In 2010, after more than 20 years of research, development and testing, an experimental 2.1 MW geothermal power plant in Soultz-sous-Forêts, 50 km north of Strasbourg (France), became the first Enhanced Geothermal Systems (EGS) plant to be connected to the electricity grid. Rather than relying on naturally occurring steam produced by underground volcanic activity, EGS injects cold water from the surface into hot rocks deep underground. The heated water (at about 200°C) is then pumped back to the surface and used to drive turbines. The advantage of EGS, if it can become commercially viable, is that sites with subsurface layers of hot rock, like the granite beneath Soultz-sous-Forêts, are much more common across the globe than the volcanic regions traditionally associated with geothermal power production. However, the energy is much more difficult to harness, not least because of the costly deep drilling technology involved.
The attractiveness of more widely available geothermal energy, however, motivated a decision in 1987 to develop a pilot EGS plant on the site of former oil wells in Soultz-sous-Forêts, in the Upper Rhine Valley, where the geological conditions were right. A public-private consortium was set up, under the umbrella name of Géothermie Soultz, with industrial partners including French and German utilities (EDF, Electricité de Strasbourg, EnBW and Pfalzwerke), the chemical multinational, Evonik and German geothermal specialists, Bestec. Several public research centres are also part of the project, including the French National Centre for Scientific Research (CNRS) and the Geological Survey (BRGM). Public funding has come from the European Commission, under its Framework 6 and 7 programmes, the French environment and energy agency, ADEME, the German project management agency, PTJ and the German Federal Environment Ministry.
The idea of tapping into the Earth’s natural geothermal energy to produce electricity is not new. The oldest geothermal plant, in Larderello (Northern Italy), first produced electricity - enough to power five light bulbs - as long ago as 1904. However, this site still relied on the availability of hot fluids relatively near to the surface. EGS, initially known as HDR (hot dry rock) geothermal energy, however, exploits the natural heat in layers of sedimentary rock over 3 000 metres below the surface. First developed in the Los Alamos National Laboratory (USA) in the 1970s, the principle involves injecting water from the surface, thus dispensing with the need for a local reservoir of groundwater to produce the steam needed to power electricity turbines.
The first challenge to develop EGS is to locate suitable subsurface rock formations. For the European pilot project, these were already known to exist in disused oil wells at Soultz-sous-Forêts. There also appeared to be an underground adjacent source of salty water, which would facilitate the production of steam. After drilling to shallow depths (2 km) in the 1990s and confirming the temperature gradient, three boreholes were finally drilled to about 5 000 metres, tapping into rock at the required 200°C. A central well is used to inject water from the surface - the underground brine source not being sufficient to power the turbines - and two adjacent collector wells, 700 metres apart in the rock, are used to pump the heated water back to the surface.
For EGS to work, though, the permeability of the naturally impervious granite has to be increased, by opening existing fractures and creating new ones. This is carried out either with hydraulic (using pressurised water) or chemical stimulation - both ‘fracking’ techniques borrowed from the oil and gas industries. It has proven essential to understand and monitor the network of fissures created, both to maximise the flow of water from the injection well to the collector wells and to manage the risk of triggering seismic activity.
A prototype commercial EGS being developed in Basel (Switzerland), at the same time as the Soultz project, was abruptly halted in 2006 when hydraulic stimulation triggered a 3.5 magnitude earthquake, causing damage to buildings. This innovative plant was situated in an industrial zone within the city and had been designed to be part of ‘green’ energy complex also incorporating a waste recycling plant. After this event, hydraulic stimulation at Soultz was abandoned in favour of chemical stimulation, using hydrochloric and organic acids.
At Soultz, once the hot water has been pumped to the surface, it is fed into an organic Rankine cycle process in a binary power unit to produce electricity. Isobutane is used as the working fluid rather than water, because of its lower boiling point. The cooled water can then be re-injected into the underground wells in a closed loop, to be reheated and pumped back to the surface. Of the 2.1 MW the plant produces, 1.5 MW of electricity is available to be fed into the grid, while the remainder is used to run the plant itself.
Now that the Soultz plant has been producing electricity continuously for almost five years, new research is being carried out to improve on existing techniques and develop new innovations. One measure of how much new ground has already been covered during the development of the Soultz project is the scientific output it has produced - some 40 PhDs and over 200 scientific publications. Now, attention is turning to the use of supercritical CO2 as a heat transfer fluid, rather than water. CO2 is potentially more efficient, with greater power output, reduced loss from pumping and cooling, and the added possibility of carbon sequestration. Another avenue being explored is to inject air into abandoned oil and gas reservoirs and exploiting the very high temperatures produced by in-situ combustion.
Following the success of the Soultz-sous-Forêts demonstration project, new partners are now being invited to invest. Meanwhile, utility companies and industrial partners are now exploring the feasibility of developing other, similar sites in Europe, including mainland France, Germany, the Czech Republic and the UK, while EGS projects are well advanced in Australia, Japan, and the USA. The French authorities have already awarded permits to carry out exploratory drilling at 20 sites in France, mostly in the former volcanic regions of Auvergne and Ardèche. Meanwhile, planning permission has recently been granted to develop a 3–4 MW EGS power plant at the Eden Project in Cornwall (UK). The energy produced would be sufficient to run the Eden project itself and to provide power and heat for around 4 000 homes nearby. And, the 100 year-old Larderello site in Tuscany has continued to evolve, incorporating new EGS technologies, making Italy one of the world leaders in geothermal heat and power production.
The main issue for EGS in Europe now is to demonstrate its long-term sustainability and to attract investors. The up-front costs of exploration, drilling and stimulation are still dissuasive, given that the technology may still be 10 to 15 years from maturity and that an EGS project can take up to 7 years to develop, from start to finish, with as much as five years for exploration, test drilling and field development before the plant itself is constructed. But, if these obstacles can be overcome, EGS has the potential to open up a largely untapped source of continuous, base-load electricity for millions of users, without the intermittency issues and storage requirements of solar and wind.
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