A geothermic power station
A geothermic power station.

Source: Stock photography (copyrighted)

Geothermal power plants use hydrothermal resources that have both water (hydro) and heat (thermal). Geothermal power plants require high-temperature (300°F to 700°F) hydrothermal resources that come from either dry steam wells or from hot water wells. People use these resources by drilling wells into the earth and then piping steam or hot water to the surface. The hot water or steam is used to operate a turbine that generates electricity. Some geothermal wells may be as deep as two miles.

Types of geothermal power plants

There are three basic types of geothermal power plants:

  • Dry steam plants use steam piped directly from a geothermal reservoir to turn generator turbines. The first geothermal power plant was built in 1904 in Tuscany, Italy, where natural steam erupted from the earth.
  • Flash steam plants take high-pressure hot water from deep inside the earth and convert it to steam to drive generator turbines. When the steam cools, it condenses to water and is then injected back into the ground to be used again. Most geothermal power plants are flash steam plants.
  • Binary cycle power plants transfer the heat from geothermal hot water to another liquid. The heat causes the second liquid to turn to steam, which is then used to drive a generator turbine.
The differences between dry steam, flash steam, and binary cycle power plants are shown in the diagrams below.
Illustration of a Dry Steam Power Plant - Geothermal steam comes up from the reservoir through a production well.  The steam spins a turbine, which in turn spins a generator that creates electricity.  Excess steam condenses to water, which is put back into the reservoir via an injection well. Illustration of a Flash Steam Power Plant - Pressurized geothermal hot water comes up from the reservoir through a production well.  The water enters a flash tank where it depressurizes and flashes to steam.  The steam then spins the turbine, which in turn spins a geneator that creates electricity.  Excess steam condenses to water, which is put back into the reservoir via an injection well. Illustration of a Binary Cycle Power Plant.  Geothermal hot water comes up from the reservoir through a production well.  The hot water passes by a heat exchanger that is connected to a tank containing a secondary hydrocarbon fluid.  The hot water heats the fluid, which turns to vapor.  The vapor spins a turbine, which in turn spins a generator that creates electricity.  The hot water continues back into the reservoir via an injection well.  This closed-loop system produces no emissions.

Source: U.S. Department of Energy, Energy Efficiency & Renewable Energy (public domain)