Geothermal

Heat from the Earth, or geothermal — geo (Earth) + thermal (heat) — energy can be harnessed at different depths for applications ranging from generating electricity to heating and cooling buildings. Geothermal resources accounted for about 5 percent of U.S. renewable energy use last year – an amount comparable to wind.

Geothermal resources range from shallow ground to hot water and rock several miles below the Earth's surface, and even farther down to the extremely hot molten rock called magma. Mile-or-more-deep wells can be drilled into underground reservoirs to tap steam and very hot water that can be brought to the surface for use in a variety of applications. In the United States, most geothermal reservoirs are located in the western states, Alaska, and Hawaii.
 
Facts

• Geothermal heat pumps can be placed virtually anywhere in the U.S. because the top few yards of earth remain constant at 50 to 60°F year round. During the winter the relatively warmer air below ground is pumped in to heat the building; in the summer the warmer air is pumped below ground to be cooled, and that excess heat can even be used to heat the building's water.
  Read more at the Department of Energy, Energy Efficiency and Renewable Energy, Geothermal Technologies Program
• Mile-or-more-deep wells can be drilled into underground reservoirs to tap steam and very hot water that drives turbines that in turn drive electricity generators.
  Read more at the Department of Energy, Energy Efficiency and Renewable Energy, Geothermal Technologies Program
• Geothermal power plants have average electricity generating capabilities of 90% or higher, compared to about 75% for coal plants.
  Read more at the Department of Energy, Energy Efficiency and Renewable Energy, Geothermal Technologies Program
• Geothermal energy is not a widely used source of electricity because upfront costs of finding the right place to drill and building the power plant are very high, despite operation and maintenance costs of the plant being as low as $0.01-$0.03 per kWh.