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Earth's molten mantle is a potentially inexhaustible source of energy that could meet 10 percent of our nation's energy needs, but cost and safety concerns have hampered the growth of geothermal energy. Now, researchers have announced plans to test a more efficient way to tap into safer, low-temperature geothermal stores using nanotechnology.
President Barack Obama has promoted geothermal energy as a component for kicking the nation's fossil fuel habit and reducing greenhouse gas emissions. Last week, the U.S. Bureau of Land Management auctioned geothermal energy rights on nearly 1,000 square kilometers of public land in the Southwest and expects 111 new geothermal plants by 2015.
These traditional geothermal power plants—which currently account for just 0.5 percent of the nation's energy supply—tap into hot springs miles beneath Earth's surface that contain water superheated to between 150 and 370 degrees Celsius. These springs arise when magma from the outer mantle, about 50 kilometers beneath the surface, intrudes into the crust, heating rock and water. At most power plants, as this high-pressure water is extracted from the reservoir, it expands and turns to steam, powering the plant's generator turbines.
But drilling for hydrothermal resources can sometimes trigger earthquakes, particularly when projects fracture hot bedrock around faults and inject their own water. The approach, called stimulation, led the Swiss government to shut down one such project in Basel in 2006 after it set off violent tremors, and a California company, AltaRock Energy, has put its project at The Geysers on hold after an article in The New York Times raised questions about its safety.
Low-temperature hot springs, sometimes below 100 degrees C, are more abundant and closer to Earth's surface, reducing the chance that drills will intersect the deep faults where large earthquakes typically begin. The water must be used in conjunction with a heat exchanger to warm special liquids like alkanes (saturated hydrocarbons such as methane) or perfluorocarbon that have a lower boiling point. As these substances evaporate, they release energy to turn turbines. Unfortunately, they release less energy than water would during this transition, making the process less efficient.
So, environmental engineer Peter McGrail and his colleagues at the U.S. Department of Energy's (DoE) Pacific Northwest National Laboratory are proposing a way to change that.
Eight months ago, his team made a surprising discovery during their work on capturing and storing carbon. They were working with cagelike nanostructures that could trap carbon dioxide when they learned that they could also increase the heat-storing capacity of alkanes by a factor of 20. "That discovery led us to the idea of trying to apply this on the geothermal basis," McGrail says.
In theory, he explains, this fluid could allow a 30 to 40 percent increase in the efficiency of power production from low-temperature geothermal sources. McGrail and his team christened the structures "metal-organic heat carriers," or MOHCs, and with $1.2 million in funding from the DoE, the lab will test electricity generation using different blends on a lab bench over the next three years.
The new proposal surprised many in the community, who are optimistic about its chances. "This is the first I have heard of this," says Peter Rose, who runs the Geothermal Program at the University of Utah's Energy and Geoscience Institute in Salt Lake City. "But it sounds like a potentially exciting approach for generating electricity from low-temperature geothermal resources."
Leonard MacGillivray, a University of Iowa chemist who has worked with similar nanostructures, says the approach is sound and the compounds are both easy to synthesize and unlikely to lead to environmental problems. "It's very intriguing," he says. "It's a nice way to take these metal-organic structures and apply them to a problem."
Of course, the technology is still in its infancy, and its feasibility will depend on such factors as the availability of energy transmission lines and public land in the vicinity of low-temperature geothermal resources.
"The hope here," McGrail says, "is if you are able to make it more economic to use shallower systems...then the chances of intersecting an active fault that can transmit an earthquake diminishes."
