If the temperature increases quickly until 1

Deep geothermal energy has won his bet. A few weeks of its scheduled end, the project of Soultz-sous-Forêts comes this summer to Alsatian deep granites its first kilowatts of electricity. "We have produced 300 kilowatts by spending 200 kilowatts, the balance is slightly positive," slides with a discreet smile Daniel Fritsch, co-managing the EEIG mining heat (). Twenty years and several tens of millions of euros to get this far, the balance may yet seem lean. However, given the present flowering of projects of deep geothermal energy in Germany, Alsace, Auvergne or Switzerland, Soultz made school. But wiping the casts.

As new plants have learned the lessons of the European experience. Their promoters were well assimilated the concept of engineering of the faults of the rock. Before Soultz, deep geothermal energy was injecting cold water in dry and hot rocks, then recover SIP of energy in another production well. The European project has shown that performance is better if it operates fractured rocks in which already runs a natural hydrothermal activity. "More than 70 of hot water up comes from the basement." "The rest comes from water that we inject under pressure," assured Albert Genter, scientific coordinator at the BRGM. Researchers do not really know the origin of these Springs, they could come from the Black Forest, seep into the Earth's crust and back the Alsatian basement.

Soultz has also validated the strategy of mechanical and chemical faults stimulation. By injecting acid water at high pressure, geologists were able to exclude some too narrow cracks or filled by mineral deposits. The environment becomes more porous and facilitates the circulation of the heat transfer fluid. Several years of experience have not been too to learn how to identify major faults, some measuring up to 10 metres, or to exploit the enormous forces involved in the basement. "The Alps grow in the meaning of the plan that form our drilling." "Certain faults want to open, stimulating and a little", explains Daniel Fritsch. Modelling and the measure taken, researchers estimate that seven large fault structures connect the three wells.

Less deep drilling

Soultz, however, does not represent the most effective device, energetically and economically. Today, new deep geothermal projects, purely commercial ambitions, are less electricity than the direct operation of the heat of the basement. Soultz can generate up to 50 megawatts of heat but less than 5 megawatts of electricity. The rocket company hopes to operate from the basement steam for drying starch produced. Other ideas would feed crops e.g. greenhouses. Geothermal energy has also revised downward the depth of drilling. Industrialists are now basements to 3,000 metres and not more the 5,000 metres in Soultz. European researchers have made in recent years that their initial strategy, based on a high water temperature of 200 C, was not good. First because the subsurface temperature gradient proved more irregular than expected. If the temperature increases quickly until 1. 000-1. 500 meters of water, in the sedimentary layers, its growth stabilized once in granite. Geologists estimate that Frac of this hard rock is more consistent temperature. Gain between 3,000 and 5,000 metres is not as important as expected.

In addition, new thermal technologies today allow to operate under high temperatures. "At 3,500 metres, it gets 142 C: it is now sufficient to run a turbine while ten years ago, it was known that do more than 200 C", says Albert Genter. Soultz has opted for a Rankine cycle to isobutane, a fluid that vaporizes more easily than water. It retrieves the energy of the ground water through a surface heat exchanger. The gas then causes a small turbine 2.5 megawatt gas, which fuels an electricity generator. Isobutane is end-of-cycle in a series of wind turbine to cool and liquefy the. Other stations, such as the Unterharching in Bavaria, operate a base of ammonia, a little more efficient but more delicate cycle. The yields of conversion of these cycles exceed hardly 15 but they lead to less deeply, operate a major criterion when 60 of the costs of a project concerning the drilling. Central Landau has directly benefited from these lessons. For six months, it produces 2 megawatts of electricity through a reserve located at 3,000 meters of water. Large 15 million euros of investment were used to develop the production pump and the gas plant. Behind the project is the small company German Bestec, a very active partner of the experience of Soultz.

50,000 earthquakes

Another argument for more shallow drilling, it is easier to stimulate the faults of the granite because pressure is more limited and the risk of creating more low microearthquakes. Soultz operations generated 50,000 earthquakes, including big ten exceeded the threshold of human perception (2 on the Richter scale). One approached the level 3, but the subject is taken very seriously. Especially since production by another experience of several medium earthquakes in Basel which forced him to stop pressure from the public. This problem has led the European Commission to finance a project on the control of the seismicity of deep geothermal energy.

The experience of Soultz will continue again a few weeks. The EEIG would push the installation to its limits to achieve a flow of hot water of 30 litres per second. It will be wait for winter to produce the maximum of 2.2 MW power, cold air promoting the thermodynamic cycle. On success, the plant could be doubled and work a few years to monitor the progress of the installation time.