Germany has a number of projects for geothermal power production. Such projects are promising wherever hot thermal water can be obtained from underground (see 7.6).23 For example, in Neustadt-Glewe a project that draws water from 2200m underground has been in operation since 2003. The turbine used there generates 1.2 million kilowatt-hours each year.24
In lieu of hot water, hot dry rock can also be used to generate environmentally friendly power. The process involves boreholes some 5-7km deep, but not far apart. The first borehole is used to inject water underground. Fissures in the rock allow the water to flow towards the second borehole, where it is drawn back up to the surface. When it passes through the rock, it heats up so much that it creates steam, which can be used to drive a turbine that generates electricity. But first, the water has to be filtered to remove residual pieces of rock. Once the water has cooled down, it is pumped back underground into the injection borehole.
The HDR method allows geothermal heat to be used to generate electricity even in countries without hot underground aquifers. Since 1987, a research project in Soultz – sous-Forets, France has been conducted on HDR. After many years of preparatory work, a 5000m deep, 1.5MW HDR system – then the deepest in the world – went into operation in the summer of 2008. A second borehole will expand the geothermal heat exchanger, bringing total output up to 3MW.25
Another project, however, shows the difficulty. In Bad Urach, a project with a hot rock of 170°C, 4.5km deep was to provide 3MW of power and 20MW of heat.26 But the difficulties encountered during drilling were so great that the project has been discontinued.
Other geothermal power projects are planned along the upper Rhine, where geological conditions are excellent.27 In most of these cases, however, hot thermal water is available. Because deep boreholes cost millions of euros, and there is no certainty that sufficient hot water will be found, the upfront risk is great in such projects.
Hot dry rock (HDR) is possible if two things hold true. First, sufficiently hot layers of rock should not be too deep if the project is to be economically feasible. In Germany, this is the case especially in the upper Rhine Graben and in Upper Swabia. Second, the rocks must allow fissures to be created and kept open between the two boreholes. Essentially, the fissures constitute a geothermal heat exchanger. To create one, water is injected into the boreholes at high pressure. If the geological conditions are right, naturally present fissures are expanded to form a sort of connecting network. This network is crucial if the fissures are to stay open.
Figure 7.7 Power from hard rock: How the hot-dry-rock method works
Source: Markus O. Haring, Geothermal Explorer Ltd