energies Article Underground Pumped-Storage Hydropower (UPSH) at the Martelange Mine (Belgium): Underground Reservoir Hydraulics Vasileios Kitsikoudis 1,*, Pierre Archambeau 1 , Benjamin Dewals 1, Estanislao Pujades 2, Philippe Orban 3, Alain Dassargues 3 , Michel Pirotton 1 and Sebastien Erpicum 1 1 Hydraulics in Environmental and Civil Engineering, Urban and Environmental Engineering Research Unit, Liege University, 4000 Liege, Belgium;
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[email protected]; Tel.: +32-478-112388 Received: 22 April 2020; Accepted: 6 July 2020; Published: 8 July 2020 Abstract: The intermittent nature of most renewable energy sources requires their coupling with an energy storage system, with pumped storage hydropower (PSH) being one popular option. However, PSH cannot always be constructed due to topographic, environmental, and societal constraints, among others. Underground pumped storage hydropower (UPSH) has recently gained popularity as a viable alternative and may utilize abandoned mines for the construction of the lower reservoir in the underground. Such underground mines may have complex geometries and the injection/pumping of large volumes of water with high discharge could lead to uneven water level distribution over the underground reservoir subparts. This can temporarily influence the head difference between the upper and lower reservoirs of the UPSH, thus affecting the efficiency of the plant or inducing structural stability problems.