Geothermal Energy

There are currently three types of geothermal power plants—dry steam, flash steam, and binary cycle.

Dry steam power plants pipe steam from underground wells to the power plant, where it is used 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.

Flash steam power plants, which use waters at temperatures greater than 360°F, are the most common. 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 power plants use the heat from lower temperature reservoirs (225° to 360°F) 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.

How Geothermal Energy is Used
Low to medium temperature waters (70° to 200°F) can be used directly to heat buildings, grow and dry crops, melt snow on sidewalks, and for fish and alligator farms. High temperature geothermal reservoirs (225° to 600°F) are generally developed to produce large-scale electricity for distribution by power providers. Our Buying Clean Electricity section provides information on buying electricity generated from geothermal energy and other renewable resources in your state.

Geothermal heat pumps allow home and business owners to take advantage of the Earth's constant temperature (around 55°F) just a few yards beneath the surface to heat and cool buildings, and to produce hot water. Learn more about geothermal heat pumps.

Where Geothermal Energy is Used
Current drilling technology limits the development of geothermal resources to relatively shallow, water- or steam-filled reservoirs, most of which are found in the western part of the United States.

Researchers are now developing new technologies for capturing the heat in deeper, "dry" rocks, which would support drilling almost anywhere.

Geothermal heat pumps can be used in almost any part of the country. The only real requirement is enough soil in which to bury the heat-exchange pipes. Geothermal heat pumps have proved most popular in areas with large heating requirements, such as the Northeast and the northern Midwest, but they have been installed in almost every state in the nation.