Most of the electricity is made by one simple action: spinning. If we can spin a magnet inside a copper coil, we can generate electric current. Most of that spinning force is achieved with steam. Usually, that means that we have to begin

by heating water. But sometimes, the earth provides its own steam. Electricity from geothermal sources accounts for less than 1% of all electric production in the United States. It is highly dependent upon the local geography and there are only a few places where it’s economical to tap that heat. There are three types of geothermal generator: dry steam, flash steam, and binary cycle.

Dry steam power plants draw from underground resources of steam. The steam is piped directly from underground wells to the power plant where it is directed into a turbine/generator unit. The Geysers in northern California is the only active dry steam power plant in the United States.

Flash steam power plants are far more common. They use geothermal reservoirs of water with temperatures greater than 360°F (182°C). This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine/generator. Any leftover water and condensed steam are injected back into the reservoir, making this a more sustainable resource.

Binary cycle power plants operate by using water at temperatures of about 225-360°F to boil working fluids with a lower boiling point. The working fluid is vaporized in a heat exchanger and used to spin a turbine. The source water and the working fluid are kept separated during the whole process and the water is simply injected back into the ground to be reheated. Of course, geothermal energy does not have to be converted into electricity to be useful. Direct use of geothermal energy can be harnessed in pipes to heat buildings, grow plants in greenhouses, dehydrate food, heat water for fish farming, pasteurize milk, or even be piped under roads and sidewalks to melt snow.

Like other renewables, geothermal does not consume fuel to generate power so it is, by definition, renewable. It also means relatively low operation costs, and low emission of pollutants. However, geothermal is different from other renewable technologies because it is not dependent on seasonal factors such as precipitation, wind resources, or exposure to the sun (solar insolation) and it provides a constant source of energy.