Overview of converting abandoned coal mines to underground pumped storage systems: Focus on the underground reservoir

The utilization of Underground Pumped Storage Power Systems (UPSP) addresses the growing need for energy storage in the face of increasing intermittent energy sources. Simultaneously, the closure of mining activities has resulted in vast underground spaces potentially becoming available for alternative purposes. This paper explores the potential of repurposing abandoned mines, particularly coal mines, as lower reservoirs for UPSPs. The challenges associated with employing abandoned mines as lower reservoirs are multifaceted. The foremost challenge stems from limited knowledge about the current state of the mines due to post-mining processes, such as weathering, dissolution, hydration, leaching, swelling, slacking, subsidence, creeping along faults, gas migration, and precipitation, along with corrosion and deterioration of the support elements. This study documents and discusses the various processes related to cyclical pumping and discharge within the context of UPSPs, encompassing hydraulic discharge processes, cyclic loading, dry and wet processes, as well as fatigue and thermal stress. These processes significantly impact the safety, productivity, and stability of the lower reservoir. To address these challenges, the paper presents different numerical solutions available to comprehend and mitigate cyclical processes in abandoned mines. Finally, it explores the economic feasibility of repurposing mines as lower reservoirs and the conditions required are examined, including favorable rock mass properties, reduced land acquisition costs, the necessity of permanent water pumping, and the potential income from excavated rock as a revenue source in case of new excavations. This research contributes to the understanding of utilizing abandoned mines for UPSPs, highlighting the challenges associated with the use of coal mines as lower reservoirs and presenting several main processes to prevent safety and productivity issues.