Hydrochemical Evolution during Thermal Cycling in a Mine Thermal Energy Storage System: Insights from the Reiche Zeche MTES site

Mine Thermal Energy Storage (MTES) systems represent a promising solution for seasonal heat storage to balance the seasonal regenerative energy supply-demand mismatch in former mining regions. MTES may induce hydrochemical changes due to repeated thermal cycling. This study investigates the hydrochemical evolution of mine water during the operation of a pilot-scale MTES system in the Research and Teaching Mine Reiche Zeche of the TU Bergakademie Freiberg (Germany) and in laboratory experiments in the BMFTR-funded project 'MineATES'.

The MTES pilot system consisted of a 20 m³ mine water filled basin located in the unsaturated zone of the mine. The Mine Water in the basin and the inflows showed typical acid mine drainage characteristics with original pH at 2.7, redox potential at 840 mV and sulphate values around 500 mg/l, up to 30 mg/l zinc and up to 26 mg/l dissolved iron. In total three heating and three cooling cycles were conducted at the test site from original 11.6 °C to water temperatures reaching 26 °C in the first two heating cycles and 39 °C in the last heating cycle. The basin was sampled weekly and the inlets into the storage basin were also monitored.

Results indicate that the mine water chemistry was mostly controlled by temperature, mine water influx, and evaporation. We observed iron precipitation during heating after high inflow periods. After the inflow was significantly reduced iron precipitation did so, too. Iron concentration decreased from 23.7 mg/l to 2.5 mg/l during the first cycle with high inflow conditions. After the inflow was reduced a decrease from 1.5 mg/l to 0.2 mg/l iron was observed in the third heating cycle. The third heating cycle reached 39 °C and induced evaporation through the gaps of the basin cover leading to an enrichment in components by the factor 2 and to gypsum formation above the water line. For future optimisation of MTES systems, results suggest a reduction of new mine water inflow to prevent repeated iron precipitation and full contact with the surrounding rock to minimize evaporation effects.