Performance assessment of a potential underground thermal storage facility: Case study of the closed mine Prosper‐Haniel in the Ruhr coal mining area in Germany

Abstract In this study, we assessed the energy efficiency for a potential underground low‐temperature thermal storage facility in the decommissioned Prosper‐Haniel mine located in the Ruhr coal mining area of Germany. As per the proposed concept, warm water is stored in the underground gallery during the summer and utilized in heat pumps to generate heat in the winter. Proper sealing enables the functioning of a closed‐loop system with seasonal replacement of cold and warm water using the communicating vessel principle. The energy efficiency of the storage facility is estimated to have an expected surplus of gained thermal energy over that spent on operation, ranging from 62% to 84%, and a thermal output of 2.4–4.2 MW in winter. The recoverable energy share during the annual cycle, ranging from 38% to 45%, depends primarily on the warm water temperature estimated at 18–22°C, and the duration of the heating period. Based on numerical modeling of heat transfer and thermal energy calculations, we estimated how the thermal output, recoverable heat, and energy efficiency of the storage facility depend on the average temperature of warm water, heat season duration, and the temperature of heat transfer fluid at the heat consumer. The developed comprehensive approach enables the optimization of key indicators of storage facility operation, considering seasonal variations in energy demand and temperature.