Geothermal energy is an inexhaustible source of energy by human standards. German engineers have developed the first deep geothermal power station in Europe to use the hot dry rock technique. It started operation in 2008 and is expected to achieve a capacity of three megawatts. The bore holes must be up to 5,000 meters deep.
Hot, dry rock (HDR) layers at depths up to five kilometers can be used for geothermal heat and power generation. In order to bring the heat from the mostly crystalline rock like marble or granite to the surface, a heat-transfer medium must be circulated through it. If the water were to be pressed through the layers without any additional preparation, the heat-exchange area and the permeability of the rock layers would be far too low. For this reason, a deep bore hole is first made through which a large quantity of water under very high pres -sure is forced into the rock. Naturally occurring cracks and gaps are thereby expanded and new cracks are created. The result: increased permeability of the rock and a “natural heat exchanger” is obtained.
A HDR plant is operated by pumping cold water to the depths through an injection drill-hole and returning I tto the surface again through a second (production) bore. The water heated by the hot rocks at these depths can be fed into district heating networks or provide steam for industrial purposes. It is particularly attractive to generate electricity from this geothermally heated water. So-called ORC turbines (ORC = Organic Rankine Cycle), which work essentially like a steam turbine, are used for this purpose.
However, due to the comparably low temperature of the heat transfer medium, between100 °C and a maximum of about 180 °C, it is necessary to use an organic liquid with a low boiling point like propane or isobutane instead of water in the steam tur-bine circuit. The electrical efficiency of this cycle is between 8 and 12 %.
Crystalline rock layers can be found underground almost everywhere in Germany. The HDR technique can therefore exploit 95 % of the geothermal potential, an amount which is sufficient to cover the entire base load of Germany’s electricity needs.
The reason this potential is not yet being exploited is the high cost of the HDR technology. The costs for a bore hole down to a depth of 5,000 meters already amount to several million Euros. Due to the high drilling costs, locations are preferred where crystalline rock and high temperatures are to be found at comparatively shallow depths. This case is true in the Upper Rhine Basin.
The total investment costs for a HDR plant located there are estimated at about $3600-$7200 kW (2,500 to 5,000 Euro/kW). The costs for generating electricity are then about 7 to15 Cents/kWh for 8,000 full-load hours per year. HDR systems and similar so-called “petrothermal systems” take advantage of the heat of the earth. This power station is the first system in Europe to generate electricity using the HDR technique. The goal is to develop standardized and cost-effective HDR systems allow the use of heat stored in rocks are therefore promoted with a higher bonus in the German Renewable Energy Sources Act (EEG).
The deeper one penetrates the interior of the Earth, the warmer it becomes. In Central Europe the temperature increases by an average of 3 °C per 100 m depth. The temperature in the upper most mantle – 40 to 400 kilometers below the surface – is approximately 1,300 °C; in the Earth’s core – about 5,100 to 6,370 km deep – it is probably 5,000 °C.
