Storing Thermal Energy in Underground Mines: In Situ Simulations at the Freiberg Living Geo-Lab

Underground mines, once vital industrial hubs, hold immense potential for innovative applications, including Mine Thermal Energy Storage (MTES). MTES repurposes partially and fully flooded mine cavities as reservoirs for storing surplus heat or cold, presenting a novel alternative to conventional Aquifer Thermal Energy Storage (ATES). While promising, MTES faces challenges such as scaling, corrosion, energy loss, and interactions between the geological matrix and technical infrastructure.

To address these challenges, TU Bergakademie Freiberg has established a living MTES geo-lab at the historic Reiche Zeche silver mine. Key features of this facility include a 21-cubic-meter water reservoir and over 90 temperature sensors embedded in Freiberg Gneiss. The pilot-scale MTES simulator setup allows for continuously monitoring heat transfer during thermal energy injection and extraction cycles realized by a mobile heat pump system. Early findings are revealing an average background rock temperature of 12 °C, a fast conductive heat transport within the rock as well as a good storage potential with elevated rock temperatures of up to 25 °C in approximately 2 meters from the water body. However, significant heat losses across system boundaries have been observed, with advective heat transport via flowing water identified as the primary contributor.

Parallel laboratory-scale experiments using column flow setups and batch reactors simulate MTES conditions, exposing rock and mine water to temperature cycles ranging from 10°C to 60°C. These experiments demonstrate significant chemical changes, including the precipitation of 90% of dissolved iron. These findings offer valuable insights into the chemical stability and thermal efficiency of MTES systems.

Two complementary methods were employed to quantify effective inflow and energy dissipation caused by mine water movement. First, a dilution test with NaCl was conducted. Second, inflowing water volume was calculated based on reservoir water level reductions. Results indicate that the calculation based on inflowing water volume provided more reliable values, while the formula used in the dilution test requires further refinement.

Additionally, numerical simulations using OpenGeoSys (OGS) software are being developed to assess the influence of fracture networks in the surrounding rock formation on heat storage and recovery performance. Preliminary results indicate that fractures enhance advective heat transport, leading to lower heat recovery ratios during cyclic operation.