Heated Exchange

University of Minnesota researchers aim to build up geothermal’s potential by pushing down CO2.

Martin Saar

Photo: Josh Kohanek
Martin Saar’s project is supported in part by the Initiative for Renewable Energy and the Environment, an IonE signature program. More >>

Martin Saar suggests a trade: Let’s bring heat up from Earth’s depths to make electricity and, in return, we can stuff a bunch of climate-changing carbon dioxide back down below.

Combining geothermal energy extraction with carbon storage could be a win-win for clean energy. Geothermal projects would become more efficient when CO2, rather than water, is used to carry heat. At the same time, greenhouse gas emissions could be stored deep underground where they can’t heat up the climate. Merging energy recovery and carbon storage in a single site improves the bottom line for both.

Saar, a geoscientist at the University of Minnesota, and graduate student Jimmy Randolph were driving north from the Twin Cities to Bemidji to conduct hydrogeology field work, when they had the “aha” moment that sent them down this research path. “We discussed the idea for several hours in the car,” Saar recalls.

A standard geothermal electricity project pumps water to raise heat from up to a mile or more underground. But as it turns out, high-temperature, high-pressure carbon dioxide—in a “supercritical” state, where it acts partially like a liquid and partially like a gas—is almost twice as efficient as water for collecting geothermal heat in these systems.

Meanwhile, researchers worldwide are trying to figure out how to bury fossil-fuel-generated CO2 permanently underground as a strategy for combating climate change. This approach raises a number of concerns, especially how to make sure the CO2 doesn’t leak out.

Nevertheless, Saar believes the time required to transition from fossil fuels to renewable energy makes carbon sequestration inevitable in the face of climate change. And sites composed of porous rock beneath an impermeable cap layer, far from drinkable groundwater, provide the right conditions for storing carbon dioxide.

Saar and Randolph’s idea is to choose sequestration sites that also have accessible geothermal energy—places where heat flow from the planet’s interior is significant enough to be captured and converted. While the majority of the CO2 that gets sent underground would be permanently stored in a geologic formation, a fraction could rise back up, carrying sufficient heat to generate electricity in a turbine. After passing through the turbine, the cooled CO2 would be reinjected, thus eliminating its release into the atmosphere.

The gains in efficiency from using carbon dioxide instead of water could make geothermal projects more feasible, says Saar. Shallower depths or lower temperatures might be hot enough to justify energy extraction, especially if CO2 sequestration was going to happen at the site anyway.

One big question remains, says Saar. “‘How hot does it really need to be?’ It looks like it doesn’t have to be as hot as with water because of the efficiency improvements, but it’s still true that the hotter, the better.”

Saar and Randolph are also trying to determine how deep the sites need to be and whether the CO2 would react with rocks to clog things up or dissolve away the underground minerals.

Despite the challenges, Saar says geothermal energy has many advantages compared with other renewable energy sources. No complicated or expensive energy storage system is required, since geothermal power plants can be turned on and off quickly. As a result, geothermal energy can provide peak electricity while also meeting the baseload demand—because, unlike solar or wind, Earth’s heat is on hand 24/7.

JESSICA MARSHALL is a St. Paul, Minn.-based science and environmental journalist, and the environment correspondent for Discovery News.