In solar thermal power plants the incoming radiation is tracked by large mirror fields which concentrate the energy towards absorbers. They, in turn, receive the concentrated radiation and transfer it thermally to the working medium. The heated fluid operates as in conventional power stations directly (if steam or air is used as medium) or indirectly through a heat exchanging steam generator on the turbine unit which then drives the generator.

To make solar high flux, with high energetic value originating from processes occurring at the sun's surface atblack body equivalent temperatures of approximately 5800 K usable for technical processes and commercial applications, different concentrating technologies have been developed or are currently under development for various commercial applications. Such solar thermal concentrating systems will undoubtedly provide within the next decade a significant contribution to efficient and economical, renewable and clean energy supply.

There are three different technologies for solar thermal power plants making use of concentrating solar energy systems:

Parabolic Dish:

A practical application of a parabolic dish is a flashlight lens, which is used to transform a point source of light into a parallel beam. The reverse is true. Since sunlight radiation is essentially parallel it may be concentrated at the focal point of the lens. As a matter of fact a tiny flashlight lens may be used as a cigarette lighter by substituting a cigarette for the bulb and by pointing the lens in the direction of the sun. A type of solar reflector dish concentrator may also be made by lining the inside of a cardboard box with aluminum foil. Large experimental parabolic dishes known as heliodynes are capable of melting steel but they operate at a low efficiency and must be continually aligned to be of any practical value.

Parabolic Trough:

Solar power plants use parabolic troughs to make super heated steam and generate electricity because their fabrication and tracking equipment is less expensive than the dish. A simple parabolic trough may be made with a sheet of cardboard lined with a piece of aluminum foil. I've heard of people using this device to roast hot dogs. Both solar reflectors can be used as cookers, but we won't be cooking hot dogs with our science projects. Right? To be scientific and collect meaningful data we should control as many variables as possible. For this reason we should construct our solar reflectors with the aid of a carefully constructed template.

Central Receiver System:

Central receiver systems use heliostats to track the sun by two axes mechanisms following the azimuth and elevation angles with the purpose to reflect the sunlight from many heliostats oriented around a tower and concentrate it towards a central receiver situated atop the tower. This technology has the advantage of transferring solar energy very efficiently by optical means and of delivering highly concentrated sunlight to one central receiver unit, serving as energy input to the power conversion system. In spite of the elegant design concept and in spite of the future prospects of high concentration and high efficiencies, the central receiver technology needs still more research and development efforts and demonstration of up-scaled plant operation to come up to commercial use. Its main attraction consists in the prospect of high process temperatures generated by highly concentrated solar radiation to supply energy to the topping cycle of any power conversion system and to feed effective energy storage systems able to cover the demand of modern power conversion systems. The solar thermal output of central receiver systems can be converted to electric energy in highly efficient Rankine-cycle/steam turbine generators, in Brayton-cycle/gas turbine generators or in combined cycle (gas turbine with bottoming steam turbine) generators. Gridconnected tower power plants are applicable up to about 200 MWe solar-only unit capacity.

Important

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