A simplified approach to numerical modelling of an underground pumped hydroelectric energy storage system

The Underground Pumped Hydroelectric Storage (UPHS) is an energy storage system in which inflation and deflation of an underground geomembrane-lined reservoir interconnected to an open water basin enable the storing and harvesting of energy, respectively, in terms of potential energy. Axisymmetric Finite-Element Analyses (FEA), including monotonic and multi-cycle changes of the reservoir volume, are used to investigate reservoir aspects that influence the capacity and efficiency of UPHS in sand, including geomembrane stiffness and interface friction. In particular, a novel simplified approach for the modelling of UPHS reservoir is proposed. This consists of rigid piston elements lifting within a skirt incompressible material with low shear stiffness and directly tied to the overburden. The simplified approach is validated against refined models having the reservoir as a lined fluid cavity undergoing volume changes. Results indicated that the simplified modelling is suitable for preliminary estimates of overburden deformations, reservoir pressure-volume curves, and energy efficiency.