If we could extract all the geothermal energy that exists underneath the United States to a depth of two miles, it would supply America’s power demands (at the current rate of usage) for the next 30,000 years. Getting at all that energy is not feasible—there are technological and economic impediments—but drawing on just 5 percent of the geothermal wealth would generate enough electricity to meet the needs of 260 million Americans. The Department of Energy’s National Renewable Energy Laboratory (NREL) asserts that reaching that 5 percent level, which would produce 260,000 megawatts of electric power and reduce our dependence on coal by one-third, is doable by 2050.
So what is holding us back? Tapping geothermal energy means facing the harsh realities of thermodynamics: Typically, geothermal electricity is generated when hot water or steam underground is piped to the surface to drive a turbine, usually through heating an intermediate working fluid that actually turns the turbine’s blades. The turbine drives a dynamo that then produces the electricity. Crucially, the temperature of the piped-up water dictates the efficiency of a turbine-based system: the hotter the better, with a minimum of about 200 degrees Fahrenheit needed. But there is a limited number of geothermal hot spots that naturally contain water and that heat it to such high temperatures at accessible depths. Probably the best example of one in the United States is The Geysers. In a valley 72 miles north of San Francisco, steam billows from the earth’s surface. (This prompted the first European visitor to the site, in 1847, to believe he had discovered the gates of hell.) An elaborate array of gleaming metal pipes brings steam up from underground to drive turbines that generate 850 megawatts of electricity.
