Pumped-storage hydropower plants with underground reservoir: Influence of air pressure on the efficiency of the Francis turbine and energy production

Abstract

Closed mines can be used as lower reservoirs of Underground Pumped-Storage Hydropower (UPSH) plants. The excavation of ventilation shafts is necessary to allow the exhalation of the trapped air which is displaced during the turbine process. This paper analyzes the influence of air pressure in the lower reservoir due to the energy losses in the ventilation shafts on the overall energy production. Analytical and three-dimensional CFD numerical models (using VOF formulation for two-phase flow) have been developed in order to know the influence of the air pressure on the turbine operation for the first time. The results obtained show that depending on the usual designs of the ventilation system, the pressure inside the reservoir may vary between 0.67 and 42 mH2O. The increase in pressure implies a reduction in the available net head and therefore, the stored energy decreases. The reduction in the generation of electricity could reach up to 62 MWh/cycle (12.5% of the available energy) for an installation of a Francis turbine with a flow rate at full load of 55 m3/s and 214 MW of power with a single air duct of 0.5 m in diameter. Economically, this would imply losses of more than 1.12 M€/year.