Castle Geyser, a cone geyser, in Yellowstone National Park.

Geothermal energy is thermal energy generated and stored in the Earth. Geothermal energy can manifest on the surface of the Earth, or near the surface of the Earth, where humankind may harness it to serve our energy needs.

Geothermal resources are reservoirs of hot water that exist at varying temperatures and depths below the Earth's surface. Wells can be drilled into these underground reservoirs to tap steam and very hot water that can be brought to the surface for a variety of uses.


Geothermal energy is broken into the following resource types:

Electricity Production

A geothermal resource requires three things to generate electricity:

Fluid- sufficient fluid must exist in fractures and pore space within the rocks.

Heat- the rocks must be hot. Heat within the Earth's crust is generated from radioactive decay of minerals and continual heat loss from when the Earth was formed 4.6 billion years ago. Though the rocks need to be hot, the rocks are cool and brittle, relative to the more recognizable magma or lava.

Permeability- fluids must come into contact with the heated rock via fractures and pore spaces. Through these voids the fluids can circulate through the rocks, heat up, and come to the surface for electricity production.

Direct Use

Geothermal reservoirs of low-to moderate-temperature water — 68°F to 302°F (20°C to 150°C) — provide direct heat for residential, industrial, and commercial uses. This resource is widespread in the United States, and is used to heat homes and offices, commercial greenhouses, fish farms, food processing facilities, gold mining operations, and a variety of other applications. In addition, spent fluids from geothermal electric plants can be subsequently used for direct use applications in so-called "cascaded" operation.

Heating and Cooling

The geothermal heat pump, also known as the ground source heat pump, is a highly efficient renewable energy technology that is gaining wide acceptance for both residential and commercial buildings. Geothermal heat pumps are used for space heating and cooling, as well as water heating. The benefit of ground source heat pumps is they concentrate naturally existing heat, rather than by producing heat through the combustion of fossil fuels.

Power Plants

Hot water or steam brought the surface can generate electricity. There are three primary geothermal plant technologies:

Dry Steam Power Plant

Dry steam power plants use steam to generate electricity. The steam travels directly to a turbine, which drives a generator, generating electricity. Dry steam power plants were the first type of geothermal power generation plants built (Lardarello, Italy, 1904). Steam technology is still effective today and currently in use at The Geysers in northern California, the world's largest single source of geothermal power.

Flash Steam Power Plant

Flash steam plants are the most common type of geothermal power generation plants in operation today. Fluid is pumped under high pressure into a tank at the surface held at a much lower pressure, causing some of the fluid to rapidly vaporize, or "flash." The vapor then drives a turbine, which drives a generator, generating electricity. If any liquid remains in the tank, it can be flashed again in a second tank to extract even more energy.

Binary Cycle Power Plant

Binary cycle geothermal power generation plants differ from Dry Steam and Flash Steam systems in that the water or steam from the geothermal reservoir never comes in contact with the turbine/generator units. Low to moderately heated geothermal fluid warms a secondary fluid with a much lower boiling point. The secondary fluid to flashes to vapor, drives a turbine, driving a generator, generating electricity. Binary cycle power plants utilize low temperature fluids (below 400F), which is the most common temperature range for a electricity-producing geothermal reservoir.