Essential hydraulic aspects associated with the operation of underground pumped-storage hydropower plants

Pumped-storage hydropower (PSH) is used to store energy by pumping water from a lower source to an upper reservoir. In flat regions, where differences in height are lacking, it is suggested that an underground pumped-storage hydropower (UPSH) plant is built. The objective of this study is to highlight the need to enhance knowledge of hydraulic processes during filling and emptying the lower reservoir of UPSH plants before their construction in abandoned coal-mine areas. This manuscript includes (i) a review of the UPSH concept development, (ii) essential hydraulic aspects associated with the operation of the lower reservoir, and (iii)Model experiments on hydraulic processes during filling and emptying the lower reservoir with a large length-to-height ratio. The model (34.27 m long, 0.212 m high) represents the geometry of coal mine caverns on an approx. scale of 1:35. This is the first physical model to describe the oscillations that occur during the operation of a cavern-shaped lower reservoir. Oscillating single waves with an approximate period of 70 s were observed during the water level rise in the filling process and the drop in the outflow process. Contained oscillations with wave periods of 62–188 s occurred after closing the valve in both processes. The water level of the contained oscillations can be described by a cosine function including an empirically determined wave damping factor, ranging between 1.06 and 1.2. The results show that the discharge determines the performance of the system and may contribute to consideration of the best method to operate tunnel galleries.