Heat from below the earth's surface, or geothermal energy, can be used to heat and cool individual buildings and neighborhoods or produce utility scale electricity. Unlike fossil fuels, geothermal energy is a renewable resource that does not emit the greenhouse gases that cause climate change. In addition, once a geothermal system is built, the marginal costs are relatively low, because the “fuel” is free.

Geothermal Heat Pumps

To heat and cool buildings, geothermal (or ground-source) heat pumps use the earth as a temperature exchange medium. The temperature underground is fairly constant (about 50 to 60 degrees Fahrenheit) and warmer than the outside air during the colder months in many climates. Water or another fluid runs through pipes buried 10 to 300 feet underground or underwater, to absorb the heat. The liquid then is compressed in a heat pump and its stored heat is released through air ducts in the building. The cooling process is just the reverse. Because the ground temperature is cooler than the air on hot days, the heat from the air is drawn from the building and carried off by the coolant in the pipes and back into the earth.

Since underground temperatures are consistent almost everywhere, geothermal energy can be used for heating and cooling across the world. Iceland has been a major proponent of this clean energy source, with about 90 percent of its housing relying on geothermal heating systems as of 2013.

Geothermal Power Plants

Geothermal power plants run continuously to provide a reliable form of base load power. To provide utility scale electricity, geothermal energy is accessed by drilling deep wells (sometimes over 5,000 feet) to tap steam and hot water from underground reservoirs. The steam drives turbines to generate electricity. Existing oil and natural gas wells (either active or tapped-out wells) provide an opportunity for co-production of geothermal power along with oil or gas without the expense of drilling separate wells. Proper management techniques must be used to prevent contamination of groundwater resources with minerals produced from geothermal wells. There have been some concerns that the drilling of wells may amplify earthquakes in surrounding areas.

In the United States, most geothermal resources suitable for electricity generation are in the western states, Alaska and Hawaii. Texas and other oil-producing states have many co-production opportunities. In 2013, U.S. geothermal power plants led the world with a capacity of about 3,442 MW but produced less than 1 percent of total U.S. electricity. The U.S. Geological Survey has identified geothermal resources that could provide another 9,000 MW of capacity with existing technology and estimates that over 500,000 MW could be generated through more advanced technology. Geothermal energy is currently the third greatest source for renewable electricity in the United States, behind hydropower and biomass.


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