Geothermal energy

Geothermal energy is the heat available below the Earth’s surface in layers of earth and rock. Geothermal energy is unimaginably abundant; for instance, as the engine for plate tectonics, it is responsible for the migration of the continents and the formation of huge mountains and volcanoes. About 99 percent of the material that makes up the Earth’s mass is hotter than 1,000 degrees Celsius. It is assumed that temperatures at the Earth’s core are over 5,000 degrees Celsius. Only in the uppermost three kilometers or so are temperatures below 100 degrees Celsius. The temperature in the Earth’s crust increases by an average of about 3 degrees Celsius per 100 meters of depth.

There are different types of geothermal energy and technologies for harnessing it. The process MTU is looking to use in Munich(Allach) is hydrothermal geothermal energy. From a depth of around 2,200 meters, a production well will pump hot water at a temperature of around 70–75 degrees Celsius from a deep groundwater reservoir up to the surface. In a closed thermal water circuit, the heat can be transferred via heat exchangers to MTU’s heating water circuit, causing the water temperature to drop to about 40 degrees. The cooled water will be returned via an injection well to the same deep groundwater reservoir from which it was drawn. There, it will heat up again, and the cycle will begin anew.

The use of geothermal energy is part of MTU’s climate strategy. The goal of our climate strategy is to reduce emissions by 60 percent by 2030 (compared to 2019 levels). The use of geothermal energy for heating is a crucial step in this process, because it is a carbon-free energy source. Using the thermal water directly means the efficiency is very high. Only the thermal water pump and the circulation pumps require electrical energy. In addition, compared with conventional energy supplies, this approach eliminates transport routes and storage because the energy source is located directly under the MTU facility.

The drilling site is located in the northeastern area of the MTU premises, close to Dachauer Strasse

There are two bore paths. Both wells start from a common drilling site (known as a drilling pad). There, they are only a few meters apart. With increasing depth, the horizontal distance between the two wells stretchs to more than 2,000 meters. The production well runs in a southerly direction, the injection well in a northerly direction. Each is about three kilometers long. In total, about six kilometers of drilling pipelines will be used.

Once the thermal water has released its heat into the MTU location’s heating circuit via a heat exchanger, all the location’s buildings can essentially be supplied with heat.

The Molasse Basin is the geological name for the region between the Danube and the Alps. As a result of the uplift and overthrust of Alpine nappes onto the European crust, the weight of the mountains developing on top of the underlying crust pushed it several kilometers downward into the Earth’s mantle. At the same time as the Earth’s crust was sinking, the basin this formed steadily filled up again with debris eroded from the Alps. This geological basin is called the Molasse Basin. At the base of this basin is a water-bearing limestone layer (the Malm karst), which was formerly at the Earth’s surface, but is now encountered at varying depths, depending on the location. By the Danube south of Regensburg, it lies only a few meters deep; on the edge of the Alps, the Molasse Basin is already over 5,000 meters deep. In the Munich area, it is located at a depth of 2,000 to 3,000 meters.

At the base of the Molasse Basin, the Malm karst forms a deep karst aquifer approximately 500 to 600 meters thick. As a geothermal energy source directly beneath MTU’s Munich location, the company can use this aquifer to supply heat. The characteristics of the Malm karst mean water can circulate well in its pores, fissures and karst cavities and absorb heat from the environment, making it an ideal hydrothermal reservoir. The Malm limestone dates from the Upper Jurassic period and is about 150 million years old. At that time, southern Bavaria was a subtropical shallow sea with reefs and lagoons. The thermal water from the Malm karst is used directly, for example, in spas located in Erding, near Munich, and in Lower Bavaria, Upper Austria and Upper Swabia.

In principle, the environmental risks associated with deep geothermal energy are very low. The water is returned to the reservoir without further modification once its heat has been transferred to the heat exchanger. In Germany, the requirements and official specifications for the protection of the environment during the construction and operation of a geothermal plant are very high and are monitored intensively. The geological conditions of the deep subsurface in the Munich area are already thoroughly understood from numerous geothermal boreholes or from former deep wells for oil and gas. This means that in the Molasse Basin, drilling challenges arising from geologic uncertainties can be very well defined. The wells will be drilled in such a way that by means of several telescopically interlocked and concreted-in steel pipes, the boreholes are secured in a stable manner for the long term. This technique prevents the inflow of water or gas from the various strata through which the wells pass. During the drilling process, only substances will be used for which a risk to drinking water can be ruled out. Rocks from the wells, the water brought to the surface and the gases dissolved in it will be regularly tested for their composition during the drilling phase.

At the drilling site, additional structural measures are in place to prevent contamination of the soil even in the event of an accident (e.g. tank leakage). This includes a concrete surface with an appropriate drainage design, temporary storage tanks, inspection chambers and oil separators. The thermal water from the Malm itself is characterized by its low content of dissolved substances. Traces of oil or gas can sometimes be found in some geological strata or even in the Malm water. For this reason, the deep water will be routed in a closed system from extraction through heat exchange to recirculation. This will prevent the escape of any gases or substances that may be hazardous to the environment.

Geothermal energy is always available. Unlike other sustainable and carbon-free energy sources, its use is not dependent on time of day, weather or climate.

A geothermal plant is not particularly large. MTU already has a combined heat and power plant on-site at its Munich location, so the geothermal system only needs to be integrated into the existing heat distribution system. The main part of the geothermal plant is located out of sight under the ground. Above ground, only the well heads, two compact pipelines and the heat distribution center are visible.

The thermal water borehole pump installed for continuous operation hangs at a depth of several hundred meters and cannot be heard at the surface. The network pumps, heat exchangers and power supply are housed in a small building and are also not a source of noise for the surrounding area.

Only during the construction and drilling phases will there be a perceptible level of noise at times; examples include individual components of the drilling rig such as pumps or vibrating screens, the unloading of pipes, or construction site traffic. Potential sources of noise will be enclosed when necessary. Any noisier work or truck transports will be performed during the day, if possible. The drilling process as such, i.e. the sound of the bit drilling through the rock, is neither audible nor perceptible at the Earth’s surface. In continuous operation, the geothermal plant complies with all the requirements of legally regulated noise protection.

Since no combustion takes place, no exhaust gases are released when geothermal energy is used. At MTU, the thermal water will be used in a closed circuit to supply heat, so the gases or other constituents dissolved in the water will remain in the thermal water circuit. The hydrothermal heat recovery approach taken by MTU is a purely physical process.

Deep geothermal energy at MTU’s Munich location will have no impact on flora and fauna, as the plant will have no effect on the near-surface area. Lowering the rock temperature in deeper areas has no effect on the microclimate or the flora and fauna at the surface; this is determined by the temperature down to a depth of around 20 meters—the area where soil organisms or plant roots are found. The soil temperature here is a result of solar radiation and percolating rainwater as well as moving groundwater. This means the geothermal energy present in the shallow subsurface is predominantly stored solar energy and is subject to seasonal temperature fluctuations.

No. Detailed borehole planning and proven drilling techniques prevent mutual interference. The near-surface groundwater is already being secured with standpipes (steel pipes reaching down to a depth of about 50 meters) as part of the drilling site construction. Due to their strength and impermeability, these standpipes ensure the stability of the well. This both prevents thermal water from penetrating near-surface groundwater and ensures no groundwater enters the drilling area.

Toxic substances, such as hydrogen sulfide or traces of oil, may be present in thermal water. However, since the thermal water cycle is conducted in a closed system and the water is reintroduced into the same deep aquifers after the heat is removed, there is no contamination of the surface with potentially toxic substances. The composition of the Malm water in the Munich area is also known from numerous geothermal wells and generally raises no critical concerns. Some spas also use this water directly for filling their baths.

The heat source itself is inexhaustible by human standards. The use of geothermal energy is therefore limited only by the lifetime of the technical equipment and structures. As the water is recirculated back into the reservoir, the hydrogeological balance is maintained. The flow of heat from the Earth’s interior will not end. This means the cooled area around the injection well will also warm up again and can regenerate.

The thermal water used for MTU’s geothermal energy is drawn from a depth of about 2,200 meters, and the cooled water is reintroduced at a comparable depth into the same geological horizon at a sufficient distance. Harnessing the water’s heat does not involve MTU extracting any substances from the water. Returning the water causes the rock to cool in the immediate vicinity of the injection well. The cooled water is reheated as it flows onward through the rock.