Harnessing geothermal power involves extracting heat from beneath the surface of the Earth. Normally, this means pumping water down through hot rocks and extracting it again. But the new analysis suggests carbon dioxide could extract heat from rocks more efficiently than water.
Karsten Pruess, a hydro-geologist at Lawrence Berkeley Laboratory in the US, carried out the study and says carbon dioxide could theoretically boost the amount of energy produced by hydrothermal plants by 50% or more. At the same time, Pruess calculates that the technique could be used to dispose of the carbon dioxide produced by conventional power plants, which contribute to global warming.
Pruess used the Soultz hydrothermal plant, in northwest France, as a model to test the idea, which was originally proposed by Donald Brown of Los Alamos National Laboratory, US, in 2000. The Soultz plant pumps water into the ground through a single borehole and uses water heated to around 200°C, recovered via another borehole, to drive turbines and generate electricity.
Pruess calculated that the plant would perform more efficiently if carbon dioxide was used instead of water. “Initially I thought this was the most harebrained idea I had ever heard,” he told New Scientist. “But the more I looked the more I liked it.” Pruess discovered that using carbon dioxide to drive turbines and generate electricity, either directly or indirectly through the use of steam, could produce 50% more energy.
Although carbon dioxide cannot carry as much heat as water, it could boost efficiency because it can move through the plant’s system much more quickly. “Carbon dioxide is to water what water is to honey,” Pruess explains. “It is much less viscous.”
In addition, less energy would be required to drive carbon dioxide through the system in the first place. Hot gas in the exit borehole would be less dense than colder gas in the entry borehole, and this density difference would help drive gas through the rock with little need for pumping. Furthermore, because some gas would leak into the rock, such a plant could be used to store carbon dioxide, Pruess says.
Carbon storage trials in the North Sea and Algeria have involved sequestering around a million tonnes of carbon – the annual output from a 150MegaWatt (MW) coal power station – beneath ground. Pruess says that, over a year, a small geothermal plant could store the same amount of carbon while also generating 50MW.
Pruess bases his calculations on the assumption that the same amount of carbon dioxide would leak into the rock as does water in existing geothermal plants. But he plans to carry out chemical tests that should help reveal whether this is really the case when hot pressurised carbon dioxide is pumped through underground rock.
Robert Pine, an expert on hydrothermal power at the Camborne School of Mines in the UK, says using carbon dioxide system is a novel idea. “But leakage could be a serious issue,” he told New Scientist. “All carbon sequestration projects I am aware of use old gas or oil fields that function as geological traps.”
While carbon dioxide is unlikely to escape from such traps, rock fractures, which are common in regions used for hydrothermal operations, could allow gas to leak out, Pine warns. “Using gas fields might be better, but because they aren’t very hot you would have to go very deep to get to the heat.”