*Article* **The Impact of Hydrogeological Features on the Performance of Underground Pumped-Storage Hydropower (UPSH)**

**Estanislao Pujades 1,\*, Angelique Poulain <sup>2</sup> , Philippe Orban <sup>3</sup> , Pascal Goderniaux <sup>4</sup> and Alain Dassargues <sup>3</sup>**


**Featured Application: This work evaluates the influence of groundwater exchanges occurring in the context of underground pumped storage hydropower using abandoned mines on the efficiency and on the environment. The findings are useful to define (1) design criteria of future underground pumped storage hydropower plants and (2) screening methodologies to choose the best places to construct them.**

**Abstract:** Underground pumped storage hydropower (UPSH) is an attractive opportunity to manage the production of electricity from renewable energy sources in flat regions, which will contribute to the expansion of their use and, thus, to mitigating the emissions of greenhouse gasses (GHGs) in the atmosphere. A logical option to construct future UPSH plants consists of taking advantage of existing underground cavities excavated with mining purposes. However, mines are not waterproofed, and there will be an underground water exchange between the surrounding geological medium and the UPSH plants, which can impact their efficiency and the quality of nearby water bodies. Underground water exchanges depend on hydrogeological features, such as the hydrogeological properties and the groundwater characteristics and behavior. In this paper, we numerically investigated how the hydraulic conductivity (*K*) of the surrounding underground medium and the elevation of the piezometric head determined the underground water exchanges and their associated consequences. The results indicated that the efficiency and environmental impacts on surface water bodies became worse in transmissive geological media with a high elevation of the piezometric head. However, the expected environmental impacts on the underground medium increased as the piezometric head became deeper. This assessment complements previous ones developed in the same field and contributes to the definition of (1) screening strategies for selecting the best places to construct future UPSH plants and (2) design criteria to improve their efficiency and minimize their impacts.

**Keywords:** energy storage; renewable energy; hydropower; mine; groundwater; numerical modelling; environmental impacts; efficiency
