Geothermal power plants produce electricity by using steam from hot underground water to turn turbines and activate a generator. Some types of geothermal power plants use the steam to directly turn the turbine, while others use the steam to heat a fluid used to turn a turbine.
The three types of geothermal power plants are binary cycle power plants, dry steam power plants and flash steam power plants.
Flash steam power plants are the most common type of geothermal power plant in the United States. Flash steam power plants use water wells, tapping into underground bodies of boiling water. As the hot water ascends up the well, it produces steam, which is redirected from the water and used to rotate turbines. Excess water and steam are returned to the reservoir in order to ensure sustainability.
Dry steam power plants extract steam directly from extremely hot underground water sources, and they use the energy to turn turbines and generate electricity. In the United States, this form of geothermal power production is only used in a volcanically active area of California.
Binary cycle power plants use slightly cooler water to heat an alternative fluid that has a lower boiling point. This "working fluid" evaporates, and it is used to turn turbines and generate electricity.