Underground thermal energy storage in mines: Field scale heating-cooling cycles and performance at the Reiche Zeche underground mine

Flooded and partially flooded mine galleries represent a largely untapped type of subsurface reservoir within underground thermal energy storage (UTES), known as mine thermal energy storage (MTES). This study presents a long-term field demonstration of a fully instrumented MTES test-bed at the Reiche Zeche underground mine in Germany. Three controlled heating–cooling cycles with a combined duration of 504 days were carried out and monitored through dense thermometry, tracer testing, and hydrochemical and materials analyses. A total of 38.0 MWh of heat was supplied. Approximately 90% of the stored energy resided in the surrounding gneiss, confirming that the rock mass functioned as the principal store while the basin water acted as a rapid carrier and exchanger interface. The rock warmed by 10.1 K at 1.8 m depth after the hottest cycle, consistent with a conduction-dominated regime. Tracer dilution determined a throughflow of 79 L h−1 with a residence time of about 10.5 days, corresponding to an advective heat-loss coefficient of 0.092 kW K−1. Warm phases triggered Fe(II) oxidation and precipitation of Fe-oxyhydroxides that dominated exchanger fouling, while hydrophobic coatings limited conductance losses to roughly 18%. Integrated field, laboratory, and numerical analyses showed that hydraulic isolation and oxygen control were the main levers for improving efficiency. The results demonstrate reproducible MTES operation under mine conditions and show that advective loss and exchanger fouling govern recoverability. The derived metrics provide a practical basis for MTES design in similar underground settings and highlight the potential of post-mining infrastructure to contribute to the underground thermal energy storage portfolio.