Renewable Energy - Geothermal

Iceland's Nesjavellir geothermal power station. Geothermal plants account for more than 25 percent of the electricity produced in Iceland. Photo: Gretar Ívarsson

Iceland's Nesjavellir geothermal power station. Geothermal plants account for more than 25 percent of the electricity produced in Iceland. Photo: Gretar Ívarsson

There are three main ways to use geothermal energy:

  • Geothermal heat pumps - About 10 feet below the ground, the temperature stays between 50° and 60° F throughout the year. Geothermal heat pumps take advantage of this constant temperature to heat or cool water. By moving water through the ground it can be heated in the winter or cooled in the summer. This water can then be used by a heat exchanger to heat or cool the air in a home. This can be a very efficient and inexpensive way to heat or cool buildings.

  • Direct use - hot water from hot springs. This water can be used with heat exchangers to heat up homes and buildings. It also can be used to heat pools.

  • Generating electricity - Geothermal energy can be used by power plants to create electricity. Power plants take advantage of extremely hot water that is between one and two miles deep in the Earth. Some power plants pipe the steam directly up to the generator. They are called dry steam power plants. Other power plants, called flash steam plants, use high pressure from deep in the Earth to create steam to drive the generator.

How Geothermal Energy Works

By Union of Concerned Scientists

How geothermal energy is captured

Geothermal springs for power plants. Currently, the most common way of capturing the energy from geothermal sources is to tap into naturally occurring "hydrothermal convection" systems, where cooler water seeps into Earth's crust, is heated up, and then rises to the surface. Once this heated water is forced to the surface, it is a relatively simple matter to capture that steam and use it to drive electric generators. Geothermal power plants drill their own holes into the rock to more effectively capture the steam.

There are three basic designs for geothermal power plants, all of which pull hot water and steam from the ground, use it, and then return it as warm water to prolong the life of the heat source. In the simplest design, known as dry steam, the steam goes directly through the turbine, then into a condenser where the steam is condensed into water. In a second approach, very hot water is depressurized or "flashed" into steam which can then be used to drive the turbine.

In the third approach, called a binary cycle system, the hot water is passed through a heat exchanger, where it heats a second liquid—such as isobutane—in a closed loop. Isobutane boils at a lower temperature than water, so it is more easily converted into steam to run the turbine. These three systems are shown in the diagrams below.

The three basic designs for geothermal power plants: dry steam, flash steam, and binary cycle. Image:    U.S. Department of Energy

The three basic designs for geothermal power plants: dry steam, flash steam, and binary cycle. Image: U.S. Department of Energy