Retrofitting Abandoned Slate Mine For Long-Term Thermal Energy Storage: Lessons Learned From the WeForming and Ard-Nrgy Projects

This paper presents the lessons learned from the WeForming and Ard-Nrgy projects, which aim to convert a former slate mine in Martelange, Belgium, into a large-capacity underground thermal energy storage facility. Considering the European climate objectives and the growing need to integrate intermittent renewable energy sources into heating networks, the reuse of abandoned flooded mines represents an innovative technical and economic opportunity. After exploring the identified cavity and conducting drilling operations, a complete pumping, heat exchange, water treatment, and fiber optic measurement system was installed. Detailed dynamic modeling, based on Modelica and the IDEAS library, was conducted to analyze the performance of three different architectures: an individual geothermal heat pump system, a centralized system without storage, and a centralized system with storage. The simulations show that underground storage can improve photovoltaic selfconsumption and energy self-sufficiency, while ensuring seasonal storage efficiency comparable to conventional UTES systems. The results also demonstrate the importance of adequately sizing the underground storage volume, which is essential for optimizing system costs and performance. The conclusion is that the geological uncertainty dominates techno-economic viability of mine-based UTES.