Carbon dioxide could make an underappreciated form of renewable energy much more efficient.

In a win-win for clean energy, researchers propose that carbon dioxide pumped deep underground for storage could be used to make geothermal energy projects more efficient.

This combined system could double the efficiency of geothermal energy capture, while also making carbon storage projects more feasible.

"I think it's really exciting because one of the big problems with most geothermal energy and carbon capture and sequestration seems to be that the economics of either doesn't quite work unless there's some sort of subsidy on the geothermal side, or some tax on carbon on the carbon capture and sequestration side," said Grant Ferguson of St. Francis Xavier University in Antigonish, Nova Scotia, who is not a part of the research.

Typical geothermal energy systems pump water deep underground in geologic hot spots. The water soaks up heat emanating from Earth's core and brings it back to the surface. The hot water is then used to generate electricity.

However, it turns out that high-pressure, high-temperature carbon dioxide is a more efficient fluid than water for capturing heat from geothermal systems.

Martin Saar and Jimmy Randolph of the University of Minnesota, Twin Cities, propose that sites where carbon dioxide will be sequestered deep underground could do double duty as geothermal energy production projects by bringing a small fraction of the high-pressure CO2 back to the surface and using it like hot water to generate electricity. The CO2 would be pumped back down after the heat was extracted.

The process could go one of two ways, Saar said.

"In places where you have active geothermal energy, you could look for the possibility of formations to sequester CO2 there also," he said. "Or perhaps there's a great area for CO2 sequestration, but maybe the heat flows aren't that great. But now we have this approach using CO2 which basically doubles the efficiency (of geothermal energy projects), so maybe now we can actually produce electricity geothermally there, which was not possible before."

Because CO2 is so much more efficient at extracting heat, the criteria for what would make a site worthwhile for extracting geothermal energy might be broader. Shallower depths or weaker sources of heat that are insufficient for water-based projects might be good enough for CO2-based extraction, especially if sequestration was going to happen at a given site, anyway.

Others have previously made use of carbon dioxide's improved efficiency at capturing heat, but Saar and Randolph are the first to propose combining carbon-dioxide-driven geothermal energy with large-scale carbon sequestration sites.

The pair presented their work at a meeting of the American Geophysical Union in San Francisco.

Saar and Randolph still have many details to work out, including how deep the sites would need to be, how much subsurface heat would be needed to make it feasible, and how the carbon dioxide would react with the minerals in the ground. These factors would vary with location.

Researchers and environmental groups have concerns over large-scale carbon capture and sequestration, including whether the sites may leak, and whether investing in storing carbon from coal-based power plants diverts efforts away from developing renewable energy sources.

Saar acknowledges that the concerns surrounding carbon capture must be worked out, irrespective of his proposal. But extensive CO2 storage is inevitable, he said, so this approach might be a way to get a bonus out of carbon storage sites.

Geothermal energy is an underappreciated form of renewable energy, he added. It is easy to turn on and off, but unlike wind and solar, geothermal energy is available any time.