In regions where the Earth's crust is warm and dry, sourcing enough water to pump through the rocks and recover heat is often difficult. Instead scientists have proposed using waste carbon dioxide, captured from the chimneys of coal-fired power stations.

The carbon dioxide can be compressed into its "supercritical" phase, which is neither liquid nor gas, but has some properties of each. "It can diffuse through rock like a gas yet can carry heat like a liquid," explained Edward Frank, from the Argonne National Laboratory, US.

Some of the supercritical carbon dioxide will seep into the rocks during the process and remain underground – potentially a form of carbon storage. However, producing and transporting supercritical carbon dioxide requires energy, and it is not clear if the energy costs will sufficiently outweigh the benefits of the geothermal energy produced.

To address this question Frank and his colleagues did a life-cycle analysis of a theoretical supercritical carbon-dioxide geothermal power plant. They considered the energy costs involved, including mining coal for the coal-fired power plant, building both the coal and geothermal power stations, and producing the supercritical carbon dioxide.

Assuming no leakage of carbon dioxide to the surface, and capture of 5% of the flow of supercritical carbon dioxide in the rock formation, the team calculated that a combined coal-fired plant and supercritical carbon-dioxide-driven geothermal plant would produce just 15% of the greenhouse-gas emissions produced by a stand-alone coal-fired plant generating the same overall amount of electrical energy.

"We found that the energy recovered by the supercritical carbon-dioxide geothermal system made up for the energy required to capture, compress, and transport the supercritical carbon dioxide," Frank told environmentalresearchweb. By altering parameters in their model the researchers were also able to investigate the effect of differing values of carbon-dioxide capture by the underlying rocks, and of leakage. The findings are published in Environmental Research Letters.

So does this mean that supercritical carbon-dioxide geothermal power plants will become mainstream power producers? Probably not: the number of locations that have both a local coal-fired power station and suitable underlying rocks for a supercritical carbon-dioxide geothermal plant is likely to be limited. However, the study by Frank and his colleagues indicated that energy gains and emissions reductions could be made by introducing this new form of geothermal power to suitable sites. Now the model findings need to be complemented with resource studies, to determine the number of regions suitable for this technology, and the total power that could be produced.