Geothermal energy is renewable heat energy from deep in the earth. Use of hydrothermal energy is economic today at a number of high-grade sites.
Earth energy is used by geothermal heat pumps.
F to more than 700deg. F, is brought from the underground reservoir to the surface through production wells, and is flashed to steam in special vessels by release of pressure. The steam is separated from the liquid and fed to a turbine engine, which turns a generator. Spent geothermal fluid is injected back into peripheral parts of the reservoir to help maintain reservoir pressure. If the reservoir is to be used for direct-heat application, the geothermal water is usually fed to a heat exchanger before being injected back into the earth.