Solar thermal power plants use the sun's rays to heat a fluid, from which heat transfer systems may be used to produce steam. The steam, in turn, is converted into mechanical energy in a turbine and into electricity from a conventional generator coupled to the turbine. Solar thermal power generation is essentially the same as conventional technologies except that in conventional technologies the energy source is from the stored energy in fossil fuels released by combustion. Solar thermal technologies use concentrator systems due to the high temperatures needed for the working fluid. The three types of solar-thermal power systems in use or under development are: parabolic trough, solar dish, and solar power tower.