- The upper layer of Earth is called the crust. It is only a few miles thick, and floats on a giant pool of liquid rock called the mantle. Over years, water can seep down deep below the surface of the earth. When this water comes into contact with magma welling up from the mantle, it heats to extremely high temperatures. Geothermal power plants use this hot water to produce electricity.
- Geothermal power plants use steam to drive a turbine, which turns a generator to produce electricity. The turbine is like a large propeller. When the steam flows through it, it creates lift along the edge of the blades, spinning the turbine. The turbine is attached to an electric generator, which is basically a spinning magnet surrounded by coils of wire. When the magnet spins, its magnetic field moves through the wire. This creates a moving electric current in the wire called alternating current, or AC. That electric current is then sent out to power homes, businesses and other buildings.
- The oldest design in geothermal power plants is the dry steam plant, in which steam from deep underground runs up a tube to spin the turbine. Then the steam is either cooled and pumped back underground or released. Flash steam power plants are a newer and more sophisticated design. Deep underground where the water is heated, the pressure is much higher. The water can remain liquid at temperatures well above the normal boiling point. Dry steam plants pump this pressurized water directly to the surface. Right before it gets to the turbine, however, the pressure is reduced. The superheated water instantly turns to steam, creating a huge burst of pressure to drive the turbine. The water is then cooled and sent back into the ground to start the process over again.
- In binary geothermal plants, the subterranean water doesn't actually drive the turbines. It is pressurized and pumped up through a tube, as in a flash steam plant, to where it meets a secondary tube. The secondary tube also has water in it, but at a lower pressure. Heat from the first tube flows into the second tube, boiling the water to drive the turbine. The water in the first tube is then pumped back down into the earth to gather more heat.
