How does a solar power plant work?

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Solar thermal electric power plants

The two main types of solar thermal power plants are Solar Chimneys and Concentrating Solar Power (CSP) plants.
Concentrated Solar Power (CSP) Plants
Solar Two, a concentrating solar power plantParabolic trough power plants are the most successful and cost-effective CSP system design at present. They use a curved trough which reflects sunlight onto a hollow tube running along above the trough. The whole trough tilts through the course of the day so that light remains focussed on the hollow tube for as long as the sun shines. A fluid passes through the tube and becomes hot. Full-scale parabolic trough systems consist of many such troughs laid out in parallel over a large area of land. A solar thermal system using this principle is in operation in California in the United States, called the SEGS system. At 330 MW, it is currently the largest operational solar thermal energy system. SEGS uses oil to take the heat away: the oil then passes through a heat exchanger, creating steam which runs a steam turbine. Other parabolic trough systems are under development which create steam directly in the tubes; this concept is thought to lead to cheaper overall designs, but the concept is still under development.
Power Towers (also know as ‘Central Tower’ power plants or ‘Heliostat’ power plants (power towers) use an array of flat, moveable mirrors (called heliostats) to focus the sun’s rays upon a collector tower (the target). The high energy at this point of concentrated sunlight is transferred to a substance that can store the heat for later use. The more recent heat transfer material that has been successfully demonstrated is liquid sodium. Sodium is a metal with a high heat capacity, allowing that energy to be stored and drawn off throughout the evening. That energy can, in turn, be used to boil water for use in steam turbines. Water had originally been used as a heat transfer medium in earlier power tower versions (where the resultant steam was used to power a turbine). This system did not allow for power generation during the evening. Examples of heliostat based power plants are the 10 MWe Solar One, Solar Two, and the 15 MW Solar Tres plants. Neither of these are currently used for active energy generation. In South Africa, a solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m².[3]
A Dish system uses a large, reflective, parabolic dish (similar in shape to satellite televison dish). It focuses all the sunlight that strikes the dish up onto to a single point above the dish, where a thermal collect is used to capture the heat and transform it into a useful form. Dish systems, like power towers, can achieve much higher temperatures due to the higher concentration of light which they receive. Typically the dish is coupled with a Stirling engine in a Dish-Stirling System, but also sometimes a steam engine is used. These create rotational kinetic energy that can be converted to electricity using an electric generator.[4] [5] [6].
A linear Fresnel reflector power plant uses a series of carefully angled plane mirrors to focus light onto a linear absorber. Recent prototypes of these types of systems have been built in Australia (CLFR) and Belgium (SolarMundo). These systems claim to offer lower overall costs because they permit the heat-absorbing element to be shared between several mirrors. The mirrors can therefore be smaller and do not require complex pivoting couplings for the fluid flowing from the absorber. The design can also permit mirrors to be placed closer together, allowing for a more efficient use of land area.

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