User:Adacore/Solar Energy

Nellis Solar Power Plant

Solar energy is energy from the Sun in the form of radiated heat and light. It drives the climate and weather and supports life on Earth. Solar energy technologies make controlled use of this energy resource.

Solar power is a synonym of solar energy or refers specifically to the conversion of sunlight into electricity by photovoltaics, concentrating solar thermal devices or various experimental technologies.

In building design, thermal mass is used to conserve heat, and daylighting techniques optimize light. Solar water heaters heat swimming pools and provide domestic hot water. In agriculture, greenhouses grow specialty crops and photovoltaic-powered pumps bring water to grazing animals. Evaporation ponds find applications in the commercial and industrial sectors where they are used to harvest salt and clean waste streams of contaminants.

Solar distillation and disinfection techniques produce potable water for millions of people worldwide. Family scale solar cookers and larger solar kitchens concentrate sunlight for cooking, drying and pasteurization. More sophisticated concentrating technologies magnify the rays of the Sun for high temperature material testing, metal smelting, and industrial chemical production. A range of prototype solar vehicles provide ground, air and sea transportation.

Energy from the Sun


Earth continuously receives 174 PW of incoming solar radiation (insolation) at the upper atmosphere. Here, 6% of the insolation is reflected and 16% is absorbed. Clouds, dust and pollutants further reduce insolation traveling through the atmosphere by 20% due to reflection and 3% via absorption, on average. These atmospheric conditions not only reduce the quantity of energy reaching the earth's surface, but also diffuse approximately 20% 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 conversion of solar energy into chemical energy via photosynthesis produces food, wood and the biomass from which fossil fuels are derived.

Solar radiation along with secondary solar resources such as wind and wave power, hydroelectricity and biomass, account for over 99.9% of the available flow of renewable energy on Earth. 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 3850 zettajoules (ZJ) per year.
 * Global wind energy at 80m is estimated at 2.25 ZJ per year.
 * Photosynthesis captures approximately 3 ZJ per year in biomass.
 * Worldwide electricity consumption was approximately 0.0567 ZJ in 2005.
 * Worldwide energy consumption was 0.487 ZJ in 2005.

Types of technologies
Solar energy technologies use solar radiation for practical ends. Technologies that use secondary solar resources such as biomass, wind, waves, and ocean thermal gradients can be included in a broader description of solar energy but only primary resource applications are discussed here. The performance of solar technologies varies widely between regions; therefore, solar technologies should be deployed in a way that carefully considers these variations.

Solar technologies such as photovoltaics and water heaters increase the supply of energy and may be characterized as supply side technologies. Technologies such as passive design and shading devices reduce the need for alternate resources and may be characterized as demand side. Optimizing the performance of solar technologies is often a matter of controlling the resource rather than simply maximizing its collection.

Architecture and urban planning


Sunlight has influenced building design since the beginning of architectural history. Fully developed solar architecture and urban planning methods were first employed by the Greeks and Chinese who oriented their buildings toward the south to provide light and warmth.

The elemental features of passive solar architecture are Sun orientation, compact proportion (a low surface area to volume ratio), selective shading (overhangs), and thermal mass. When these features are tailored to the local climate and environment they can produce well lit spaces that stay in a comfortable temperature range. Socrates' Megaron House is a classic example of passive solar design. The most recent approaches to solar design use computer modeling to tie together solar lighting, heating, and ventilation systems in an integrated solar design package. Active solar equipment such as pumps, fans, and switchable windows can also complement passive design and improve system performance.

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 using these methods at an estimated cost of US$1 billion, giving estimated total annual benefits of US$530 million from reduced air-conditioning costs and healthcare savings.< ref name="Heat Islands">

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 accelerated ripening 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 plants in controlled environments, enabling year-round production and the growth of specialty crops and other plants not naturally suited to the local climate. Primitive greenhouses were first used during Roman times to grow cucumbers year-round for the Roman emperor Tiberius. The first modern greenhouses were built in Europe in the 16th century to conserve exotic plants brought back from explorations abroad. Greenhouses remain an important part of horticulture today, while plastic transparent materials have also been used to similar effect in polytunnels and row covers.

Solar lighting
The history of lighting is dominated by the use of natural light. The Romans recognized 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.

Daylighting systems collect and distribute sunlight to provide interior illumination. These systems directly offset energy use by replacing artificial lighting and indirectly offset non-solar energy use by reducing the need for air-conditioning. 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 also consider exterior shading devices. 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 flux and time-of-use. When daylighting features are properly implemented they can reduce commercial lighting related energy requirements by 25%.

Hybrid solar lighting (HSL) is an active solar method of using sunlight to provide illumination. HSL 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% of the direct sunlight received.

Although daylight saving time is promoted as a way to use sunlight to save energy, recent research is limited and reports contradictory results: several studies report savings, but just as many suggest no effect or even net costs, particularly when gasoline consumption is taken into account. Electricity use is greatly affected by geography, climate, and economics, making it hard to generalize from single studies.

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

Water heating
Solar hot water systems use sunlight to heat water. Approximately 14% of the total energy used in the United States is for water heating. When sited in low latitudes (below 40 degrees), solar heating system can provide around 60 to 70% of domestic hot water use with temperatures up to 60 °C. Worldwide, solar water heaters annually deliver approximately 600 kWh per kW installed. The most common types of solar water heaters are evacuated tube collectors (44%) and glazed flat plate collectors (34%) generally used for domestic hot water; and unglazed plastic collectors (21%) used mainly to heat swimming pools.

As of 2007, the total installed capacity of solar hot water systems is approximately 154 GW. China is the world leader in the deployment of solar hot water with 70 GW installed as of 2006 and a long term goal of 210 GW by 2020. Israel is the per capita leader in the use of solar hot water with 90% 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 GW as of 2005. The 0.25 EJ produced by solar water heaters worldwide in 2006 compares to 15 EJ used annually to heat water in the U.S.

Heating, cooling and ventilation
In the United States, heating, ventilation, and air conditioning (HVAC) systems account for over 25% (4.75 EJ) of the energy used in commercial buildings and nearly 50% (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, in the most general sense, is any material that has the capacity to store heat. In the context of solar energy, thermal mass materials are used to store heat from the Sun. Common thermal mass materials include stone, cement, and water. These materials have historically been used in arid climates or warm temperate regions to keep buildings cool by absorbing solar energy during the day and radiating their stored heat to the cooler atmosphere at night, but they can also be used in cold temperate areas to maintain warmth. The size 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 without consuming energy.

A solar chimney (or thermal chimney) is a passive solar ventilation system composed of a vertical shaft connecting the interior and exterior of a building. As the chimney warms, the air inside is heated causing an updraft that pulls air through the building. Performance can be improved by using glazing and thermal mass materials in a way that mimics greenhouses. These systems have been in use since Roman times and remain common in the Middle east.

Deciduous trees and plants can be used to provide heating and cooling. When planted on the southern elevation of the building, the leaves can provide shade during the summer while the bare limbs allow light and warmth to pass during the winter.

Process heat
Concentrating solar technologies such as parabolic dish, trough and Scheffler reflectors can provide process heat for commercial and industrial applications. The first commercial process heating project was the Solar Total Energy Project (STEP) in Shenandoah, Georgia where a field of 120 parabolic dishes provided 50% of the process heating, air conditioning and electrical requirements for a clothing factory. This system generated 400 kW of electricity, 3 MW of thermal energy in the form of steam, and had a thermal storage system which allowed for peak-load shaving. A prototype Scheffler reflector is currently being constructed in India for use in a solar crematorium.

Evaporation ponds are shallow pools that concentrate dissolved solids through evaporation. The use of evaporation ponds to obtain salt from sea water is one of the oldest applications of solar energy. Modern uses include concentrating brine solutions used in leach mining and removing dissolved solids from waste streams. Evaporation ponds represent one of the largest commercial applications of solar energy in use today.

Clothes lines, clotheshorses, and clothes racks dry clothes through evaporation. These devices use wind and sunlight instead of electricity or natural gas. Florida legislation specifically protects the 'right to dry' and similar solar rights legislation has been passed in Utah and Hawaii.

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) makes 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, including an 860 m² collector in Costa Rica used for drying coffee beans and a 1300 m² collector in Coimbatore, India, used for drying marigolds.

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 the generation 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 reach temperatures up to 315 °C but require direct light to function properly and must be repositioned to track the Sun.

The solar bowl is a unique concentrating technology used by the Solar Kitchen in Auroville, India that uses a stationary spherical reflector. This reflector focuses light along a line perpendicular to the sphere's surface and a computer control system moves the receiver to intersect this line. Steam is produced in the solar bowl's receiver at temperatures reaching 150 °C and then used for process heat in the kitchen.

A reflector developed by Wolfgang Scheffler in 1986 is used in many solar kitchens. Scheffler reflectors are flexible parabolic dishes that combine aspects of trough and power tower concentrators. Polar tracking is used to follow the Sun's daily course and the curvature of the reflector is adjusted for seasonal variations in the incident angle of sunlight. These reflectors can reach temperatures of 450-650 °C and have a fixed focal point which improves the ease of cooking. 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 2,000 large Scheffler cookers had been built worldwide.

Desalination and disinfection


Solar distillation is the production of potable water from saline or brackish water using solar energy. The first recorded use was by 16th century Arab alchemists. In 1589, Giambattista della Porta distilled water from crushed leaves. The first large-scale solar distillation project was constructed in 1872 in the Chilean mining town of Las Salinas. This 4,700 m² still could produce up to 22,700 L per day and operated for 40 years. Individual still designs include single-slope, double-slope (or greenhouse type), vertical, conical, inverted absorber, multi-wick and multiple effect. These stills can operate in passive, active or hybrid modes. Double slope stills are the most economic for decentralized domestic purposes while active multiple effect units are more suitable to large-scale applications.

Solar water disinfection (SODIS) is a method of disinfecting water by exposing water-filled plastic PET bottles to several hours of sunlight. Exposure times vary according weather and climate from a minimum of six hours to two days during fully overcast conditions. SODIS is 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. Over two million people in developing countries use SODIS for their daily drinking water needs.

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, or photovoltaic, photovoltaic cell is a device that converts light into direct current using the photoelectric effect. The first solar cell was constructed by Charles Fritts in 1883. Although the prototype selenium cells converted less than 1% of incident light into electricity, both Ernst Werner von Siemens and James Clerk Maxwell recognized the importance of this discovery. 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. These early solar cells cost 286 $/watt and reached efficiencies of 4.5-6%.

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

The high cost of solar cells limited terrestrial uses throughout the 1960s. This changed in the early 1970s when prices reached levels that made PV generation competitive in remote areas without grid access. Early terrestrial uses included powering telecommunication stations, off-shore oil rigs, navigational buoys, and railroad crossings. These and other off-grid applications have proven very successful and accounted for over half of worldwide installed capacity until 2004.

The 1973 oil crisis stimulated a rapid rise in the production of PV during the 1970s and early 1980s. Economies of scale which resulted from increasing production along with improvements in system performance brought the price of PV down from 100 $/watt in 1971 to 7 $/watt in 1985. Steadily falling oil prices during the early 1980s led to a reduction in funding for photovoltaic R&D and a discontinuation of the tax credits associated with the Energy Tax Act of 1978. These factors moderated growth to approximately 15% per year from 1984 through 1996.

Since the mid-1990s, leadership in the PV sector has shifted from the U.S. to Japan and Germany. Between 1992 and 1994 Japan increased R&D funding, established net metering guidelines, and introduced a subsidy program to encourage the installation of residential PV systems. As a result, PV installations in the country climbed from 31.2 MW in 1994 to 318 MW in 1999, and worldwide production growth increased to 30% in the late 1990s.

In 1990 Germany introduced Feed-in Tariffs to support the development of renewable energy sources, but it was the revision of the tariff structure as part of the Renewable Energy Sources Act in 2000 that has made Germany the leading PV market worldwide. Installed PV capacity has risen from 100 MW in 2000 to approximately 4,150 MW at the end of 2007. Since adopting a similar feed-in tariff structure in 2004, Spain has become the third largest PV market, while France, Italy, South Korea, and the U.S. have also seen rapid growth recently due to various incentive programs and local market conditions. Cumulative worldwide capacity as of year end 2007 is 10.6 GW and average costs of solar modules range from 3.75 - 4.75 $/watt. .

Concentrating solar energy
Concentrated sunlight has been used to perform useful tasks since 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 1866, Auguste Mouchout used a parabolic trough to produce steam for the first solar steam engine. And subsequent developments led to the use of concentrating solar-powered devices for irrigation, refrigeration and locomotion.

Concentrating Solar Power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated light is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exist; the most developed are the solar trough, parabolic dish and solar power tower. These methods vary in the way they track the Sun and focus light. In all these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage.

A solar trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The reflector is made to follow the Sun during the daylight hours by tracking along a single axis. Trough systems are the most developed CSP technology. The SEGS plants in California and Acciona's Nevada Solar One near Boulder City, Nevada are representatives of this technology.

A parabolic dish system consists of a stand-alone parabolic reflector that concentrates light onto a receiver positioned at the reflector's focal point. The reflector tracks the Sun along two axes. Parabolic dish systems give the highest efficiency among CSP technologies. The Big Dish in Canberra, Australia is a representative of this technology.

A solar power tower consists of an array of tracking reflectors (heliostats) that concentrate light on a central receiver atop a tower. Power towers are less advanced than trough systems but offer higher efficiency and better energy storage capability. The Solar Two in Daggett, California and the Planta Solar 10 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 m deep) that collects and stores solar energy. Solar ponds were first proposed by Dr. Rudolph Bloch in 1948 after he came across reports of a lake in Hungary in which the temperature increased with depth. This effect was due to salts in the lake's water, which created a "density gradient" that prevented convection currents. A prototype was constructed in 1958 on the shores of the Dead Sea near Jerusalem. The pond consisted of layers of water that successively increased from a weak salt solution at the top to a high salt solution at the bottom. This solar pond was capable of producing temperatures of 90 °C in its bottom layer and had an estimated solar-to-electric efficiency of two percent.

Thermoelectric devices convert a temperature difference between dissimilar materials into an electric current. First proposed as a method to store solare energy by solar pioneer Mouchout in the 1800s, 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 the US space program as an energy conversion technology for powering deep space missions such as Cassini, Galileo and Viking. Research in this area is focused on raising the efficiency of these devices from 7–8% to 15–20%.

Solar chemical
Solar chemical processes use 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.

Hydrogen production technologies have been a significant area of solar chemical research since the 1970s. Aside from electrolysis driven by photovoltaic or photochemical cells, several thermochemical processes have also been explored. 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%). A more conventional approach uses process heat from solar concentrators 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 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 decompose zinc oxide (ZnO) at temperatures above 1200 °C. This initial reaction produces pure zinc which can subsequently be reacted with water to produce hydrogen.

Sandia's Sunshine to Petrol (S2P) technology uses the high temperatures generated by concentrating sunlight along with a zirconia/ferrite catalyst to break down atmospheric carbon dioxide into oxygen and carbon monoxide (CO). The CO may then be used to synthesize fuels such as methanol, gasoline and jet fuel.

Photoelectrochemical cells or PECs consist of a semiconductor, typically titanium dioxide or related titanates, immersed in an electrolyte. When the semiconductor is illuminated an electrical potential develops. There are two types of photoelectrochemical cells: photoelectric cells that convert light into electricity and photochemical cells that use light to drive chemical reactions such as electrolysis.

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.

Solar mechanical
Solar mechanical technologies use sunlight to produce a mechanical effect. There are many such technologies covered in 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 consists of a glass bulb containing a set of vanes mounted on a spindle. Each vane has a dark side and a white side. When illuminated, the dark side becomes warm due to absorbing light but the white side reflects light and stays cool. The vanes rotate due to the motion of gases from the hot to the cool side of each vane.

Passive solar tracking devices use imbalances caused by the movement of a low boiling point fluid to respond to the movement of the Sun. Tracking PV systems can generally produce 25% more electricity than fixed tilt PV systems. Shading systems that respond to the movement of the Sun can also be used in buildings to maximize natural lighting during winter, lessen summer glare, and reduce cooling loads associated with unwanted solar gain.

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 3021 km across central Australia from Darwin to Adelaide. In 1987, when it was founded, the winner's average speed was 67 km/h. The 2007 race included a new challenge class using cars which could be a practical proposition for sustainable transport with little modification. The winning car averaged 90.87 km/h. The North American Solar Challenge and the planned South African Solar Challenge are comparable competitions that reflect an international interest in the engineering and development of solar powered vehicles.

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) and subsequent designs, cumulating in the Helios which set the altitude record for a non-rocket-propelled aircraft of 29524 m in 2001. The Zephyr, developed by BAE Systems, is the latest in a line of record breaking solar aircraft, making a 54 hour flight in 2007, and month long flights are envisioned by 2010.

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 relatively high.

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

Thermal and electrical storage
Storage is an important issue in the development of solar energy because modern energy systems usually assume continuous availability of energy. Solar energy is not available at night, and the performance of solar power systems is affected by unpredictable weather patterns; therefore, storage media or back-up power systems must be used.

Thermal mass systems can store solar energy in the form of heat at domestically useful temperatures for daily or seasonal durations. Thermal storage systems generally use readily available materials with high specific heat capacities such as water, earth and stone. Well designed systems can lower peak demand, shift time-of-use to 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 an initial temperature of 290 °C up 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 in its 68 m³ storage tank with an annual storage efficiency of about 99%.

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, which has a high enthalpy of fusion. 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%.

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

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 cheap and available. Batteries used in off-grid applications should be sized for three to five days of capacity.

Excess electricity can also be fed into the transmission grid. Net metering programs 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, effectively acting as a battery. For large scale use of renewable energy the most practical storage is hydro-storage, although vehicle-to-grid is also being developed, which will become viable when more plug-in hybrids and electric cars are in use.

Development, deployment and economics


Beginning with the surge in coal use which accompanied the Industrial Revolution in the late 18th century, energy consumption has steadily transitioned from wood and biomass to fossil fuels. The early development of solar technologies starting in the 1860s was driven by an expectation that coal would soon become scarce but solar development stagnated in the early 20th century in the face of the increasing availability, economy, and utility of fossil fuels such as coal and petroleum.

The 1973 oil embargo and 1979 energy crisis caused a reorganization of energy policies around the world and brought renewed attention to developing solar technologies. Deployment strategies focused on incentive programs such as the Federal Photovoltaic Utilization Program in the US and the Sunshine Program in Japan. Other efforts included the formation of research facilities in the USA (SERI, now NREL), Japan (NEDO), and Germany (Fraunhofer Institute for Solar Energy Systems ISE).

Between 1970 and 1983, photovoltaic installations grew rapidly, but dropping oil prices in the early 1980s moderated the growth of PV from 1984 to 1996. Since 1997, PV development has accelerated due to supply issues with oil and natural gas, global warming concerns (see Kyoto Protocol), and the improving economic position of PV relative to other energy technologies. Incentives first in Japan and then Germany resulted in increased production and reduced prices. Photovoltaic electricity is now competitive with conventional electricity rates in many locations. Due to sale of renewable energy credits and other incentives, Nellis Air Force Base is obtaining photovoltaic power for about 2.2 cents/kWh and grid power for 9 cents/kWh. Typical payback periods for installing photovoltaics are 15 to 25 years. Photovoltaic production growth has averaged 40% per year since 2000 and installed capacity reached 10.6 GW at the end of 2007. By 2006 more polysilicon was used for photovoltaics than for computer chips.

Commercial solar water heaters began appearing in the United States in the 1890s. These systems saw increasing use until the 1920s but were gradually replaced by cheaper and more reliable heating fuels. As with photovoltaics, solar water heating attracted renewed attention as a result of the oil crises in the 1970s but interest subsided in the 1980s due to falling petroleum prices. Development in the solar water heating sector progressed steadily throughout the 1990s and growth rates have averaged 20% per year since 1999. Solar water heating has a levelized cost of energy (LCOE) of 8-12 cents/kWh. Although generally underestimated, solar water heating is by far the most widely deployed solar technology with an estimated capacity of 154 GW as of 2007.

Commercial concentrating solar power (CSP) plants were first developed in the 1980s. CSP plants such as SEGS project in the United States have a LCOE of 12-14 cents/kWh. The 11 MW PS10 power tower in Spain, completed in late 2005, is Europe's first commercial CSP system and a total capacity of 300 MW is expected to be installed in the same area by 2013.