Efficiency analysis of underground pumped storage hydropower plants

Large-scale energy storage systems, such as underground pumped-storage hydropower (UPSH) plants, are required in the current energy transition to variable renewable energies to balance supply and demand of electricity. In this paper, a novel method to determinate the round trip energy efficiency in pumped storage hydropower plants with underground lower reservoir is presented. Two Francis pump-turbines with a power output of 124.9 and 214.7 MW (turbine) and a power input of 114.8 and 199.7 MW (pump), respectively, have been selected to investigate the overall operation of UPSH plants. Analytical models and two-phase 3D CFD numerical simulations have been carried out to evaluate the energy generated and consumed, considering a typical water mass of 450,000 t and a maximum gross pressure of 4.41 MPa. The results obtained in both analytical and numerical models show that unlike conventional pumped-storage hydropower plants, the round trip energy efficiency depends on the pressure inside the underground reservoir. The round trip energy efficiency could be reduced from 77.3% to 73.8% when the reservoir pressure reaches -100 kPa. In terms of energy balance, the energy generation decreases down to 3,639 MWh pw (-1) and the energy consumption increases up to 4,606 MWh year(-1) compared to optimal conditions.