In the United States, geothermal energy has been used to generate electricity on a large-scale since 1960. Through research and development, geothermal power is becoming more cost effective and competitive with fossil fuels.
Heat from the Earth—geothermal energy—heats water that has seeped into underground reservoirs. These reservoirs can be tapped for a variety of uses, depending on the temperature of the water. The energy from high temperature reservoirs (225º–600ºF) can be used to produce electricity. There are currently three types of geothermal power plants:
Dry steam plants use steam from underground wells to rotate a turbine, which activates a generator to produce electricity. There are only two known underground resources of steam in the United States: The Geysers in northern California and Old Faithful in Yellowstone National Park. Since Yellowstone is protected from development, the power plants at The Geysers are the only dry steam plants in the country.
The most common type of geothermal power plant, flash stream plants use waters at temperatures greater than 360ºF. As this hot water flows up through wells in the ground, the decrease in pressure causes some of the water to boil into steam. The steam is then used to power a generator, and any leftover water and condensed steam is returned to the reservoir.
Binary cycle plants use the heat from lower-temperature reservoirs (225º–360ºF) to boil a working fluid, which is then vaporized in a heat exchanger and used to power a generator. The water, which never comes into direct contact with the working fluid, is then injected back into the ground to be reheated.