Traditionally, geothermal energy has been heat energy stored within the earth's crust, hot springs and geysers providing evidence of this stored energy. Until the beginning of this century, the utilization of geothermal heat has been limited to the use of warm water as in several Roman baths in England and geothermal hot springs began to enjoy wide use throughout the world as therapeutic treatment. More extensive use has been made of geothermal energy for both power and non-power applications in more recent times. Typically, the useful geothermal heat for electric power generation is available in the form of a hot brine with temperatures ranging from 300 to 400 deg F or 150 to 200 deg C.

The principle characteristics of geothermal electric generating plants as compared to a fossil plant are that:
(1) Significantly larger flow of steam is required by a geothermal plant to produce a KW of electrical power because of the significantly lower steam pressure,
(2) The small output of an individual geothermal plant which is limited by the availability of hot water/steam that is able to be created in a thermal pool of hot water below the earths surface.
(3) The number of different systems may be employed for the extraction of steam from geothermal sources.

For example, the heat contained in the hot geothermal brine may be recovered as steam by flashing the brine to a lower pressure and either utilizing the steam directly in a steam turbine or by transferring the heat to a second working fluid (which may be steam or an organic fluid) to produce power by expansion in a turbine.