Spend time in the French village of Soultz-sous-Forêts and you’re likely to experience a manmade earthquake. The vibrations – some as high as 2.87 on the Richter scale – are the most conspicuous element of a renewable energy research program that may succeed where others have failed.
By fracturing granite bedrock located five kilometers below the surface and pumping in super-saline water, a team of French, German, and Swiss engineers are extracting the rock’s thermal energy, and they plan to use it to produce pollution-free electricity. At least they will if the local residents put up with a little more shaking.
The project is the most advanced effort to date to deliver on the promise of so-called hot-rock mining. Since the 1970s, geothermal engineers have tried many times to push enough fluid through hot rocks to capture energy at a commercial scale. Now the Soultz project has achieved the highest flow rates in the world through some of the hottest rocks. By this time next year, they expect to be transforming this heat into at least 1.5 megawatts of renewable power for the grid.
The concept of hot-rock mining is deceptively simple. Two or more wells are drilled into hot bedrock, and the intervening bedrock is fractured with hydraulic blasts. Brine is then pumped into one or more injection wells, and it flows through the rock to one or more production wells, heating up as it travels. When the salty water reaches the surface of a production well, its heat is bled off to produce power or to be used for area heating, then returned to the injection wells.
Despite its simplicity, this concept has failed several times. In the 1970s, a pioneering project initiated by Los Alamos National Laboratory demonstrated that one could fracture rock and circulate brine to extract heat. But that project could never get enough brine in – and therefore enough heat out – to make the process competitive with conventional power plants burning fossil fuels such as coal or natural gas.
Gunnar Grecksch, a geophysicist and hot-rock fracturing expert at the Leibniz Institute for Applied Geosciences in Hanover, Germany, says follow-on efforts in the U.K. and Japan failed for the same reason: the fracturing of the rocks was never sufficient. “Flow resistance is still the key problem,” he says. “In none of these projects were the flow rates in the range you need for a commercial system.”
The Soultz project was initiated in 1987 and funded by the European Commission. Since 2001, it has been managed by a consortium of European energy companies, including Shell and Electricité de France. French, Germany, and Swiss research agencies support the science.