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Underground pumped storage hydropower (UPSH) plants may be an alternative to store subsurface energy with lower environmental impacts than conventional pumped storage hydropower (PSH) plants. Network of tunnels in closed mines (i.e. coal mines) could be used as water lower reservoir of UPSH plants. The amount of storable energy depends on the water mass and the net head between upper and lower reservoirs. Depending on the direction of the water flow rate, pumping or turbine modes may be used to produce or consume electrical energy. Filling and emptying processes during the operation stage in the underground reservoir are complicated due to the presence of two fluids (water and air) interacting inside the network of tunnels. This paper explores the underground reservoir during the operation stage considering a water flow rate of 55 m3 s-1 . Two-phase three dimensional CFD numerical models using Ansys Fluent have been developed in order to know the behaviour of the air flow on tunnels and ventilation shaft. Static pressure and air velocity have been analyzed in the simulations at the exit of the ventilation shaft as well as the junction zone between the ventilation shaft and the tunnels network. The results obtained show that a static pressure up to 8,600 Pa and air velocities up to 80 m s-1 could be reached in turbine mode considering a vent shaft with 1 m in diameter. The static pressure increases up to 258,000 Pa if a ventilation shaft of 0.5 m in diameter is considered.

Numerical modelling of water subsurface reservoirs during the operation phase in underground pumped storage hydropower plants