Geothermal energy is produced by the heat of Earth’s molten interior. This energy is harnessed to generate electricity when steam (or hot water which is later converted to steam) from deep underground is used to drive a turbine on an electric power generator. Moderate to low temperature geothermal resources are also used to heat buildings directly and to provide space heating through district heating systems.
Although the United States generates more electricity from geothermal energy than any other country in the world, in 2008 it only accounted for 0.4% of the total U.S. energy supply.
Although the United States generates more electricity from geothermal energy than any other country in the world, in 2008 it only accounted for 0.4% of our total energy supply. Most U.S. geothermal reservoirs are located in the western United States, Hawaii, and Alaska, where the boundaries between massive plates in the Earth’s crust yield a concentration of geological activity.
Compared to other renewable energy sources, such as wind and biomass, electricity generation from geothermal resources in the United States is expected to grow very modestly over the next couple of decades. Conventional geothermal fields, which produce hot steam that can be used directly in turbines, have limited potential for expansion. However, we may be able to create geothermal reservoirs through a process called enhanced geothermal development, in which holes are drilled deep into the ground, fracturing hot, dry rock in regions heated by the Earth’s molten interior. Water is injected and heated by the rock, creating hot water or steam that can be used in electric power plants. A wealth of energy could potentially be captured in this way but the process remains an expensive option and still poses major engineering challenges. Thus it remains to be seen whether enhanced geothermal can eventually make a significant contribution to our energy supply.
Nevertheless, geothermal energy represents an attractive domestic resource. Compared to other renewable resources, such as wind and solar, it’s not intermittent. Additionally, the environmental impacts of geothermal energy are relatively benign compared to other energy sources. Geothermal power plants do not burn fossil fuels, and therefore have very low emissions. They emit only about one sixth of the carbon dioxide that a relatively clean natural-gas-fueled power plant releases, and only 1% of the carbon dioxide that a coal-fired power plant emits. Although water from geothermal reservoirs may need to be replenished from time to time, and as heat is “mined” it may be necessary to suspend operations temporarily in order for a field to return to equilibrium, geothermal power plants could potentially provide a significant low-emissions resource over the long term.