Storing heat in mines
What happens to mines that are no longer in use? Could they be used to store heat? Researchers at the International Geothermal Centre are hoping to find out whether mines could be put to this use and are conducting tests in the former Prosper-Haniel hard-coal mine in the Ruhr region to investigate whether it is suitable for heat storage.
|Project status||Numerical modelling of the pit structure has been completed.|
|Storage effect||Temperature change (sensible heat), water (pit water)|
|Storage structure||Pit structure of a former mine|
|Number of cycles||Half-yearly|
|Charging temperature||90 °C|
|Discharge temperature||80 °C|
|Storage capacity||1.7 GWh p.a.|
|Energy storage density||210 kJ/m3|
|Project duration||December 2014 to November 2017|
The idea of drawing thermal energy from existing and disused coal mines is not new. Up to now, the possibility of storing heat in a former hard-coal mine has not yet been studied using a pilot plant. The aim of thermal storage in the pit structure of a mine is to store solar-generated heat and exhaust heat produced in a seasonal manner by industrial processes and power plants (mainly from combined heat and power generation) in the pit structure and to use this energy for heating residential and commercial buildings in wintertime.
There is currently no seasonal heat storage system in a former hard-coal mine in North Rhine-Westphalia or in Germany as a whole. The Geo-MTES (Mine Thermal Energy Storage) project by the International Geothermal Centre at Bochum University of Applied Sciences in cooperation with RAG and delta h Ingenieurgesellschaft is aiming to play a pioneering role in implementing such a system. The main aim of the feasibility study is the development of a heat-storage concept that is implementable from both an engineering and economic viewpoint for a new form of use of the former Prosper-Haniel mine for thermal storage.
The concept envisages the storage in the pit structure of exhaust heat from industrial processes and power plants and/or of solar energy generated on nearby disused mine sites; this heat, which cannot be used directly when it is produced, is then to be used during the colder half of the year when it is needed for heating residential and commercial buildings – using the existing district heating network in certain cases. Appropriate infrastructural measures need to be carried out in the pit structure and suitable systems for connection routes and pumping need to be developed in order to create this type of underground thermal storage system. The prerequisite for all this is the presence of a completely accessible mine, which ideally will still be in service. The Prosper-Haniel hard-coal mine is fully accessible up to the end of 2018, which allows targeted heat-storage concepts to be developed and put in place. Undisturbed rock temperatures of 30 to 50 degrees Celsius inside the Prosper-Haniel mine – thanks to pit depths that extend down to 1,200 metres and more – could serve as a basis for a seasonal heat-storage system. There are 141 kilometres of underground routes and a total mining area of 165 km2 in the Prosper-Haniel mine, and thus a heat-storage system with a large volumetric capacity is feasible inside the pit structure.
Pit water table depths of 600 metres can be expected in the future in most of the central and northern parts of the Ruhr area, which means that the energy required for lifting would be too great compared to the thermal energy obtained – even at water temperatures of up to 35 degrees Celsius. One possible method of increasing efficiency would be to significantly raise the pit water temperatures by storing seasonal heat in the pit structure. However, this has not yet been implemented.
The Prosper-Haniel mine:
The fact that the Prosper-Haniel mine is an active mine that is still fully accessible is a major advantage with regard to geothermal harnessing of the mine and its use as a heat-storage system.
- On the one hand, improved water flow paths have been created by coal mining and the creation of underground access routes (rock and seam drifts) in the carboniferous rock, which is otherwise relatively dense. These improve the capacity of the underground system for heat transport.
- On the other hand, it is comparatively easy to make use of the pit structure from an engineering viewpoint as a result of the shafts and drifts that are currently still open.
The high population density in the area around the mine means that there will almost certainly be a good market to take up the heat stored underground. This heat can be used to supply surrounding residential areas and commercial facilities – for example, through a connection with the “Fernwärmeschiene Ruhr” district heating network or integration into the “InnovationCity Ruhr” process. A pilot storage concept is described here in greater detail:
- Storage concept in the 7th mining level (doublet within shaft 10): The 7th shaft (see green marking in Fig. 1), which is adjacent to shaft 10, could be used as a seasonal heat-storage system for the possible implementation of a pilot plant.
Surplus heat from a biogas power plant could be stored here in summer and then used in winter for heating and providing hot water for a local “mini-heating network” in Bottrop-Kirchhellen. When designing a thermal storage system, two dams would have to be built in the 7th mining level to implement hydraulic decoupling from the planned pit water drainage.
When compared with seam drifts and mining areas, significantly fewer convergences (higher stabilities) can be expected in the 7th mining level as this is a rock drift. Within the thermal storage system, undisturbed rock temperatures of up to 50 degrees Celsius can be anticipated, as this mining level is at a depth of 1,159 metres below sea level. To increase the efficiency of the storage system, a turbine is to be fitted to the injection pipes so as to recover a fraction of the pump energy. If feed-in with a temperature difference of 50 K is assumed, around 1.7 GWh p.a. of heat would result at the present time.
The idea of drawing thermal energy from existing and disused coal mines has been the subject of research for some time now, even though relatively little research effort has focused on this area. Up to now, the possibility of storing heat in a former hard-coal mine has not yet been studied using a pilot plant.
The thermal harnessing of pit water from existing water drainage measures – such as in Essen (in the former Zollverein mine) or in Bochum (in the former Robert Müser mine) – shows the greatest efficiency, as no additional pumping costs need to be taken into account here. Further expansion of such projects is only being carried out to limited extent at the moment. The “open” usage concept of the Mijnwater project in the Netherlands was feasible because the pit structure was already largely flooded after decommissioning of the mine. With a pit water depth of less than 150 metres, the ratio of thermal energy obtained to expended energy (pump energy) can be regarded as favourable despite the low pit water temperatures of around 28 degrees Celsius. However, the pit water must be heated to a higher temperature level using heat pumps. In contrast with the Mijnwater project, the depths of the pit water table of over 600 metres in certain cases will increase significantly in the long term in most of the central and northern parts of the Ruhr area; as a result, the energy required for lifting is too high compared to the thermal energy obtained – even at water temperatures of up to 35 degrees Celsius. One possible method of increasing efficiency would be to significantly raise pit water temperatures by storing seasonal heat in the pit structure. However, this has not yet been implemented.
The development of storage capacities is a high priority in the context of the further expansion of renewable energy sources. Mines that have not been made use of so far offer great potential for heat storage, particularly in the Ruhr area, as a significant amount of unused exhaust heat is available on a seasonal basis for other uses from power plants and industrial processes. The latest studies show that an unharnessed potential of around 500 PJ of heat is released to the environment each year in Germany. For this reason, basic data must be obtained relating to heat storage in former mines in order to further advance and establish these technologies. If the pilot plant is implemented in a manner that is feasible from both an engineering and financial viewpoint, the results with regard to the design and operation of a seasonal heat-storage system in a former hard-coal mine could be scaled-up and applied to other locations in Germany and worldwide.