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Energy

Tapping Rocks for Power

A European consortium is drawing closer to building a megawatt-scale power plant that uses bedrock heat.

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.

The key to its success to date has been painstaking geological analysis, which ensures they position their wells to hit the right rocks. In 1997, after ten years of work, the project demonstrated impressive flow rates, moving brine heated to 140 degrees Centigrade at a rate of 25 liters per second and a depth of 3.6 kilometers. And the resistance was less than half that encountered at Los Alamos.

That positive result emboldened the project’s leaders to push their wells deeper, into 200-degree Centigrade granite five kilometers deep – and last fall they finally turned on the taps. Daniel Fritsch, project coordinator, says the system “could probably do 40 to 50 liters per second” with the addition of pumps that will be installed in the wells this summer – another kind of technological challenge given the punishing temperatures involved, which few pumps are capable of withstanding. Then the plan is to build a pilot electrical plant by early 2007 to generate 1.5 megawatts, about the same output as one of today’s towering wind turbines. But the hot-rock plant won’t go idle every time the wind dies down, and should produce about three times more energy per year.

Fritsch says that to cover the cost of its equipment and to generate a profit, however, the project should produce closer to five megawatts. To produce more power, however, they must more than double the flow rate, to around 100 liters/second, which could be a challenge due to the large amount of shaking their blasts cause on the surface. Lawsuits from some disgruntled citizens claiming property damage have limited Fritsch’s willingness to use stronger hydraulic blasts. To many local people, though, it seems like much ado about nothing. Local journalist Bernard Stéphan, who lives two kilometers from the project’s ground zero, says his home has not been affected by the blasts. And Soultz-sous-Fôrets mayor Alfred Schmitt says “There is no problem.”

Nevertheless, instead of using stronger hydraulic blasts to open the rocks further, Fritsch plans to complement the blasting with a new method: pouring acid in the wells. The idea is to dissolve salt deposits in the fractures immediately surrounding the wells. Fritsch says that tests in Italy with acid have improved the functioning of some geothermal wells by a factor of 10.

Peter Fairley is a Technology Review contributing writer based in Paris.

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