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The power of rocks
Vast slabs of hot rock, buried miles underground, could provide an energy alternative to coal, gas, and oil -- and do less damage to the environment
By Gareth Cook, Globe Staff | January 29, 2007
As the nation searches for environmentally friendly sources of electricity, some researchers say they have an answer: rock power.

The continental United States rests on vast slabs of blistering granite heated by the earth's mantle and other geological processes. Using a technology called heat mining, it should be increasingly possible to tap that energy and use it to generate electricity with virtually no pollution.

Already the country is the world's leading producer of geothermal power, which taps steam from underground to turn turbines. This renewable resource generates more electricity than wind and solar combined. But geothermal power today is largely restricted to isolated spots in the West where rocks are hot at relatively shallow depths.

Heat mining -- which takes advantage of advances in drilling and geological understanding -- could make geothermal power practical across much broader swaths of the country.

"What was thought to be impossible 10 years ago is now possible," said Roy Baria , a British geophysical consultant. "The technology has moved significantly."

Last week the Massachusetts Institute of Technology released a study concluding that heat mining could generate enough energy by 2050 to replace the coal-fired and nuclear power plants that are likely to be retired over the next several decades.

While the nation has focused on higher-profile technologies like wind power, scientists say innovative projects overseas show that the United States has a vast and largely overlooked underground treasure.

"The public needs to understand that these are not just Western resources," said Joseph Moore , who was not involved in the MIT study and is a research professor at the Energy and Geoscience Institute at the University of Utah . "And unlike wind and solar, it is always available on demand."

All geothermal systems are built on a simple fact : The deeper you go, the warmer it gets. More and more homes and businesses are tapping into that warmth, using it to save on their heating bills by installing heat pumps connected to relatively shallow wells.

But it takes much higher temperatures and deeper wells to generate electricity efficiently. Power companies have traditionally placed plants at hydrothermal sites: geologically active places with plenty of water percolating through shattered rock, where simply drilling a well will bring up large amounts of steam.

These hydrothermal sites are relatively rare, though, while hot rock is everywhere.

With heat mining, engineers tap the massive amounts of heat energy stored in this deep rock, away from natural hydrothermal sites. In theory, this is straightforward. Engineers drill a deep well into hot rock. They pump down water under very high pressure to expand cracks in the rock, creating a chamber where water can flow and absorb the rock's heat. Finally, they drill another well to tap the superheated water.

Heat mining is challenging to carry out in practice, however, according to Jefferson W. Tester , who led the MIT study and is a professor of chemical engineering there. Tester was a part of a team at Los Alamos National Laboratory that first attempted heat mining. The ideal rock for heat mining is typically harder and is tougher to drill through than the sedimentary rock where oil and gas are usually found. And the high temperatures strain the drilling equipment.

Deep rock is hot across the planet, but the temperature varies depending on the local geography. In the western United States the earth's crust is generally thinner, and heat migrates up from the mantle. Even under the eastern part of the country, the rock is hot, in part due to the decay of trace radioactive elements, but to reach rock that is hot enough requires drilling deeper.

The MIT study includes a map of the continental United States showing the heat of rock at different depths. At 4 miles deep, large sections of the West reach about 400 degrees, which is ideal for heat mining. In most of the East the temperature is below 300 degrees. But at a little more than 6 miles below the surface, the East has many pockets that exceed 400 degrees. It is possible to drill that deep with today's technology, but it is expensive.

The cutting-edge projects in heat-mining are now abroad. An international consortium based in Soultz, France, near the German border, has shown that the technology is close to commercial viability, Tester and other scientists said.

At Soultz, the team used a three-well technique, according to Baria, who worked there. They pumped water into one well, creating a fractured volume that expanded in two directions. They then drilled wells as deep as about 3 miles to either end to tap the superheated water.

The Soultz project demonstrated that it is possible to generate more power than it takes to pump the water, and it is also possible to heat large volumes of water, said Baria, who helped create the MIT report.

There are large commercial heat mining projects now underway in Europe and Australia with the encouragement of government.

But American companies have been slow to pursue it because the technology is still risky, according to Susan Petty , a member of the MIT study team who is president and CEO of Black Mountain Technology , a geothermal consulting company based in Seattle. For example, an Australian company is setting up a heat mining site in Cooper Basin in Australia, but they recently lost a piece of equipment down their well. The well, which cost millions of dollars to drill, is now useless.

The MIT report, which was sponsored by the Department of Energy , called on the government to build demonstration plants to prove that the technology is workable. The report suggested that work begin in the West, where hot rock is more accessible, and then consider a site in the eastern United States.

The report concluded that the environmental effects are minimal, although heat mining can cause very minor earthquakes, and the superheated water brought up from the depths can contain some pollutants.

But the report found that there are no major technological hurdles preventing heat mining from becoming more widespread.

Technological improvements should bring the costs down over time, making it more economical. Even more important, scientists said, would be improvements in geological sensors and computer models so that engineers could have a better understanding of what was happening in the rock and how to improve the flow of water.

Although heat mining was invented by Americans, much of the work is now being done overseas. If that continues, the scientists predicted, the United States will be left behind in the race to tap the full power of rock.

"The country that leads in the technology," said Baria, "will dominate."

Gareth Cook can be reached at cook@globe.com.