In power tower concentrating solar power systems, a large number of flat, sun-tracking mirrors, known as heliostats, focus sunlight onto a receiver at the top of a tall tower. A heat-transfer fluid heated in the receiver is used to heat a working fluid, which, in turn, is used in a conventional turbine generator to produce electricity. Some power towers use water/steam as the heat-transfer fluid. Other advanced designs are experimenting with high temperature molten salts or sand-like particles to maximize the power cycle temperature.
Two large-scale power tower projects are currently operating in the U.S. The Ivanpah Solar Electric Generating System is the largest concentrated solar thermal plant in the country. Located in California's Mojave Desert, the plant is capable of producing 392 megawatts of electricity using 173,500 heliostats, each with two mirrors that focus sunlight onto three solar power towers. Crescent Dunes Solar Energy Project is a 110 megawatt solar thermal power plant located in southwestern Nevada. The project includes 10,347 heliostats and is the largest power tower system with direct thermal energy storage, which means the heat transfer fluid is also the storage material.
Aside from the U.S., Spain has several power tower systems. Planta Solar 10 and Planta Solar 20 are water/steam systems with capacities of 11 and 20 megawatts, respectively. Gemasolar, previously known as Solar Tres, produces nearly 20 megawatts of electricity and utilizes molten-salt thermal storage.