Considerations on the existing capacity and future potential for energy storage in the European Union's hydropower reservoirs and pumped-storage hydropower

Water storage and water reservoirs are key to the Water-Energy-Food-Ecosystem (WEFE) nexus, especially when they store water for hydropower. However, there is not a uniform view on existing energy storage capacity and on the potential for future deployment of pumped-storage hydropower (PSH) and conventional reservoir storage hydropower (RSHP) across energy system models. Furthermore, knowledge of energy storage capacity in PSH and RSHP remains rather fragmented and expressed in different metrics that need to be interpreted and reconciled. This paper addresses such barriers by systematically reviewing the literature and datasets (which are commonly used for regional assessments and modelling) related to the energy storage capacity, hydropower and reservoirs, reconciling various metrics and providing an internally consistent dataset related to the current energy storage capacity and potential for capacity expansion, with focus on the European Union (EU). The estimated theoretical storage capacity in this paper (approximated by the product of storage volume and head) of the aggregated EU's RSHP and PSH is 62 TWh (6.3 TWh + 55.7 TWh from PSH and RSHP, respectively). The technical storage capacity (i.e., the theoretical with some reduction correction factors) is 2.3 TWh for PSH and 24.9 TWh for RSHP. The reported PSH one is 1.3 TWh by IHA, and 71 TWh by ENTSO-E for PSH + RSHP (the latter including cascade effects for Sweden), while the actually usable one for PSH may be in the range 500–600 GWh. According to the literature review, new PSH and RSHP developments are possible (the available theoretical potential is 35–230 TWh from closed-loop PSH, +50 TWh from open-loop PSH neglecting the cascade effect, and +2.6 TWh from new reservoirs in valleys with melting glaciers), but the most suitable sites have already been exploited in the EU, and the remaining ones may be less economic. Hence interconnecting existing reservoirs, exploiting abandoned mines (+141 GWh and +3 TWh, respectively, technical potential), modernisation and increasing storage capacity of existing PSH could be much more cost-effective.