Through the production well the thermal water is pumped from the aquifer (the water-bearing layer) to the surface with the help of a pump. Generally, a distinction is made between so-called line shaft pumps, which are driven by a shaft from the surface, and submersible centrifugal pumps. In the latter, the motor is installed inside the borehole together with the actual pump unit. In the Molasse Basin, only submersible centrifugal pumps are used due to the hydraulic conditions. The submersible centrifugal pump is several meters long and – depending on the location of the at-rest water level – installed at a depth of several hundred meters in the borehole. In Munich and the surrounding area, the at-rest water level is approx. 300-500 meters below the surface. After the hydraulic tests have been completed, the pump is usually installed about 300 meters deeper than the at-rest water level. Depending on the yield of the thermal water-bearing layer, pumping rates of over 100 liters per second can be achieved. At the surface, heat energy is extracted from the thermal water via heat exchangers. The thermal water is then returned to the subsurface via the injection well. The geothermal doublet, consisting of the production well and the injection well, allows a closed circuit of the geothermal water. This means that the geothermal water does not come into contact with (sub)surface water or working fluids and is re-injected into the same aquifer after the energy transfer. There it can be reheated again. The amount of energy (heat and/or electricity) that can be generated by the geothermal system depends on the temperature of the thermal water and the rate of pumping.
Schematische Darstellung eines geothermalen Systems für die Wärmeversorgung mit den unter- und oberirdischen Kreisläufen (Bildquellen: Geothermie Unterhaching, edelstahl-waermetauscher.de)
A geothermal plant can produce either heat, electricity or both. Electricity production requires a thermal water temperature of at least 110 °C. However, the efficiencies for electricity production are comparatively low and making direct use for heat supply much more efficient.
For electricity production at thermal water temperatures below 200 °C, the heat of the thermal water is transferred via a heat exchanger to a working fluid with a lower boiling point than water. The working fluid can either be an organic liquid or a water-ammonia mixture. Unlike water, the vapor pressure of these working fluids is lower, allowing the materials used to release larger amounts of steam at lower temperatures. The steam is used to drive a turbine and produce electricity. Depending on the working fluid used, organic liquids are referred to as an ORC (organic Rankine cycle) plant and water-ammonia mixtures are referred to as a Kalina plant. At thermal water temperatures above 200 °C, a working fluid step can be skipped and the steam can be used directly from the thermal water to drive the turbine. In Germany, however, thermal water temperatures are below this level, so electricity is generated exclusively by ORC or Kalina plants.
Schematic representation of electricity generation from geothermal sources using ORC technology (sources: deutschlandfunkkultur.de, produktion.de, SWM)
For heat production, the energy of the thermal water is transferred directly to a district heating network via the heat exchanger. The heat exchanger is often a plate heat exchanger with titanium plates, because these are particularly corrosion resistant. The district heating network transports the heat from the geothermal plant to the consumers in a second closed circuit. The district heating pipes consist of a steel pipe encased in insulation and filled with water. The pipes are often laid under roads. In the house, the in-house hot water circuit takes heat energy from the district heating network through the heat transfer station to be used for hot water and heating. Depending on the size of the geothermal plant, 1000 up to 10000 households can be supplied with heat.