During the last decades, the Asturian Central Coal Basin (ACCB) has been a highly exploited coal mining area by means of underground mining and its network of tunnels extend among more than 30 mines. Parts of this infrastructure will soon become available for alternative uses since most of the coal mining facilities in Spain will fade out in 2018. Increasing penetration of renewable energy sources into the electrical grid, with intermittent and fluctuating supply, leads to excessive frequency variations, so the development of energy storage technologies are required, such as Pumped Storage Hydroelectricity (PSH). Reduced environmental impacts, deep, non-flooded shafts and abundance of water from underground runoff , make coal mines in ACCB suitable for the development of Underground Pumped-Storage Hydropower projects (UPSH). The network of tunnels of a mine facility has an unusual geometry for a water storage system. Although there are numerous studies for the construction of UPSH plants, until now there have been no known projects of this type under operation. Filling and emptying processes during the operation of the turbine-pump are complex due to the presence of two fluids interacting inside the tunnels, water and air. This paper explores the viability of a network of tunnels as an underground water reservoir. Two-phase three-dimensional CFD models have been developed in order to know the flow behavior in the tunnels. The pressure and velocity results that have been obtained in the simulations confirm that the use of underground mines as a lower reservoir of a UPSH is technically possible.

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The increasing penetration of variable renewable energies (VRE) in the European electricity mix requires flexible energy storage systems (ESS), such as pumped storage hydropower (PSH). Disused mining voids from deep closed mines may be used as subsurface reservoirs of underground pumped-storage hydropower (UPSH) plants. Unlike conventional PSH plants, the air pressure in UPSH plants is variable and it differs from the atmospheric conditions. In this paper, the hydraulic transient process of an UPSH plant operating in pumping mode was investigated and a preliminary thermodynamic analysis of the closed surge tank was carried out. Analytical and CFD three-dimensional numerical simulations based on the volume of fluid (VOF) model with two-phase flow have been performed for analyzing the transient process. In the transient simulation, air and water are considered as ideal gas and compressible liquid, respectively. Different guide vanes closing schemes have been simulated. The obtained results show that the dimensioning of underground reservoir, surge tank, and air ducts is essential for ensuring the hydraulic performance and optimizing the operation of UPSH plants. The static pressure in the air duct, surge tank and lower reservoir reaches −1.6, 112.8 and −4 kPa, respectively, while a heat flux of −80 W was obtained through the surge tank walls.