Energy Utilization

The energy utilization potential of an underground mine depends on its characteristics. A wide variety of applications are possible, including heating and cooling using heat pumps and mine water, climate control through mine ventilation, electricity production and storage, or the storage of heat in the form of hot water storage. [1]

The high heating and cooling capacity provided by the occasionally large amounts of water available is of particular significance for the use of mining water. For heating applications, an increase in the temperature is generally required, for which heat pump technology is employed. For cooling, there are two options: the water can be used as is, in which case no heat pump is necessary (termed “passive cooling”), or the system temperature can be further decreased using a heat pump (termed “active cooling”).

The innerworkings of a mine differ based on the type of raw material being mined and the deposit itself. In the figure below, the common sections of a mine, from which water can be taken for geothermal use and/or used water can be deposited, are indicated.

Possible Chances for the mine water use in an old mine (Grafik: TU Bergakademie Freiberg)

The figure above shows an example of a mine, which has been decommissioned and flooded. As long as the tunnels and galleries in drainage system for the active mine were designed for safe, long-lasting use or even if they were converted to such during decommissioning, these adits pose a well-suited option for extracting water for energy usage. In principle, it is also possible to use the water in the Rösche  of the tunnels or to take out the rising water in the mine shafts, which can be either partially or completely filled. The advantage of taking water from the shafts is that water from large depths flows upward due to temperature-related density differences. Furthermore, cavities filled with standing water can form, surrounded by water-permeable rock. Upon water extraction, the inflowing water refills these cavities after a certain regeneration time. Also, deeper, flooded mines structures, which can be found under the deepest water-bearing adits, can be used for providing energy. These have the advantage that the water temperature is generally higher at greater depths. In addition to the previously mentioned withdrawal sites, water that must be removed for the purpose of mining activities or after mine operation have concluded also poses a particularly suitable source. For example, mine waters must be pumped off if, as a result of mining-related subsidence of the surface and after flooding of the mine, the resulting groundwater level is above the surface or if groundwater has the potential to be contaminated by heavily mineralized or otherwise polluted mine waters.

 

Gefluteter Bergwerksbereich (Grafik: TU Bergkademie Freiberg)

There are also many possibilities for releasing the water after energy generation.  One option is an open system in which water from the mine is deposited in surface water bodies; here, care must be taken not to pump the mine dry. Water can also be released into the water-bearing adits from which it was pumped. If using the same gallery, the respective suction and discharge lines can be installed together in one large drill hole. Additionally, it is possible to discharge the water in other areas of the mine, from which the water flows back to the pumping site through permeable rock; however, the necessary thermal regeneration in this scenario also needs to be considered. Finally, there is also the option of discharging the water in the deeper workings of the mine which do not drain directly to the surface. In combination with the fourth withdrawal option, deep water with increased temperatures can also be utilized at the highest extractable water mass flowrate because the ground water level does not sink.

Literature

[1] Grab T., Storch T., Groß U. (2018) Energetische Nutzung von Grubenwasser aus gefluteten Bergwerken. In: Bauer M., Freeden W., Jacobi H., Neu T. (Hrsg) Handbuch Oberflächennahe Geothermie. Springer Spektrum, Berlin, S. 523–586