The German Geothermal Energy Industry
- Heat and electricity generation via the Hot Dry Rock process
Geothermal energy is available around the clock and is not subject to seasonal changes, the weather or climatic conditions. In many countries around the world, geothermal energy is already being used to generate electricity or used directly in heating networks. Geothermal energy forms a solid basis for environmentally friendly and cost-effective energy generation, particularly in regions with geologically favourable conditions (e. g. regions of volcanic activity, temperatures > 200 °C). The geothermal energy available in the Earth's crust originates mainly from radioactive decay - the residual heat from the time of our planet's formation. In addition to this, a portion of the sun's energy is also stored in the uppermost strata of the earth (up to a depth of 2 m).
In countries such as Germany, Italy, Indonesia, Mexico and the USA, the use of geothermal energy has been an integral part of energy strategy for many years now. In addition to making efficient use of available high temperature sources, the German geothermal industry also focuses on developing technologies which can work efficiently at lower temperature ranges of approx. 120 - 200 °C.
Technologies and applications
Depending on the drilling depth, there are two geothermal energy possibilities - deep geothermal energy and shallow or near-surface geothermal energy.
Deep geothermal energy
Deep geothermal energy can be utilised both to generate electricity in power plants and to feed heat into larger heating networks for industrial production or the heating of buildings. Deep geothermal energy is further divided into hydro-geothermal energy, HDR systems and deep geothermal probes. In hydro-geothermal energy, hot water is used directly from subsurface reservoirs located at great depths. Depending on the temperature, hydro-geothermal energy can be used for the generation of heat or electricity.
The so called HDR (hot dry rock) process utilises the geothermal energy found in deep strata (approx. 3,000 to 7,000 m) in which there are very few, if any, natural water resources. In this process, deep boreholes are drilled, through which water is circulated via a carefully created system of cracks and fissures. The hot water is returned to the surface via a second borehole where, in the form of steam, it drives a turbine to generate electricity or, alternatively, is fed into the heating network. German companies offer a wide range of solutions for the effective utilisation of lower temperature levels as well. The Kalina Cycle and the Organic Rankine Cycle (ORC) are ideally suited for electricity production at lower temperatures levels between 120 and 200 °C. The downstream utilisation of such systems can increase returns from high-enthalpy reservoirs considerably.
- Schematic representation of a heat pump system
Shallow geothermal energy
Shallow geothermal energy is ideally suited for both the heating and cooling of buildings. Such energy is obtained from the uppermost strata of the Earth at depths of up to 400 metres. Various systems such as geothermal heat collectors, geothermal heat probes, energy piles and other ground-contact concrete units are used to harness the average temperatures to be found in the earth's crust which range from 7 - 12 °C in the first 100 to 150 metres below the surface. When used for heating, heat pumps are utilised in order to increase these low temperature levels to the building temperature required. This is done by extracting ground heat in a cyclical process. However, by bypassing the heat pump, the constant temperatures present below ground can also be used to directly cool buildings. If the ground is not able to provide adequate cooling, heat pumps can be operated in reverse to supply the cooling capacity which is lacking.
Geothermal heat collectors are normally laid horizontally at a depth of 80 - 160 cm and are subject to the prevailing weather conditions at the surface. In order to gather the stored ground heat, a heat transfer medium flows through the collectors.
Geothermal probes are the most widely used type of plants in central and northern Europe. They are installed at depths of between 50 and 400 metres for utilising shallow geothermal energy. They have a small footprint and use a constant temperature level. Plastic pipes are integrated into the cir- cuits and connected to the building's cooling and heating system. A heat transfer medium then circulates through the pipes, absorbing heat from the surrounding earth and transferring it to the heat pumps.
With energy piles, deep concrete piles, diaphragm walls or other static, subterranean structures are fitted out with plastic pipes through which water flows in order to absorb geothermal heating or cooling energy. The cold water in the concrete piles is warmed by the geothermal heat. Thanks to its interconnection with a heating pump, the warm water heats the building. In the summer, the system described above can be used to gently cool the building.