Energy storage systems are indispensable for a reliable, economical and environmentally friendly energy supply. However the expansion of the only well-known short- and medium-term storage technology, pumped storage plants (PSW), is limited. Not enough suitable sites are available; one of the limiting factors is the need of specifically required topographies. The available height difference and the size of the storage reservoirs define the storage capacity. The alternative idea of underground pumped storage plants would significantly expend the extension options and many of the currently unusable sites could then be adapted. The function of underground pump storage plants is the equivalent to the function of traditional plants, though ventilated underground cavern systems are used as a storage reservoir. Due to the new design of the reservoirs the hydrodynamic processes during filling and emptying operations are not known yet. Objective of the DFG project is a systematic description of the hydrodynamic processes in underground pumped storage reservoirs (UPSR), their knowledge is relevant for the hydraulic design. Therefore, at the Institute of Hydraulic Engineering and Water resources Management (IWW) a hybrid modelling to determine the effect of opera-tional and structural factors on the flow processes in the reservoirs is performed. In this context hybrid implies the combination of experimental model tests in the laboratory of the IWW and numerical 3D simulations with OpenFOAM.