The term geothermal has come to be associated with two technologies which are only tangentially related; first, power can be produced by drilling into the ground to a depth where the rock is hot enough to boil water. The other use of the term geothermal is associated with ground source heat pumps which need only drill down a few feet to a temperature of fifty to sixty degrees Fahrenheit.
Utility scale power can be produced by drilling into the ground to a depth where the rock is hot enough to boil water to produce steam. The steam is then used to drive a turbine to generate electricity just as a nuclear reactor or a coal fired power plant produces steam to turn turbines. Electricity production from geothermal heat requires drilling several kilometers into the earth and is consequently very expensive, but in certain locations heat is near enough to the surface to make its utilization practical.
Heat at the core of the earth is approximately 6000 degrees Celsius, hence a temperature gradient exists: twenty five degrees C per kilometer. The heat is due to at least two factors, residual heat from the accretion of the planet over four billion years ago and radioactive decay of certain elements such as Uranium and Thorium.
To economically produce power, hot rock must be within three or four kilometers of the surface. This only occurs in geologically active regions, such as areas with earthquakes and/or volcanoes. In these locations fissures in the earth’s crust allow movement of magma near enough to the surface to be exploited for power production.
The simplest design for a geothermal power plant takes advantage of hydrothermal convection. Cool water from the surface seeps underground, is heated and then rises back to the surface. The heated water, now steam, is obtained by drilling wells to capture the steam and directing it to turbines for energy production. The water from the condensed steam can then returned to continue the cycle.
Although the heat is essentially free, the cost of drilling and maintenance of equipment can be high. Subterranean steam extracts caustic materials which corrode even the most inert metals. A limiting factor for energy production can be the rate of heat transfer through rock. As heat is extracted from rock surrounding the well site, heat must be transferred through the rock, limiting the rate of heat extraction.
The United States leads the world in geothermal electric capacity. The US has about 2.7 GigaWatts installed, a quarter of world capacity. Twenty six plants in one location called the geysers,
north of San Francisco, accounts for three quarters of the total US production. For comparison, one nuclear reactor has a capacity of just under one GW.
Parts of Alaska, Washington, Oregon, California, and much of Nevada and Hawaii have potential for geothermal electricity production and much of the Rocky Mountain area could extract useful heat for direct uses such as space heat for apartment buildings, schools, and other large facilities.