Mine water from abandoned and flooded former coal mines can be used as a low-temperature geothermal energy source for heating buildings and industrial complexes. The idea of obtaining thermal energy from an inoperative colliery has already been pursued for a long time, although to a rather limited extent. Well-known projects concerning the utilization of mine waters included:
The thermal utilization of the mine water from existing mine drainage stations, as they are realized in Essen or Bochum (Germany), show the highest economic efficiency, as no additional pumping costs are being generated. Due to the lack of suitable customers and a not yet existing final planning security concerning the future locations of mine drainage stations after the end of active coal mining (end of 2018) and the renaturation of the Emscher river, potential expansion currently takes place to a limited extent. The “open” utilization plan of the Mijnwater-project could be realized in the Netherlands, as the mine workings are already flooded after being closed down. In the case of a mine water table <80 m below ground, the proportion between the thermal energy obtained and the input energy (pumping energy) is to be assessed as positive, despite the low temperature of the mine water of about 28°C. Nevertheless, the mine water must be brought to a higher temperature level with the use of heat pumps.
Fig. 1. A conceptual model of underground heat storage in an abandoned coal mine (HEATSTORE final report 2019)
One of the main aims of the HEATSTORE (Geothermica) project is to create a technically and fully functional seasonal Mine Thermal Energy Storage (MTES) pilot plant to use geothermal heat of the abandoned coal mine Markgraf II in the area of Bochum in western Germany. As for today, a pilot plant has not yet been established, in which the possibility of thermal energy storage in a former hard coal mine has been considered. The conceptual idea of the project is based on the storage of unutilized surplus heat during the summer periods from solar thermal collectors within the abandoned coal mine and using the stored heat during the winter season for heating purposes (Hahn et al., 2019). The location of the project is shown in Figure 2.
Fig. 2. The location of the coal mine Markgraf II (HEATSTORE final report 2019)
Future MTES installations need to have a large mine water volume available, to store vast amounts of geothermal heat. At the same time, systems based on the abandoned coal mines have to be reliable, cost-efficient and be integrated into an existing urban framework. To meet economical requirements, MTES systems need to be operating for a period between 40 and 50 years. Depending on the utilized geothermal heat source and its application, different heat capacities, mass flows, and temperature levels would be encountered within the mine thermal energy storage. All affected components of the system need to be suitable for the intended operations and their resulting thermo-mechanical stresses.
This wraps up another #GeothermalFactsandStats blog post from our ongoing series dedicated to the promotion of all things geothermal. Follow us on all major social media platforms to keep up with our latest developments, and check back weekly for new posts.
Author: Michal Kruszewski