Geothermal Energy
Geothermal energy is available around the clock and is not subject to seasonal changes, the weather or climate conditions. In many countries around the world, geothermal energy is already used to generate electricity or used directly in heating networks. Particularly in regions with geologically favourable conditions (e.g. regions in the so-called “Pacific Ring of Fire” and those with volcanic activity and temperatures > 200 °C), geothermal energy forms a solid basis for environmentally-friendly, cost-effective and sustainable energy generation. The geothermal energy available in the Earth’s crust originates mainly from radioactive decay processes in the Earth’s core or from residual heat from the time of our planet’s formation. Some energy from solar radiation is also stored in the Earth’s uppermost strata. In countries such as Germany, Italy, Indonesia, the Philippines, Mexico, the USA and Iceland, the use of geothermal energy has been an integral part of energy strategy for many years. Recently, interest in the use of geothermal energy for power generation in Africa, e.g. in Kenya, has increased sharply.
The German geothermal industry covers the entire range of geothermal technologies, from shallow geothermal energy up to hydrothermal and petrothermal deep geothermal energy for heating, cooling and power generation.
Technologies and applications
Depending on drilling depth, there are two main geothermal energy possibilities; deep geothermal energy and shallow or near-surface geothermal energy.
Deep geothermal energy
There is no precise definition, but a minimum depth of 400 m (a boundary set in the German Association of Engineers (VDI) Guideline 4640) is usually accepted as defining deep geothermal energy in Germany. Deep geothermal energy can be used both to generate electricity in power plants and to feed heat into larger heating networks for industrial production or for heating buildings. One advantage of this kind of energy supply over other renewable energies is that deep geothermal energy is not subject to seasonal or daily fluctuations, making it always available and without the need to be secured. In Germany, geothermal energy is suitable for covering the base load. Deep geothermal energy is further divided into hydro geothermal energy, petrothermal geothermal energy (Hot Dry Rock systems) and deep geothermal probes.
Hydro geothermal energy uses hot water drawn directly from underground reservoirs located at great depths. The water-bearing rock layer, or productive layer, should have a very large vertical and lateral distribution to ensure long-term productivity. Depending on the thermal water’s flow rate and temperature, hydro geothermal energy can be used to generate heat and/or electricity.
GFZ German Research Centre for Geosciences
The principle behind petrothermal geothermal energy using the Organic Rankine Cycle (ORC)
The use of deep heat reservoirs with few or no water resources is referred to as petrothermal geothermal energy. Crystalline and dense sedimentary rock at depths of three to six kilometres with high temperatures (over 150 °C) can serve as reservoirs. These are accessed via two or more boreholes drilled deep into solid rock. Hydraulic and chemical stimulation processes (Enhanced Geothermal Systems, EGS) are used to make cracks and fissures in the rock. Cold water is then pumped at high pressure down an injection well into the rock, where it is heated and returns to the surface via a second borehole. This hot water in turn heats a working fluid with a low boiling point (so-called Kalina Cycle and Organic Rankine Cycle, ORC), producing steam for a turbine. Heat can also be fed into district heating networks via a heat exchanger. In Germany, some 90 % of geothermal energy is generated using petrothermal systems, which represent the largest part of the potential of geothermal power generation.
A deep geothermal probe is a closed system for producing geothermal energy consisting of a single borehole ranging in depths from over 400 m to several thousand metres. A deep geothermal energy probe works in a manner similar to that of probes in a shallow geothermal energy system (see below). Since deep geothermal systems achieve higher temperatures than shallow geothermal systems, the use of a heat pump is rarely required, yet simultaneous use for cold storage is not possible. The extracted energy is used directly for heating, enabling the full potential of thermal energy utilisation, ranging from process heat for industry and commercial uses at higher temperatures to agricultural use at lower temperatures. Power generation is not economical even at high temperatures due to the smaller heat exchange surface of the probe.
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Natural near-surface temperature distribution at various depths.
Shallow geothermal energy
Shallow geothermal energy is obtained at depths of up to 400 m. Because the Earth maintains a far more even temperature than air or water, it is an optimal energy source for cooling and heating buildings. At depths of about 15 m, and depending on geological conditions down to a maximum of 40 m, temperatures in the Earth’s uppermost strata are subject to seasonal fluctuations and influenced by solar radiation. Temperatures just above the annual average temperature on the Earth’s surface prevail at these levels. From this depth on, the temperature increases at a geothermal gradient of approx. 3 °C per 100 m of depth, reaching a temperature of 20 – 25 °C at a depth of 400 m. The heat derived from the ground also depends on the qualities of the ground and rock.
Various systems, such as geothermal heat collectors, geothermal heat probes, energy piles and other ground-contact concrete units are used to harness geothermal energy. Usually, heat obtained from shallow depths is augmented by heat pumps to provide buildings with heat or hot water. When used for heating, grounded heat pumps increase the ground temperature to the temperature required in the building, drawing heat from the ground in a circulatory process. However, the constant temperatures prevailing underground can also be used to cool the building directly without the use of a heat pump. If the ground is not providing adequate cooling, heat pumps can be operated in reverse to supply the required cooling capacity.
- Illustration of ambient heat systems using geothermal heat collectors and geothermal probes via a heat pump.
Geothermal heat collectors are placed horizontally at a depth of 80 – 160 cm, with operating temperatures greatly influenced by prevailing surface weather conditions. They require more space than energy probes; for a single family home, a surface area of some 200 – 250 m² is required. A heat transfer medium circulating through the collector pipes transports ground heat to a heat pump.
Geothermal probes are the most widely-used types of plants in Central and Northern Europe. These are installed at depths of 50 to 160 m to make use of shallow geothermal energy. They require little space and utilise a constant temperature. Plastic pipes (HDPE) are joined into circuits and connected to a building’s cooling and heating system. A heat transfer medium then circulates through the pipes, absorbing heat from the surrounding earth and transporting
it to a heat pump. This method allows systems of various sizes – from small residential units to complete residential estates – to be supplied with heating or cooling.
Energy piles require deep concrete piles, diaphragm walls or other underground concrete components fitted out with plastic pipes through which water flows as a transport medium, absorbing the geothermal heating or cooling energy. Geothermal heat warms the cold water in the concrete piles, and the warm water passes through a heat pump to heat the building. This system can be used for gentle cooling in summer.
