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