| Solar power is the technology of | | | | on this small yacht at sea can charge |
| obtaining usable energy from the light | | | | the 12 V batteries at up to 9 A in full, |
| of the Sun. Solar energy has been used | | | | direct sunlightSolar cells, also |
| in many traditional technologies for | | | | referred to as photovoltaic cells, are |
| centuries and has come into widespread | | | | devices or banks of devices that use the |
| use where other power supplies are | | | | photovoltaic effect of semiconductors to |
| absent, such as in remote locations and | | | | generate electricity directly from |
| in space. | | | | sunlight. Until recently, their use has |
| Solar energy is currently used in a | | | | been limited because of high |
| number of applications: | | | | manufacturing costs. One cost effective |
| Heating (hot water, building heat, | | | | use has been in very low-power devices |
| cooking) | | | | such as calculators with LCDs. Another |
| Electricity generation (photovoltaics, | | | | use has been in remote applications such |
| heat engines) | | | | as roadside emergency telephones, remote |
| Desalination of seawater. | | | | sensing, cathodic protection of pipe |
| Its application is spreading as the | | | | lines, and limited "off grid" home power |
| environmental costs and limited supply | | | | applications. A third use has been in |
| of other power sources such as fossil | | | | powering orbiting satellites and other |
| fuels are realized. | | | | spacecraft. |
| Solar radiation reaches the Earth's | | | | Total peak power of installed PV is |
| upper atmosphere at a rate of 1,366 | | | | around 5,300 MW as of the end of |
| watts per square meter (W/m2).[1] | | | | 2005.[citation needed] This is only one |
| However, not all of this energy is | | | | part of solar-generated electric power. |
| available for use at ground level. While | | | | For solar reflector plants see below. |
| traveling through the atmosphere, 6% of | | | | Declining manufacturing costs (dropping |
| the incoming solar radiation | | | | at 3 to 5% a year in recent years) are |
| (insolation) is reflected and 16% is | | | | expanding the range of cost-effective |
| absorbed resulting in a peak irradiance | | | | uses. The average lowest retail cost of |
| at the equator of 1,020 W/m2.[2] Average | | | | a large photovoltaic array declined from |
| atmospheric conditions (clouds, dust, | | | | $7.50 to $4 per watt between 1990 and |
| pollution) further reduce insolation by | | | | 2005[citation needed]. With many |
| 20% through reflection and 3% through | | | | jurisdictions now giving tax and rebate |
| absorption.[3] Atmospheric conditions | | | | incentives, solar electric power can now |
| not only reduce the quantity of | | | | pay for itself in five to ten years in |
| insolation reaching the earth's surface | | | | many places. "Grid-connected" systems - |
| but also affect the quality of | | | | that is, systems with no battery that |
| insolation by diffusing incoming light | | | | connect to the utility grid through a |
| and altering its spectrum. | | | | special inverter - now make up the |
| The image on the right shows the average | | | | largest part of the market. In 2003 |
| global irradiance calculated from | | | | worldwide production of solar cells |
| satellite data collected from 1991 to | | | | increased by 32%. Between 2000 and 2004 |
| 1993. For example, in North America the | | | | the increase in worldwide solar energy |
| average insolation lies between 125 and | | | | capacity was an annual 60%. 2005 was |
| 375 W/m2 (3 to 9 kWh/m2/day). [4] This | | | | expected to see large growth again, but |
| is the available power, and not the | | | | shortages of refined silicon have been |
| delivered power. Photovoltaic panels | | | | hamering production worldwide since late |
| currently convert about 15% of incident | | | | 2004. Analysts have predicted the |
| sunlight into electricity; therefore, a | | | | similar supply problems during 2006 and |
| solar panel in the contiguous United | | | | 2007. |
| States on average delivers 19 to 56 W/m2 | | | | Solar thermal electric power plants |
| or 0.45-1.35 kWh/m2/day. [5] The dark | | | | Solar Two, a concentrating solar power |
| disks on the second image on the right | | | | tower (an example of solar thermal |
| are an example of the land areas that, | | | | energy applied to electrical power |
| if covered with 8% efficient solar | | | | production).Main article: Solar thermal |
| panels, would produce slightly more | | | | energy |
| energy in the form of electricity than | | | | Solar thermal energy can be used to heat |
| the total world primary energy supply in | | | | a heat exchanger to high temperature and |
| 2003. [6] While average insolation and | | | | the heat is used to produce electric |
| power values offer insight into solar | | | | power or for other industrial purposes. |
| power's potential on a regional scale, | | | | Power towers |
| locally relevant conditions need to be | | | | Power towers (also know as 'central |
| assessed to determine the solar | | | | tower' power plants or 'heliostat' power |
| potential of a specific site. | | | | plants (power towers)) use an array of |
| A recent concern is global dimming, an | | | | flat, moveable mirrors (called |
| effect of pollution that is allowing | | | | heliostats) to focus the sun's rays upon |
| less sunlight to reach the Earth's | | | | a collector tower (the target). The high |
| surface. It is intricately linked with | | | | energy at this point of concentrated |
| pollution particles and global warming, | | | | sunlight is transferred to a substance |
| and it is mostly of concern for issues | | | | that can store the heat for later |
| of global climate change, but is also of | | | | useage. |
| concern to proponents of solar power | | | | Concentrating collector with steam |
| because of the existing and potential | | | | engine |
| future decreases in available solar | | | | Solar energy converted to heat in a |
| energy. The order of magnitude is about | | | | concentrating collector can be used to |
| 4% less solar energy available at sea | | | | boil water into steam (as is done in |
| level over the timeframe 1961–90, | | | | nuclear and coal power plants) to drive |
| mostly from increased reflection from | | | | a steam engine or steam turbine. The |
| clouds back into outer space. | | | | concentrating collector can be an trough |
| After passing through the Earth's | | | | collector, parabolic collector, or power |
| atmosphere, most of the sun's energy is | | | | tower. |
| in the form of visible and Infrared | | | | Concentrating collector with Stirling |
| radiations. Plants use solar energy to | | | | engine |
| create chemical energy through | | | | A parabolic solar collector |
| photosynthesis. Humans regularly use | | | | concentrating the sun's rays on the |
| this energy burning wood or fossil | | | | heating element of a Stirling engine. |
| fuels, or when simply eating the plants. | | | | The entire unit acts as a solar |
| Types of technologies | | | | tracker.Solar energy converted to heat |
| Many technologies have been developed to | | | | in a concentrating (dish or trough |
| make use of solar radiation. Some of | | | | parabolic) collector can be used to |
| these technologies make direct use of | | | | drive a Stirling engine. The Stirling |
| the solar energy (e.g. to provide light, | | | | engine is a type of heat engine which |
| heat, etc.), while other technologies | | | | uses a sealed working gas (i.e. a closed |
| produce electricity. | | | | cycle) and does not require a water |
| Solar design in architecture | | | | supply. |
| Solar heating systems | | | | A solar Stirling system holds the record |
| Solar hot water systems use sunlight to | | | | for converting solar energy into |
| heat water. These systems may be used to | | | | electricity (30 percent at 1,000 watts |
| heat domestic hot water or for space | | | | per square meter). Such concentrating |
| heating. These systems are basically | | | | systems produce little or no power in |
| composed of solar thermal collectors and | | | | overcast conditions and incorporate a |
| a storage tank.[8] The three basic | | | | solar tracker to point the device |
| classifications of solar water heaters | | | | directly at the sun. |
| are: | | | | Solar updraft tower |
| Active systems which use pumps to | | | | A solar updraft tower is a relatively |
| circulate water or a heat transfer | | | | low-tech solar thermal power plant where |
| fluid. | | | | air passes under a very large |
| Passive systems which circulate water or | | | | agricultural glass house (between 2 and |
| a heat transfer fluid by natural | | | | 8 km in diameter), is heated by the sun |
| circulation. These are also called | | | | and channeled upwards towards a |
| thermosiphon systems. | | | | convection tower. It then rises |
| Batch systems using a tank directly | | | | naturally and is used to drive turbines, |
| heated by sunlight. | | | | which generate electricity. |
| A Trombe wall is a passive solar heating | | | | Energy tower |
| and ventilation system consisting of an | | | | An energy tower is an alternative |
| air channel sandwiched between a window | | | | proposal to the solar updraft tower. The |
| and a sun-facing wall. Sunlight heats | | | | energy tower is driven by spraying water |
| the air space during the day causing | | | | at the top of the tower, evaporation of |
| natural circulation through vents at the | | | | water causes a downdraft by cooling the |
| top and bottom of the wall and storing | | | | air thereby increasing its density, |
| heat in the thermal mass. During the | | | | driving windturbines at the bottom of |
| evening the trombe wall radiates stored | | | | the tower. It requires a hot arid |
| heat.[9] | | | | climate and large quantities of water |
| A transpired collector is an active | | | | (seawater may be used for this purpose) |
| solar heating and ventilation system | | | | but it does not require the large glass |
| consisting of a perforated sun-facing | | | | house of the solar updraft tower. |
| wall which acts as a solar thermal | | | | Solar pond |
| collector. The collector pre-heats air | | | | A solar pond is a relatively low-tech, |
| as it is drawn into the building's | | | | low cost approach to harvesting solar |
| ventilation system through the | | | | energy. The principle is to fill a pond |
| perforations. These systems are | | | | with 3 layers of water: |
| inexpensive and commercial models have | | | | A top layer with a low salt content |
| achieved efficiencies above 70 percent. | | | | An intermediate insulating layer with a |
| Most systems pay for themsleves within | | | | salt gradient, which sets up a density |
| 4-8 years. | | | | gradient that prevents heat exchange by |
| Solar cooking | | | | natural convection in the water. |
| Solar Cookers use sunshine as an | | | | A bottom layer has with a high salt |
| alternative to fire for cooking.A solar | | | | content which reaches a temperature |
| box cooker traps the Sun's energy in an | | | | approaching 90 degrees Celsius. |
| insulated box; such boxes have been | | | | The different densities in the layers |
| successfully used for cooking, | | | | because of their salt content prevent |
| pasteurization and fruit canning. Solar | | | | convection currents developing which |
| cooking is helping many developing | | | | would normally transfer the heat to the |
| countries, both reducing the demands for | | | | surface and then to the air above. The |
| local firewood and maintaining a cleaner | | | | heat trapped in the salty bottom layer |
| environment for the cooks. The first | | | | can be used for different purposes, such |
| known western solar oven is attributed | | | | as heating of buildings, industrial |
| to Horace de Saussure in 1767, which | | | | processes, or generating electricity. |
| impressed Sir John Herschel enough to | | | | There is one in use at Bhuj, Gujarat, |
| build one for cooking meals on his | | | | India and another at the University of |
| astronomical expedition to the Cape of | | | | Texas El Paso. |
| Good Hope in Africa in 1830. Today, | | | | Solar chemical |
| there are many different designs in use | | | | Solar chemical refers to a number of |
| around the world. | | | | possible processes that harness solar |
| Solar lighting | | | | energy by absorbing sunlight in a |
| Solar lighting or daylighting is the use | | | | chemical reaction in a way similar to |
| of natural light to provide | | | | photosynthesis in plants but without |
| illumination. Daylighting offsets energy | | | | using living organisms. No practical |
| use in electric lighting systems and | | | | process has yet emerged. |
| reduces the cooling load on HVAC | | | | A promising approach is to use focused |
| systems. Although difficult to quantify, | | | | sunlight to provide the energy needed to |
| the use of natural light also offers | | | | split water into its constituent |
| physiological and psychological | | | | hydrogen and oxygen in the presence of a |
| benefits. Builiding orientation, | | | | metallic catalyst such as zinc. |
| exterior shading, sawtooth roofs, | | | | While metals, such as zinc, have been |
| clerestory windows, light shelves, | | | | shown to drive photoelectrolysis of |
| skylights and light tubes are among the | | | | water, more research has focused on |
| many daylighting features. These | | | | semiconductors. Further research has |
| features may be incorporated in existing | | | | examined transition metal compounds, in |
| structures but are most effective when | | | | particular titanium, niobium and |
| integrated in a solar design package | | | | tantalum oxides.[citation needed] |
| which accounts for factors such as | | | | Unfortunately, these materials exhibit |
| glare, heat gain, heat loss and | | | | very low efficiencies, because they |
| time-of-use. Achitectural trends | | | | require ultraviolet light to drive the |
| increasingly favor daylighting as a | | | | photoelectrolysis of water. Current |
| cornerstone of sustainable design. | | | | materials also require an electrical |
| Daylight saving time (DST) can be seen | | | | voltage bias for the hydrogen and oxygen |
| as a method of utilising solar energy by | | | | gas to evolve from the surface, another |
| matching available sunlight to the hours | | | | disadvantage. Current research is |
| of the day in which it is most useful. | | | | focusing on the development of materials |
| DST energy savings have been estimated | | | | capable of the same water splitting |
| to reduce total electricity use in | | | | reaction using lower energy visible |
| California by .5% (3400 MWh) and peak | | | | light. |
| electricity use by 3% (1000 MW). | | | | It is also possible to use solar energy |
| Photovoltaics | | | | to drive industrial chemical processes |
| The solar panels (photovoltaic arrays) | | | | without a requirement for fossil fuel. |