Evaluation of the mine thermal energy storage potential with a stochastic discrete fracture matrix model

Decarbonizing the industrial and building heating and cooling sectors is a crucial step toward achieving carbon neutrality, necessitating innovative and sustainable solutions for the over-seasonal storage of excess heat energy. With Germany alone having more than 10,000 old mines, repurposing these sites to implement a controlled thermal energy storage strategy, known as mine-based thermal energy storage (TES), has emerged as a potential solution. To effectively utilize such partially flooded artificial cavities, it is crucial to fully understand the heat transport and storage behavior in these systems.

In this work, a three-dimensional hydro-thermo-component (HTC) model was developed using the open-source simulation code OpenGeoSys (OGS). The model was initially verified against analytical solutions for the single fracture flow of heat and solute transport, respectively. Subsequently, stochastic discrete fracture matrix (DFM) geometries and meshes were generated using the computational suite Frackit, based on data from a pilot heat storage site in a water-filled mining cavity in Freiberg, Germany. This test site is geologically characterized as the Freiberg gneiss, a metamorphic fractured rock formation.

The developed setup allows for investigating the thermal energy storage capacity and the energy recovery efficiency based on process simulations in OGS. The study evaluated the thermally affected zone in the fractured formation and quantified the amount of heat stored and recovered during cyclic operation. In addition, the solute transport distance within the surrounding rock can be evaluated under different hydraulic conditions. The general modeling workflow provides a basis for conducting techno-economic feasibility analysis of mine-based TES systems.