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.
But geothermal isn’t just for utilities. Homeowners looking to go green and lower their utility bills can install a residential system, essentially a scaled-down version of the binary-cycle system. A series of pipes is installed underground. Water circulating in the loop is heated naturally underground and then transferred to a heat exchanger which concentrate the energy and releases it inside the home as heat. In summer and in warmer climates, the process is reversed to fill the home with geothermal-cooled air.