per year.
• Winds can theoretically supply 6 ZJ of energy per year.^[11]
• Biomass captures approximately 1.8 ZJ of solar energy per year.^[12][2]
• Worldwide energy consumption was 0.471 ZJ in 2004.^[13]

The upper map (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).^[14] 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.^[15]

Types of technologies

There are many technologies for harnessing solar energy within these broad classifications: active, passive, direct and indirect.

• Active solar systems use electrical and mechanical components such as tracking mechanisms, pumps and fans to capture sunlight and process it into usable outputs such as heating, lighting or electricity.
• Passive solar systems use non-mechanical techniques to control the capture of sunlight and distribute this energy into usable outputs such as heating, lighting, cooling or ventilation. These techniques include selecting materials with favorable thermal properties to absorb and retain energy, designing spaces that naturally circulate air to transfer energy and referencing the position of a building to the sun to enhance energy capture. In some cases passive solar devices can have mechanical movement with the important distinction that this movement is automatic and directly powered by the sun.
• Direct solar generally refers to technologies or effects that involve a single-step conversion of sunlight that results in a usable form of energy.
• Indirect solar generally refers to technologies or effects that involve multiple-step transformations of sunlight that result in a usable form of energy.

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.^[16]

Urban heat islands (UHI) are metropolitan areas with significantly higher temperatures than the surrounding environment. These higher temperatures are the result of urban materials such as concrete and asphalt that have lower albedos and higher heat capacities than the natural environment. The albedo of an object indicates the percentage of light it reflects. Asphalt has an albedo of around 10 percent, while the average albedo of the Earth is 30 percent.^[17] 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.^[18]

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. It wasn't until the 20th century that artificial lighting took over as the main source of interior illumination. The 1973 oil and 1979 energy crises brought attention to conservation measures such as natural lighting but interest waned on both occasions with the restoration of energy supplies. Approximately 22 percent (8.6 EJ)^[19] of the electricity used in the United States is for lighting. When daylighting techniques are appropriately applied, natural light can supply interior lighting for a significant portion of the day.^[6]

Daylighting is a passive solar method of using sunlight to provide illumination. Daylighting directly offsets energy use in electric lighting systems and indirectly offsets energy use through a reduction in cooling loads.^[20] Although difficult to quantify, the use of natural lighting also offers physiological and psychological benefits compared to artificial lighting. Daylighting features include building orientation, window orientation, exterior shading, sawtooth roofs, clerestory windows, light shelves, skylights and light tubes.^[21] 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. Hybrid solar lighting systems collect sunlight using focusing mirrors that track the sun. The collected light is transmitted via optical fibers 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.^[6]

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.^[22] 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.^[22] DST, like other solar energy technologies, has not proven successful in all regions.

Solar thermal

Solar thermal applications make up the most widely used 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.^[23]

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.^[24][25] Approximately 14 percent (15 EJ) of the total energy used in the United States is for water heating.^[26] In many climates, a solar heating system can provide 50 to 75 percent of domestic hot water use.

As of 2005, the total installed capacity of solar hot water systems is 88 GWth and growth is 14 percent per year.^[27] China is the world leader in the deployment of solar hot water systems with 80% of the market.^[28] Israel is the per capita leader in the use of solar hot water with 90 percent of homes using this technology.^[27] In the United States heating swimming pools is the most successful application of solar hot water.^[25]

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.^[29] 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

Heating, cooling and ventilation (HVAC) systems of buildings are closely interrelated. All seek to provide thermal comfort, acceptable indoor air quality, and reasonable installation, operation, and maintenance costs. Conventional HVAC systems account for roughly 40 percent of the energy used in the United States and European Union.^[30] Many 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. More advanced thermal mass systems can be also be used for ventilation.

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.^[31]

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.^[32]

A transpired air collector is a perforated sun-facing wall. The wall absorbs sunlight and pre-heats air as much as 22 °C as it is drawn into the ventilation system. These systems are highly efficient (up to 80 percent) and can pay for themselves within 3 to 12 years in offset heating costs.^[33]

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.^[34] 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

A solar pond is a pool of salt water 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.^[35] 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 En Boqeq, Israel, and another used for industrial process heat at the University of Texas El Paso.^[36]

Salt evaporation ponds use solar energy to concentrate brine solutions used in leach mining, remove dissolved solids from waste streams, or obtain salt from sea water. An evaporation pond consists of a shallow layer of water that can evaporate at a rate of 3-6 mm/day. The use of evaporation ponds to obtain salt from sea water is one of the oldest applications of solar energy, and evaporation ponds remain one of the largest commercial applications of solar energy used today.^[37]

Concentrating solar technologies have been investigated as a means of providing process heat to help offset heat from costly electrical resistance systems for producing aluminum.^[38]

Cooking

Solar cookers (or solar ovens) use sunlight for cooking, drying and pasteurization. Solar cookers offset fuel costs, reduce demand for firewood, and improve air quality by removing a source of smoke. The most common designs are box cookers, concentrating cookers and panel cookers.

Solar box cookers consist of an insulated container with a transparent lid. Horace de Saussure developed this design in 1767 after observing: "It is a known fact, and a fact that has probably been known for a long time, that a room, a carriage, or any other place is hotter when the rays of the sun pass through glass." These cookers can be used effectively with partially overcast skies and can reach temperatures of 50-100 °C. These are the cheapest and most widely used cooker design.^[39][40]

Concentrating solar cookers use a parabolic reflector to concentrate light on a container positioned at the reflector's focal point. These designs cook faster and at higher temperatures (up to 315 °C). As with other concentrating technologies, these cookers require direct light and must be repositioned periodically to "track" the sun.^[40]

Solar Panel cookers (SPC) use flat reflectors to concentrate sunlight on a container within a transparent covering. Roger Bernard is credited with introducing panel cookers in 1994. This design uses partial concentration and will maintain effective operation with limited repositioning.

Desalination and disinfection

A solar still uses solar energy to distill water. A few basic types of solar stills are cone shaped, boxlike, and pit. For 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. The most sophisticated of these are the box shaped types; the least sophisticated are the pit types.

Solar water pasteurization uses solar energy to disinfect water. The basic pasteurization process consists of heating water to 60-70 °C and holding the temperature steady for a specified period depending on the organisms present. The most heat resistant organisms will be rendered inert by a temperatures of 70 °C for ten minutes, 75 °C for one minute, and 80 °C for five seconds.^[41][42]

Solar water disinfection (SODIS) is another method of disinfecting water using sunlight. The basic process involves filling a clear container 3/4 with water, shaking the container vigorously for 20 seconds, topping off the container, and placing it in the sun. Shaking the container aerates the water, which encourages disinfection. As sunlight shines into the container, the UV-A radiation causes the dissolved oxygen to become highly reactive. This reactive form of oxygen kills microorganism directly and interferes with the reproduction cycle of bacteria. As the container warms, harmful organisms are also destroyed by heat treatment. Although endorsed by the World Health Organization, SODIS is not as effective as pasteurization and the completeness of disinfection is not easily measurable.^[43][44][45]

Solar power

Solar power plants use a variety of methods to collect sunlight and convert this energy into electricity. Traditionally, concentrating solar thermal power plants have been the most common type; however, multi-megawatt photovoltaic sites are seeing more-rapid deployment. Experimental solar power plants also have been built using technologies other than concentrating solar or photovoltaics, but no recent breakthroughs have been reported.

While PV has advantages in terms of simplicity, the high temperatures produced by solar thermal systems also can provide process heat and steam for a variety of secondary commercial applications (cogeneration).

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.^[46] This breakthrough marked a fundamental change in how power is generated. The subsequent development of solar cells during the 1950s raised the efficiency of solar cells from 6 percent up to 10 percent^[47] but commercial applications were limited to novelty items due to the high costs of solar cells ($300 per watt).^[46]

In 1958, photovoltaic modules were used successfully as a power source for the Vanguard I satellite, followed by a string of additional solar-powered Russian and American satellites. Despite NASA's early focus on nuclear power, solar power had become the established source of power for satellites by the late 1960s. Solar power also played an essential part in the success of early commercial satellites such as Telstar and Syncom.^[46]

For terrestrial applications, photovoltaic costs remained above $100 per watt throughout the 1960s, limiting commercial success. However, work by Dr. Elliot Berman during the early 1970s lowered the costs of solar cells from $100 to $20 per watt. This price reduction made solar cells competitive in a range of applications, especially for remote (off-grid) sites. Uses included cathodic protection of pipe lines and power for off-shore oil rigs, railroad crossings and lighthouses.^[46]

The development of solar power was significantly affected by the 1973 oil and 1979 energy crises. These crises prompted a search for alternatives to oil, and incentive programs such as the Federal Photovoltaic Utilization Program in the U.S. and the Sunshine Program in Japan were direct results. An additional result was the establishment of research facilities such as the Solar Energy Research Institute (now NREL) in the U.S., Japan's New Energy and Industrial Technology Development Organization (NEDO) and the Fraunhofer Institute for Solar Energy Systems ISE in Germany.^[48] These developments and others helped PV production expand quickly from 500 kW in 1977 to 5 MW in 1981 and 9 MW in 1982.^[49]

Unfortunately for the industry, as oil prices began to fall in the early 1980s so too did the growth rate of PV. Historically-low oil prices from 1986-1999 helped keep funding for solar power research relatively low and largely removed solar power from the public consciousness.^{[50] [51]}Despite the lack of attention annual growth of PV ranged from 10 to 20 percent throughout the 1980s and 1990s and worldwide installation of PV reached 1000 MW in 1999.^[52]

To take advantage of electromagnetic radiation from the sun, solar panels can be attached to individual houses or buildings. The panels should be mounted perpendicular to the arc of the sun to maximize usefulness. The easiest way to use this electricity is by connecting the solar panels to a grid tie inverter. However, these solar panels also may be used to charge batteries or other energy storage device. Solar panels produce more power during summer months because they receive more sunlight during the day and at a more-direct angle of incidence.

Total peak power of installed PV is around 6,000 MW as of the end of 2006. Installed PV is projected to increase to over 9,000 MW in 2007.^[53][54]

Declining manufacturing costs (dropping at three to five percent a year in recent years) are expanding the range of cost-effective uses. The average lowest retail cost of a large photovoltaic array declined from $7.50 to $4.00 per watt between 1990 and 2005.^[55] With many jurisdictions now giving tax and rebate incentives, solar electric power can now pay for itself 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.

In 2003, worldwide production of solar cells increased by 32 percent.^[56] Between 2000 and 2004, the increase in worldwide solar energy capacity was an annualized 60 percent.^[57] 2005 was expected to see large growth again, but shortages of refined silicon have been hampering production worldwide since late 2004.^[58] Analysts have predicted similar supply problems for 2006 and 2007.^[59] but there are only small amount of solar cell companies in the worldwide market.

Photovoltaics is gaining credence among private investors as having the potential to grow into the next big industry. Many companies and venture capitalists are investing in photovoltaic development and manufacturing. This trend is particularly visible in Silicon valley, California.^[60][61][62] Nanotechnology manufacturing using CIGS solar cells promises to produce electricity at a cost of around 5¢/kWh and sell for $0.36 per peak watt,^[63] approximately one tenth of the average 2007 prices for solar panels.^[64]

Deployment of solar power depends largely upon local conditions and requirements. All industrialised nations share a need for electricity and it is believed that solar power will increasingly be used as an option for supply. The Very Large Scale Photovoltaic Power Generation (VLS-PV) proposal argues that "PV systems could generate many times the current primary global energy supply."^[65] To compensate for night time energy demands they would need to be complemented with pumped storage.

Concentrating solar

Concentrated sunlight has been used to perform useful tasks since the ancient Chinese. A classic but apocryphal legend claims Archimedes used polished shields to concentrate sunlight on the invading Roman fleet and repel them from Syracuse in 212 BC. Leonardo Da Vinci conceived using large scale solar concentrators to weld copper in the 15th century. In 1866, the French engineer Auguste Mouchout successfully powered a steam engine with sunlight. This was the first known example of a concentrating solar-powered mechanical device. Over the following 50 years, inventors such as John Ericsson, and Frank Shuman developed solar-powered devices for irrigation, refrigeration and locomotion. The progeny of these early developments are the concentrating solar thermal power plants of today.^[66]

Concentrating Solar Thermal (CST) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. CST technologies require direct insolation to function; therefore, these technologies are at a disadvantage in significantly overcast locations.^[67] The three basic CST technologies are the solar trough, solar power tower and parabolic dish. A fourth but rarely used concentrating technology is known as a solar bowl. 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.

Line focus/Single-axis

A solar trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. These systems use single-axis tracking to follow the sun. A working fluid (oil, water) flows through the receiver and is heated to 400 °C before transferring its heat to a distillation or power generation system.^[68][69] Trough systems are the most developed CST technology. The Solar Electric Generating System (SEGS) plants in California and Plataforma Solar de Almería's SSPS-DCS plant in Spain are representatives of this technology.^[68]

Point focus/Dual-axis

A solar power tower consists of an array of flat reflectors (heliostats) that concentrate light on a central receiver atop a tower. These systems use dual-axis tracking to follow the sun daily and seasonally, as focusing is critical. A working fluid (air, water, molten salt) flows through the receiver where it is heated to 1000 °C before transferring its heat to 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.^[70] The Solar Two in Daggett, California and the Planta Solar 10 (PS10) in Sanlucar la Mayor, 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. These systems use dual-axis tracking to follow the sun. A working fluid (hydrogen, helium, air, water) flows through the receiver where it is heated to 1500 °C before transferring its heat to a sterling engine for power generation.^[69][70] Parabolic dish systems display the highest solar-to-electric efficiency among CST 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.

Fixed reflector/Dual axis receiver

A solar bowl consists of a fixed parabolic reflector that concentrates light onto a receiver which tracks the focus of light as the sun moves across the sky. The solar bowl in Marseilles, France and another in Auroville, India are representatives of this technology.

Updraft tower

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.^[71]

Solar vehicles

Development of a practical 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)^[72] 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).^[73]

Helios, named for the Greek sun god, was a prototype solar-powered unmanned aircraft. AeroVironment, Inc. developed the vehicle under NASA's Environmental Research Aircraft and Sensor Technology (ERAST) program. On 13 August, 2001, it set an unofficial world record for sustained altitude by a winged aircraft. It sustained flight above 29,250 m (95,965 ft) for 40 minutes and reached 29,524 m (96,864 ft) altitude in the process.

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.

The first practical solar boat was constructed in 1975 in England.^[74] By 1995, solar passenger boats began appearing and are now used extensively.^[75] The first crossing of the Atlantic Ocean by a solar-powered boat was in the winter of 2006/2007 by the catamaran sun21.^[76]

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.^[77]

Solar chemical

Solar chemical processes convert solar energy into chemical energy. Industrial solar chemical technologies use light (photochemical) and heat (endothermic) to drive chemical, thermochemical or thermoelectric reactions. Solar chemical reactions can be used to store solar energy or offset energy that would otherwise be required from an alternate source.

Photoelectrochemical cells use light to produce hydrogen in a process similar to the electrolysis of water. Thermoelectric devices convert a temperature difference between dissimilar metals into an electric current between those metals. This current can be use to produce hydrogen and oxygen through the electrolysis of water. The solar pioneer Mouchout envisioned using the thermoelectric effect to store solar energy for later use; however, his experiments toward this end never progressed beyond primitive devices.^[78]

Concentrating solar thermal technologies can be used to drive high-temperature chemical processes.

• Zinc Oxide (ZnO) can be decomposed at high temperatures (1200-1750 °C). The resulting pure zinc can be marketed directly or the zinc can be reacted with water at 350 °C to produce ZnO and hydrogen.^[79][80]

• Water can be directly dissociated at high temperatures (2300-2600 °C). These process have so far been limited due to their high level of complexity and low solar-to-hydrogen efficiency (1-2%).^[81] An alternate path of research is investigating solar thermochemical cycles that can be used to dissociate water at lower temperatures. Thermochemical cycles are at the prototype stage.^[82][83]

Solar biological

Photosynthesis is by far the most important photochemical interaction. Most life on earth depends on the ability of plants to photosynthesize light in the visible, ultraviolet and infrared regions of the electromagnetic spectrum. While easy to overlook, the efficient use of solar energy is a founding principle of agriculture. This principle takes special priority during times of shortage as seen during the Little Ice Age when Medieval European farmers employed elaborate field orientation and thermal mass techniques to convert as much sunlight into food as possible.^[85] Agricultural advancement continues to maximize the availability of light by carefully timing planting cycles, tailoring row orientation, staggering heights between rows, optimizing plant density, and many other methods.^[86]

Greenhouses carefully 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.^[87] In the 16th century the first modern greenhouses were built in Italy to conserve tropical plants that explorers returned with. Greenhouses remain an important part of modern horticulture. The largest greenhouse complex in the world is in Willcox, Arizona where 262 acres of tomatoes and cucumbers are entirely grown under glass.

Solar mechanical

Solar mechanical technologies use sunlight to produce a mechanical effect. They were widely investigated by solar pioneers such as Auguste Mouchout, John Ericsson, Charles Tellier and Frank Shuman. These devices generally concentrated sunlight on a boiler to produce steam which was then used by a steam engine to perform useful work. Most of these technologies were displaced early in the 20th century as increasingly cheap fossil fuels made them economically noncompetitive but several solar mechanical technologies have since been developed.^[88]

• 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.
• 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% over fixed tilt PV systems.^[89]
• Passive solar shading systems also reposition with the sun according to the movement of balancing fluids. They are used in buildings to maximize natural ligting during winter, and reduce summer glare and cooling loads.^[90]

Energy storage

Storage is an important issue in the development of solar energy because availability is a vital requirement of modern energy use. Solar energy is most often stored as heat in the thermal mass of buildings but batteries, water tanks, and other storage methods can be used to store energy in thermal and chemical forms.

Thermal mass systems use various methods and materials (adobe, earth, concrete, water) to store solar energy for short or long durations (seasonal thermal store). Thermal mass can be used to lower peak demand, shift time-of-use to off-peak hours and reduce overall heating and cooling requirements.

Solar energy can be stored thermochemically with phase change materials. Suitable materials may be organic (paraffins, fatty acids) or inorganic (salts, metals, alloys).^[91]

• 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.^[92][91]

• 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.^[93]

Solar energy can be stored at high temperatures using molten salts. Salts are an effective storage medium because they are nonflammable, 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 steam-electric power plant. 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.^[94]

Rechargeable batteries can be used to store excess electricity from a photovoltaic system. This type of storage system consists of a photovoltaic power source connected to a battery bank via a charge controller and inverter. Lead acid batteries are the most common type of battery associated with photovoltaic systems because of their relatively low upfront costs and high availability. Lead acid batteries have an energy density of 110-140 kJ/kg, a charge/discharge efficiency of 70-92 percent and cost $150-200 per kWh ($45 to $55 per MJ). Lead 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.^[95] Newer batteries can be deep discharged for over 25,000 cycles.^[96]

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

Solar energy is an attractive solution to global warming.^[97]

"The Stone Age did not end for a lack of stones, and the oil age will end not for a lack of oil." — Sheik Yamani, Saudi oil minister, 1973

"We stopped using stone because bronze and iron were superior materials, and likewise we will stop using oil when other energy technologies provide superior benefits." — Bjørn Lomborg, The Skeptical Environmentalist (New York: Cambridge University Press, 2001), p. 120^[98]

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

• Government grants for research in solar technology to make production cheaper and generation more efficient.^[99]

• Implementation of incentives at the federal and state levels to encourage consumers to consider solar power. Examples include government tax subsidies, partial copayment schemes and various rebates over purchase costs of solar devices. These are meant to take some of the onus off consumers and reduce risks associated with high initial deployment investments.^[100]

• Adoption of an energy policy where consumers can connect their solar power systems to the local grid, and reverse feed the grid with unconsumed power (the state power authority guarantees an attractive purchase price). This has been especially popular in Germany and Japan.^[101]

• Development of solar loan programs, with attractive return rates, to buffer the initial deployment costs and entice consumers to purchase solar PV systems. The most famous example is the solar loan program sponsored by UNEP helping 100,000 people finance solar power systems in India.^[102] Success in India's solar program has led to similar projects in other developing areas such as Tunisia, Morocco, Indonesia and Mexico.

General Electric's Chief Engineer predicts grid parity for photovoltaics without subsidies in sunny parts of the United States by around 2015. Other companies predict an earlier date.^[103]

Solar conferences

• 3rd International Solar Cities Congress from 17-21 February 2008 in Adelaide, Australia.^[104]

Solar energy associations

• See also: Photovoltaic Industry Associations

Worldwide

• ISES: International Solar Energy Society International NGO supporting renewable and sustainable technologies.

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, Australia, China, and the USA.

• 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

See also

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Notes

References

Books and journals