Solar energy

The Solar Two 10 MW solar power facility, showing the power tower (left) surrounded by the sun-tracking mirrors.

Available solar energy (left) greatly exceeds both potential wind power (center) and global energy consumption (right).

Solar energy is energy from the Sun in the form of radiated heat and light. This energy drives the climate and weather and supports 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 most of the available flow of renewable energy.

Solar energy technologies harness the sun's energy for practical ends. These technologies date from the time of the early Greeks, Native Americans and Chinese, who warmed their buildings by orienting, or positioning them toward the sun. Solar technologies provide heating, lighting, dry clothes and provide electricity. Even solar powered airplanes have been demonstrated.

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.

Energy from the Sun

Main articles: Insolation and Solar radiation

About half the incoming solar energy is absorbed by water and land; the rest is reradiated back into space.

Annual average insolation at Earth's surface. The black dots represent the land area required to replace the total world energy supply with electricity from solar cells.

Earth continuously receives 174 PW of incoming solar radiation (insolation) at the upper atmosphere. When it meets the atmosphere, 6% of the insolation is reflected and 16% is absorbed. Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation traveling through the atmosphere by 20% due to reflection and 3% via absorption. 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 ZJ per year.
• Winds can potentially supply 2.25 ZJ of electricity per year.
• Biomass captures approximately 1.8 ZJ of solar energy per year.
• Worldwide energy consumption was 0.488 ZJ in 2005.

The output of a solar panels will vary according to their conversion efficiency and the amount sunlight the received. For example, in United States, the average insolation at ground level over an entire year (including nights and periods of cloudy weather) is 10.8 to 32.4 MJ/m²/day. At present, photovoltaic panels typically convert about 15% of incident sunlight into electricity; therefore, a solar panel in the contiguous United States, on average, delivers 1.6 to 4.85 MJ/m²/day. By contrast, typical solar water heating systems operating at 60% efficiency will deliver 6.5 to 19.5 MJ/m²/day.

Types of technologies

Solar energy technologies utilize solar radiation 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, heat and light buildings and generate electricity. The qualities and performance of solar technologies will vary regionally. The words of the first century Roman architect Vitruvius are appropriate to all solar technologies.

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.

Architecture and urban planning

Main articles: Passive solar building design and Urban heat island

Darmstadt University of Technology won the 2007 Solar Decathlon with this passive house designed specifically for the humid and hot subtropical climate in Washington, D.C.

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. Solar design was largely abandoned in Europe after the Fall of Rome but continued unabated in China where cosmological traditions associate the south with summer, warmth and health.

The elemental features of passive solar architecture are Sun orientation, compact proportion, 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 within 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 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 US$1 billion. Estimated annual benefits are US$170 million from reduced air-conditioning costs and US$360 million in smog related health savings.

Agriculture and horticulture

Main articles: Agriculture, Horticulture, and Greenhouse

Greenhouses like these in the Netherland's Westland municipality grow a wide variety of vegetables, fruits, and flowers.

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 specialty crops. Primitive greenhouses were first used during Roman times to grow cucumbers year-round for the Roman emperor Tiberius. In the 16th century the first modern greenhouses were built in Europe to conserve exotic plants brought back from explorations abroad. Greenhouses remain an important part of horticulture today where they are used to cultivate fruits, vegetables, and flowers that can be relatively exotic when considered against the local climate. One of the world's largest greenhouse complexes in Willcox, Arizona grows 106 ha of tomatoes and cucumbers year-round. Plastic transparent materials have also been utilized to similar effect in polytunnels.

Solar lighting

Main articles: Daylighting and Daylight saving time

Daylighting features such as this oculus at the top of the Pantheon in Rome have been in use since antiquity.

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% (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% (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% 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% and total electricity use by 12,250 GJ. DST has been estimated to reduce early spring and late fall peak loads by 1.5% and total daily electricity use by 3,600 to 11,800 GJ.

Solar thermal

Main article: Solar thermal energy

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

Main articles: Solar hot water and Solar combisystem

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Solar water heaters face the equator and are angled according to latitude to maximize solar gain.

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. Today, approximately 14% (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% of domestic hot water use.

As of 2007, the total installed capacity of solar hot water systems is approximately 128 GW and growth is 15-20% per year. 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.

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 will deliver water temperatures of 20-120 °C and can be expected to have efficiencies above 60% during normal operating conditions. The most common types of solar water heaters are batch systems, flat plate collectors and evacuated tube collectors. Common applications include heating swimming pools, domestic hot water, space heating and thermal storage.

Heating, cooling and ventilation

Main articles: Solar heating, Thermal mass, Trombe wall, Solar chimney, and Solar air conditioning

MIT's Solar House#1 built in 1939 utilized seasonal thermal storage for year round heating.

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. These materials prevent the overheating of internal environments during the day and radiate their stored heat to the cooler atmosphere at night. 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. These materials have historically been used in arid climates or warm temperate regions to keep buildings cool but they can also be used in cold temperate areas to keep buildings warm. 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. The water content of trees will also help moderate local temperatures.

Process heat

Main articles: Solar pond, Salt evaporation pond, and Solar furnace

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STEP project parabolic dishes used for steam production and electrical generation

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 food processing facility in Modesto, California uses 5,000 m² of parabolic troughs to provide heat for a manufacturing line. The system is expected to produce 4.3 GJ per year which will meet a significant portion of the facilities process heating needs. A prototype Scheffler reflector is currently being constructed in India for use in a solar crematorium. This 50 m² reflector will generate temperatures of 700 °C and displace 200-300 kg of firewood used per cremation.

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.

Cooking

Main article: Solar cooker

The Solar Bowl above the Solar Kitchen in Auroville, India concentrates sunlight on a movable receiver to produce steam for 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. Solar kitchens may also use solar bowl or Scheffler reflectors.

The solar bowl is a unique concentrating technology used by the Solar Kitchen in Auroville, India. Contrary to nearly all concentrating technologies that use tracking reflector systems, the solar bowl 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 where 2,000 meals are prepared daily.

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

Main articles: Solar still, Solar water disinfection, and Desalination

A SODIS application in Indonesia demonstrates the simplicity of this approach to water 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

Main article: Photovoltaics

Solar energy powers the International Space Station.

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 the early 1880s. Although these 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 silicon 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. The chemical battery aboard the Vanguard was exhausted within a few weeks but the solar cells allowed the satellite to continue transmitting for over a year. The successful operation of solar cells on this mission was duplicated in 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 continue to remain vital to the telecommunications infrastructure today.

Photovoltaic solar panels on a house roof.

While not a barrier to space applications, the high cost of solar cells limited terrestrial uses throughout the 1960s. 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, navigational buoy's, railroad crossings and lighthouses. PV remains highly competitive in locations with limited grid access and it is becoming more cost effective than running power lines in a growing number of situations. Applications include powering outdoor lighting, callboxes and roadside billboards.

The world oil shock in 1973 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 design and 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 helped moderate the growth of photovoltaics from 1984 through 1996.

Concentrating solar energy

Main articles: Solar thermal energy, Parabolic trough, and Solar power tower

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Solar troughs are the most widely deployed and cost-effective solar thermal technology.

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Dish engine systems eliminate the need to transfer heat to a boiler by placing a Stirling engine at the focal point.

The PS10 solar power tower near Seville concentrates sunlight from a field of heliostats on a central tower.

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 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

Main articles: Solar updraft tower, Solar pond, and thermogenerator

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. 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. 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% up to 15-20%.

Solar chemical

Main article: 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%). 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.

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.

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.

Solar mechanical

Headworks of a passive tracker tilted to meet the morning Sun.

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 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, and reduce summer glare and cooling loads.

Solar vehicles

Main articles: Solar vehicle, Electric boat, and Solar balloon

Australia hosts the World Solar Challenge where solar cars like the Nuna3 race through a 3,021 km (1,877 mi) course from Darwin to Adelaide.

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 kilometres (1,877 mi) across central Australia from Darwin to Adelaide. In 1987, when it was founded, the winner's average speed was 67 kilometres per hour (42 mph). 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 kilometres per hour (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.

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.

Helios UAV in solar powered flight

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 metres (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.

Thermal and electrical storage

Main articles: Thermal mass, Thermal energy storage, Phase change material, and Grid energy storage

Solar Two's thermal storage system allowed it to generate electricity during cloudy weather and at night.

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, a storage medium 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. 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%.

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.

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% to minimize cycling and prolong battery life.

Excess electricity can also be fed into the transmission grid to meet electrical demands elsewhere. 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.

Development, deployment and economics

Main article: Deployment of solar power to energy grids

Years	Total installation	Yearly production	$/Watt
1970-1983	~100 kW → 59 MW	none → 20 MW	100 → 7.75
1984-1996	59 MW → 699 MW	20 MW → 89 MW	7.75 → 4.00
1997-2007	699 MW → 10.6 GW	89 MW → 3 GW	4.00 → 3.40

11 MW Serpa solar power plant in Portugal

Historically, solar resources have made up the largest part of energy consumption in the form of wood and other biomass; however, energy consumption since the industrial revolution has transitioned to fossil fuels. Solar pioneers Mouchot, Ericsson, and Shulman were in part driven by the expectation that coal would soon become scarce.

One cannot help coming to the conclusion that it would be prudent and wise not to fall asleep regarding this quasi-security. Eventually industry will no longer find in Europe the resources to satisfy its prodigious expansion...Coal will undoubtedly be used up. What will industry do then?

— Auguste Mouchout - 1860

Despite the expectation of impending scarcity, the steadily increasing availability, economy and utility of fossil fuels discouraged the development of solar technologies during most of the 20th century. Currently coal, petroleum, and natural gas account for approximately 85% of world energy consumption.

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 solar 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 and prices fell; however, dropping oil prices weakened the demand for solar technologies and from 1984 through 1996 growth was steady but moderate.

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.

See also

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

References

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External links

• Energy transitions past and future, Encyclopedia of Earth
• Energy Education a2z from the Energy Education Foundation
• Solar calculator
• Build It Solar, The Renewable Energy site for Do-It-Yourselfers
• NASA photovoltaic info