Dear EarthTalk: How are heating, cooling and electricity produced by geothermal energy? I don’t understand how it works.
—Delano Stewart, Wyandanch, NY
The term "geothermal" is derived from the Greek words for Earth (geo) and heat (therme). In essence geothermal energy is power harnessed from the Earth itself. Heat from the Earth’s core, which averages about 6,650 degrees Fahrenheit, emanates out toward the planet’s surface. Heated springs and geysers up to three miles underground can be accessed by special wells that bring the hot water (or steam from it) up to the surface where it can be used directly for heat or indirectly to generate electricity by powering rotating turbines. Since the water under the Earth’s surface is constantly replenished, and the Earth’s core will continue to generate heat indefinitely, geothermal power is ultimately clean and renewable.
Today there are three main methods for harvesting utility-grade geothermal energy: dry steam, flash steam and binary-cycle. The dry steam process brings steam up directly from below to drive turbines that power electricity generators. Flash steam plants bring the hot water itself up from below; it is then sprayed into a tank to create steam to drive the turbines. These two methods are the most common, generating hundreds of megawatts of electricity across the American West, Europe and elsewhere. But expansion is limited as these plants only work in tectonic regions where it is easier to access ground heated water.
The binary-cycle technology extracts close-to-the surface warm (not necessarily hot) water and combines it with a second ("binary") fluid, like butane or pentane, which has a low boiling point. This fluid is then pumped through a heat exchanger, where it is vaporized and sent through a turbine before being recycled back into the system. Binary-cycle geothermal plants already pump out dozens of megawatts of electricity in California, Nevada and Hawaii, among other places.