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Utility-Scale and Direct Use Geothermal Energy Generation

Geothermal energy technology can be applied at varying scales, including utility-scale generation and direct use.

Geothermal energy technologies can be broadly defined as those activities, applications, or devices designed to harness energy from geothermal resources in order to perform useful work. Geothermal energy technologies can be applied at various scales ranging from utility-scale generation to direct use applications. While a growing industry, geothermal energy currently contributes only a small fraction of the U.S. energy mix.

What Is Utility-Scale Geothermal Energy Generation?

Utility-scale geothermal energy generation requires hot geothermal reservoirs with a temperature above 200°F (93°C); however, new technologies are proving that lower-temperature water (e.g., below 165°F [74°C]) can also be used for commercial purposes. Utility-scale power plants range from small (300 kW) to large (50 MW and greater). Generation capacity is guided by the number of turbines within a power plant. Electricity generated by geothermal resources is transmitted from the geothermal power plant to many users through the transmission grid. Continued growth of utility-scale geothermal energy generation is expected as renewable energy becomes a more important component of the U.S. energy mix.

What Is a Capacity Factor?

The capacity factor of a geothermal energy power plant is the ratio of the actual output of the plant over a period of time and its output if it had operated at full capacity all the time. Geothermal energy has a higher capacity factor (89% to 97%) than many other renewable energy sources. Unlike wind and solar resources, which are more dependent upon weather fluctuations and climate changes, geothermal resources are available 24 hours a day, seven days a week. However, the capacity factor can be influenced by changes in the resources (e.g., a clogged well), equipment, transmission variability, power market fluctuations, and other factors.

What Is Direct Use?

Direct use is the utilization of low- to moderate-temperature geothermal resources (68°F to 302°F [20°C to 150°C]) for commercial, residential, agricultural, or public facilities, or energy needs other than the commercial production of electricity. Direct use includes using heat energy from naturally occurring hot water or using other technology (e.g., heat pumps) to capture the heat from the Earth.

Potential Configurations for GHPs
Potential Configurations for GHPs
Source: Oak Ridge National Laboratory
Potential Configurations for GHPs
Source: Oak Ridge National Laboratory
Click to enlarge

Although many parts of the country experience seasonal temperature extremes, temperatures a few feet below the Earth's surface remain fairly constant (about 45°F [7°C] to 75°F [21°C], depending on latitude). As a result, the below-ground temperature is warmer than the air above it during the winter and cooler than the air in the summer. Geothermal heat pumps (GHPs) take advantage of this by exchanging heat with the Earth through a ground heat exchanger. The GHP uses the constant temperature of the Earth as the exchange medium instead of the outside air. In the winter, the GHP draws heat from the Earth and, in the summer, the GHP can be used to discharge heat to the Earth. GHP systems have very high efficiencies (300% to 600%) compared to air-source heat pumps (which have efficiencies on the order of 175% to 250%) and use 25 to 50% less electricity than conventional heating or cooling systems. GHPs have been in use since the late 1940s.

Other modern direct use systems access geothermal reservoirs by drilling into them from the surface to develop a steady stream of hot water. Water is brought up through a well and a mechanical system, consisting of piping, a heat exchanger, and controls delivers the heat directly for its intended purpose. A disposal system then injects the cooled water underground or disposes of it on the surface. Direct use applications include:

  • Heating pools, spas, greenhouses, aquaculture facilities, and buildings;
  • Melting snow on sidewalks and driveways; and
  • Drying agricultural products.

Direct use applications in the United States have been growing at a rate of about six percent per year and are abundant throughout the western states.