The Reiche Zeche Geo-Lab for in-situ simulation of mine thermal energy storage (MTES): Design and insights.

The (over-)seasonal storage of excess heat and cold in the subsurface is considered a promising solution to the manifold challenges of the energy transition. Many underground thermal energy storage (TES) systems are focusing on natural aquifers. In parallel, there has also been increasing attention on using artificial cavities in (partially) flooded underground mines. This special form is known as mine thermal energy storage (MTES). Like other underground TES systems, MTES faces several challenges. Many former mines are actively dewatered to keep a defined flooding level and, therefore, significant water flows can be present, especially in the main tunnels and shafts. The unintentional transport of stored heat energy out of the original storage area, whether through heat advection or conduction, ultimately leads to reduced recovery rates and suboptimal efficiency. Adoption of localized and hydraulically (more or less) isolated mine sections instead of entire levels may provide a solution to this technical challenge. An MTES Geo-Lab has been recently designed and established as part of the R&D project "MineATES", funded by the German Federal Ministry of Education and Research (BMBF). The in-situ Geo-Lab is located in the former silver mine "Reiche Zeche (Himmelfahrt Fundgrube)" at the TU Bergakademie Freiberg in Saxony, Germany, and focuses on the controlled simulation of TES cycles on a manageable scale. Specifically, a cuboid-shaped experimental reservoir (water capacity of approx. 21 cubic meters) in the northern field of the Reiche Zeche’s first level, slowly flown through by acidic precipitation water (pH values between 2 and 3), was chosen. The prevalent geological formation in this area is the Freiberg gneiss. Given the pilot-scale of the study site, heat losses across system boundaries are expected to be of an experimentally manageable magnitude – and are intentionally so. The immediate vicinity of the experimental reservoir has been equipped with an extensive thermal monitoring system. This includes more than 90 temperature sensors embedded at various distances from the reservoir walls, with some up to two meters deep and distributed across 18 boreholes. First measurements showed a background temperature in the rock of approx. 11.5°C on average. This monitoring system enables continuous tracking of transient temperature distributions in the surrounding rock, facilitating the quantification of heat losses and efficiency reductions during periodic heat/cold injection and extraction experiments, emulating real-world TES cycles. Furthermore, the Geo-Lab is equipped with multiple sampling points to monitor hydrochemical parameters over time.