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The vacuum significantly reduces convective heat loss. A fluid (also called heat transfer fluid) passes through thereceiver and becomes very hot. Common fluids are synthetic oil, molten salt and pressurized steam. The fluidcontaining the heat is transported to a heat engine where about a third of the heat is converted to electricity. Full-scale parabolic trough systems consist of many such troughs laid out in parallel over a large area of land. Since 1985a solar thermal system using this principle has been in full operation in California in the United States. It is calledthe Solar Energy Generating Systems (SEGS) system. Other CSP designs lack this kind of long experience andtherefore it can currently be said that the parabolic trough design is the most thoroughly proven CSP technology.
Parabolic dishSolar Parabolic dishWith a parabolic dish collector, one or more parabolic dishes concentrate solar energy at a single focal point, similarto the way a reflecting telescope focuses starlight, or a dish antenna focuses radio waves. This geometry may beused in solar furnaces and solar power plants. The shape of a parabola means that incoming light rays which areparallel to the dish's axis will be reflected toward the focus, no matter where on the dish they arrive. Light from thesun arrives at the Earth's surface almost completely parallel, and the dish is aligned with its axis pointing at the sun,allowing almost all incoming radiation to be reflected towards the focal point of the dish. Most losses in suchcollectors are due to imperfections in the parabolic shape and imperfect reflection. Losses due to atmosphericscattering are generally minimal. However, on a hazy or foggy day, light is diffused in all directions through theatmosphere, which significantly reduces the efficiency of a parabolic dish. In dish stirling power plant designs, astirling engine coupled to a dynamo, is placed at the focus of the dish. This absorbs the energy focused onto it andconverts it into electricity.Power towerA power tower is a large tower surrounded by tracking mirrors called heliostats. These mirrors align themselves andfocus sunlight on the receiver at the top of tower, collected heat is transferred to a power station below. This designreaches very high temperatures. High temperatures are suitable for electricity generation using conventionalmethods like steam turbine or a direct high temperature chemical reaction such as liquid salt.By concentratingsunlight, current systems can get better efficiency than simple solar cells. A larger area can be covered by usingrelatively inexpensive mirrors rather than using expensive solar cells. Concentrated light can be redirected to a
suitable location via optical fiber cable for such uses as illuminating buildings. Heat storage for power productionduring cloudy and overnight conditions can be accomplished, often by underground tank storage of heated fluids.