Stability analysis of the underground infrastructure for pumped storage hydropower plants in closed coal mines.

• Stability of closed mines for energy storage in underground hydro plants is analyzed. • 3D numerical model is used to analyze the stability of the underground infrastructure. • Excavation of underground caverns and tunnels is technically feasible. • Excavation and operation stages are studied and approved for two mine scenarios. • Air pressure in the lower reservoir may cause stability problems in the tunnels. Electricity storage systems are necessary to increase the efficiency of variable renewable energies. Mine water in closed underground coal mines can be used for underground pumped-storage hydropower plants. Subsurface energy storage systems require the excavation of a powerhouse cavern and a network of tunnels as lower water reservoir. To prevent build-up of high air pressures in the network of tunnels during the water filling process it is necessary to excavate ventilation shafts. In this paper, fluid dynamics and geomechanical behavior are combined in the lower reservoir in order to know the feasibility of the underground infrastructure. Depending on the diameter of the ventilation shafts and water flow rate, air pressures up to 420 kPa can be reached. The stability of the powerhouse cavern and the effect of air pressure on the tunnels and shafts during the operation phase of the turbine are analyzed. Coal mines in Northern Spain have been selected as a case study to investigate the behavior of the underground infrastructure of a hydropower plant. Three-dimensional numerical models were developed and analyzed for different rock mass scenarios. The results obtained show that the effect of air pressure on the tunnels and shafts is moderate. The direction of air flow switches during operation time and long-term fatigue damage could be produced to the contour of excavations. The excavation of the powerhouse cavern is technically feasible in case of the application of a proper designed support system. [ABSTRACT FROM AUTHOR]