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.
California, Nevada, Idaho, and Oregon all have enough high-temperature hot spots to potentially meet a significant portion of their electrical demand—as much as 60 percent in the case of Nevada—but rarely are the temperatures as high as at The Geysers, which produces steam of 400 degrees and hotter. Most of the time, developers have to look as far as six miles below ground to locate hot, flowing liquids. Finding suitable drill sites can be a big headache.
Doug Glaspey, chief operating officer of U.S. Geothermal, an Idaho-based company that just finished building a 13-megawatt geothermal electrical plant in southern Idaho, says he wishes he had “X-ray vision, so I could see where the reservoirs are. The highest-risk part of this business, bar none, is searching for reservoirs. Drilling a well costs two to three million dollars per well. If it fails, you got nothing.” Moreover, once companies hit a good hot spot, they still have to set up a power plant or a heating system, which requires big up-front costs and multiple wells. Glaspey estimates that it costs “$3.5 million to $4 million per megawatt” to build a geothermal power station.
In addition, geothermal power plants have energy efficiencies of just 8 to 15 percent, less than half that of coal plants. High up-front expenses plus relatively low efficiency makes the cost of geothermal electricity about double that of coal, which sells for around five cents per kilowatt-hour.
Gerald Nix, recently retired geothermal technologies manager at the NREL, believes that improving exploration and drilling technologies could make geothermal power cheaper than coal, however. Current attempts to refine these technologies fall under the banner of engineered geothermal systems (EGS), which can squeeze heat out of spots where the rock is not porous or permeable enough for water to circulate, or where there is not enough water in the first place. EGS uses techniques such as reopening old fissures in the rock, and then pumping water through the fracture. EGS could contribute at least 100,000 megawatts to the U.S. geothermal power budget by 2050, according to a 2006 report, “The Future of Geothermal Energy,” written by a team led by MIT chemical engineering professor Jefferson Tester. What is desperately needed to advance EGS, Tester says, are large-scale demonstration projects. “It’s not as if we don’t know how to drill holes and fracture rocks,” he says, “but we have to demonstrate EGS on a scale that would be useful for commercial enterprise.”
Uses for geothermal energy go beyond generating electricity:Geothermal sources not hot enough to make electricity efficiently can heat buildings by circulating water through pipes. This country has a swath of such lower-temperature hot spots. Draw a line from North Dakota to Texas, and nearly every state west of that line has sources with temperatures of at least 200 degrees Fahrenheit.