To generate power from geothermal systems, three elements are needed:
Heat—Abundant heat found in rocks deep underground, varying by depth, geology, and geographic location.
Fluid—Sufficient fluid to carry heat from the rocks to the earth’s surface.
Permeability—Small pathways to move fluid through the hot rocks.
The United States generates the most geothermal electricity in the world—more than 3.7 gigawatts. That’s enough to power about 2.7 million U.S. homes.
The presence of hot rocks, fluid, and permeability underground creates natural geothermal systems. Small underground pathways, such as fractures, conduct fluids through the hot rocks. In geothermal electricity generation, this fluid can be drawn as energy in the form of heat through wells to the earth’s surface. At the surface, that energy is converted to steam, which drives turbines that produce electricity.
Conventional hydrothermal resources naturally contain all three elements. Sometimes, though, these conditions do not exist naturally—for instance, the rocks are hot, but they lack permeability or sufficient fluid flow. Enhanced geothermal systems (EGS) use human-made reservoirs to create the proper conditions by injecting fluid into the hot rocks. This reopens fractures and enhances the size and connectivity of fluid pathways. Once created, an EGS functions just as a natural geothermal system does: The now-available fluids carry energy to the surface through wells, driving turbines and generating electricity. By overcoming natural limitations in the subsurface, EGS can expand geothermal energy nationwide. The 2019 GeoVision analysis concluded that, with advancements in EGS, geothermal could power more than 40 million U.S. homes by 2050 and provide heating and cooling solutions nationwide.
Geothermal Power Plants
Geothermal power plants draw fluids from underground reservoirs to the surface to produce steam. This steam then drives turbines that generate electricity. There are three main types of geothermal power plant technologies: dry steam, flash steam, and binary cycle. The type of conversion is part of the power plant design and generally depends on the state of the subsurface fluid (steam or water) and its temperature.
Dry Steam Power Plant
Dry steam plants use hydrothermal fluids that are already mostly steam, which is a relatively rare natural occurrence. The steam is drawn directly to a turbine, which drives a generator that produces electricity. After the steam condenses, it is frequently reinjected into the reservoir.
Dry steam power plant systems are the oldest type of geothermal power plants, first used in Lardarello, Italy, in 1904. Steam technology is still relevant today and is currently in use in northern California at The Geysers, the world's largest single source of geothermal power.
Flash Steam Power Plant
Flash steam plants are the most common type of geothermal power plants in operation today. Fluids at temperatures greater than 182°C/360°F, pumped from deep underground, travel under high pressures to a low-pressure tank at the earth’s surface. The change in pressure causes some of the fluid to rapidly transform, or “flash,” into vapor. The vapor then drives a turbine, which drives a generator. If any liquid remains in the low-pressure tank, it can be “flashed” again in a second tank to extract even more energy.
Binary-Cycle Power Plant
Binary-cycle geothermal power plants can use lower temperature geothermal resources, making them an important technology for deploying geothermal electricity production in more locations. Binary-cycle geothermal power plants differ from dry steam and flash steam systems in that the geothermal reservoir fluids never come into contact with the power plant’s turbine units. Low-temperature (below 182°C/360°F) geothermal fluids pass through a heat exchanger with a secondary, or "binary," fluid. This binary fluid has a much lower boiling point than water, and the modest heat from the geothermal fluid causes it to flash to vapor, which then drives the turbines, spins the generators, and creates electricity.
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