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The Great Forgotten Clean-Energy Source: Geothermal


By Prachi Patel-Predd
Discover Magazine
April 3, 2008


The U.S. Uses Less Than 1 Percent of Our Available Geothermal Energy.

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.

 

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