Geothermal Energy

Geothermal energy is a form of renewable energy that uses heat from the earth to generate electricity. Geothermal energy is renewable because the earth’s core generates a nearly unlimited amount of heat.

Geothermal energy has been used by utilities in the Unites States since 1960, when Pacific Gas and Electric built the first large-scale geothermal electric generating plant, an 11-MW dry steam geothermal facility near San Francisco, California.

U.S. Geothermal Resources
U.S. geothermal resource map
Source: U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy

Types of geothermal power plants

Geothermal power plants are generally subdivided into three types: dry steam plants, flash plants and binary cycle plants. The plant type selection is based on the temperature available in the well fields.

Dry steam plants route steam directly from their geothermal source to the turbine to generate electricity. This is the oldest form of geothermal power with its first use dating back to Lardello, Italy in 1904.

Flash plants use water in a liquid form but at higher temperatures, typically 347° to 572°F. Because these plants are dealing with a liquid, they do not have geothermal steam available and must create it. These plants create steam by routing the water to a facility where the pressure is lowered so that the fluid “flashes” into steam and is then used to spin the turbine.

Binary cycle plants generate electricity from the lowest range of water temperatures, 194° to 347°F. Binary cycle plants use two liquids, one from the geothermal source to generate heat and one to spin the turbine. These plants generate electricity by routing the geothermal water through a closed loop heat exchanger to heat a working fluid into steam, which is then used to spin the turbine.

Low temperature geothermal resources represent the most abundant geothermal resource and, therefore, binary cycle plants are the type of plant with the greatest opportunity for additional development. In addition, future plants may incorporate some means of harnessing the heat of the earth’s hot dry rock and magma as a heat source to generate steam.

Potential issues

The primary challenge with geothermal power is identifying and developing a suitable location that will produce at a cost competitive with other generation sources in the local market. In addition to the need to find sites with the necessary amount of geothermal heat, sites that do not have a geothermal water source must supply the water themselves. Geothermal plants can conserve water by using a closed loop process or, in cases where geothermal water is present, by re-injecting that water back into the reservoir at a depth below groundwater levels.

Usage in the U.S.

The U.S. produces more electricity via geothermal energy than any other nation. As of 2014, seven states within the U.S. had active geothermal plants. The electricity produced at these sites accounts for 0.4 percent of total U.S. electricity generation. Geothermal electricity producing states:

  • California
  • Nevada
  • Utah
  • Hawaii
  • Oregon
  • Idaho
  • New Mexico