Hot-rock mining.

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 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.