University of Minnesota researchers have developed an innovative approach to tapping heat beneath the Earth’s surface. The method is expected to not only produce renewable electricity far more efficiently than conventional geothermal systems, but also help reduce atmospheric carbon dioxide.
The approach, called CO2-plume geothermal system, or CPG, was developed by Earth sciences faculty member Martin Saar and graduate student Jimmy Randolph in the university’s College of Science and Engineering. The researchers have applied for a patent and plan to form a start-up company to commercialize the new technology.
Established geothermal methods involve extracting hot water from rock formations several hundred feet from the Earth’s surface at the few natural hot spots around the world, then using the hot water to turn power-producing turbines. The CPG system uses high-pressure CO2 instead of water as the underground heat-carrying fluid.
CPG provides a number of advantages over other geothermal systems, Randolph said. First, CO2 travels more easily than water through porous rock, so it can extract heat more readily. As a result, CPG can be used in regions where conventional geothermal electricity production would not make sense from a technical or economic standpoint.
“This is probably viable in areas you couldn’t even think about doing regular geothermal for electricity production,” Randolph said. "In areas where you could, it’s perhaps twice as efficient."
CPG also offers the benefit of preventing CO2 from reaching the atmosphere by sequestering it deep underground, where it cannot contribute to climate change. Because pure CO2 is less likely than water to dissolve the material around it, CPG reduces the risk of a geothermal system not being able to operate for long times due to “short-circuiting” or plugging the flow of fluid through the hot rocks.
Saar and Randolph first thought of the idea behind CPG in the fall of 2008 while driving to northern Minnesota together to conduct unrelated field research. The two had been conducting research on geothermal energy capture and separately on geologic CO2 sequestration.
“We connected the dots and said, ‘Wait a minute – what are the consequences if you use geothermally heated CO2?’” recalled Saar. “We had a hunch in the car that there should be lots of advantages to doing that.”
The researchers have recently applied for DOE funding to move CPG forward to the pilot phase.