Using CO₂ to Extract Geothermal Energy

Carbon dioxide captured from power plants could make geothermal energy more practical.

Carbon dioxide generated by power plants may find a second life as a working fluid to help recover geothermal heat from kilometers underground. Such a system would not only capture the carbon dioxide and keep it out of the atmosphere, it would also be a cost-effective way to use the greenhouse gas to generate new power.

Hot air: The Soultz-sous-Fôrets geothermal plant in Alsace, France, pumps water into fractured rock to extract heat and thus generate electricity. Researchers backed by $16 million in federal stimulus funds seek to prove that such geothermal plants could generate 50 percent more heat by cycling carbon dioxide underground instead. Credit: Géothermie Soultz

Backers of this as-yet-unproven concept secured a big endorsement and much-needed cash with the U.S. Department of Energy's recent award of $338 million in federal stimulus funds for geothermal energy research. Some $16 million of the funds will be shared by nine carbon dioxide-related projects led by Lawrence Berkeley National Laboratory and other national labs, Sunnyvale, CA-based combinatorial chemistry firm Symyx Technologies, and several U.S. universities.

The idea: Carbon dioxide that's cycled through hot regions kilometers underground can efficiently bring heat to the surface, where it can be used to generate electricity. The likelihood is that the process would leave lots of carbon dioxide underground, and thus out of the atmosphere, according to Symyx project leader and materials scientist Miroslav Petro. "You're sequestering CO₂ and at the same time generating power from it."

The concept was first proposed as a way to improve systems that pump water deep underground to fracture hot rocks, then bring the heated water up via a second well to generate power, and then cycle the water back down. The technology has been thwarted to date because it's so difficult to fracture rock to get at the geothermal heat and sustain its flow. The European Union's Soultz-sous-Fôrets project in Alsace, France, the most advanced such project worldwide, has taken 20 years to reach just 1.5 megawatts of power generation (enough to supply roughly 1,500 homes). And the process has antagonized nearby communities because of the small earthquakes sparked by the aggressive fracturing required.

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In 2000, Los Alamos National Laboratory physicist Donald Brown proposed replacing water with supercritical carbon dioxide, a pressurized form that is part gas, part liquid. Supercritical CO₂ is less viscous than water and thus should flow more freely through rock. Brown noted that a siphoning effect should help cycle the carbon dioxide, thanks to the density difference between the supercritical CO₂ pumped down and the hotter gas coming up, slashing power losses from pumping fluid. Plus, Brown argued, instead of using precious fresh water resources, a carbon dioxide-based project could sequester the equivalent of 70 years worth of CO₂ emissions from a 500 megawatt coal power plant.

Six years later, Lawrence Berkeley hydrogeologist Karsten Pruess performed the first detailed modeling of the technology. Pruess projected that a project such as Soultz-sous-Fôrets could produce approximately 50 percent more heat with carbon dioxide than with water. Most of the DOE-funded projects seek to test Pruess's optimism.