Hydropower remains a flexible and stabilizing component of the energy system, offering substantial storage capacity through large reservoirs and regulated rivers. However, the increasing share of intermittent electricity production from wind and solar in Sweden and neighboring countries challenges Northern Europe’s storage capacity, highlighting the need for new technologies and innovative solutions to secure a sustainable electricity system. Pumped hydropower storage (PHS) is a highly flexible and efficient storage method, yet it is currently only applied at a few locations in Sweden, with an overall minimal installed capacity to date. This implies a potential for future utilization, and thereby a need to investigate the environmental impacts associated with its implementation in detail.

Thermal stratification is a key determinant of lake ecosystem health. Even small shifts in its onset or breakdown can significantly alter ecological processes, influencing nutrient and carbon recycling and, in turn, all higher trophic levels. To investigate how exactly PHS operations affect the temperature regime, a three-dimensional hydrodynamic model was developed to simulate diurnal thermal dynamics induced by pumping. Implemented in Delft3D FM, the model represents the upper reservoir of a PHS plant under initial conditions of thermal stratification. Paper A presents a methodology for setting up a generic model for Swedish PHS reservoirs, making it a versatile tool for comparative studies in Sweden and internationally.

Using this framework, Paper B numerically analyzes how PHS operations influence thermal stratification and examines how the initial thermocline phenology shapes ecological effects. It demonstrates that thermocline phenology may influence the ecological response of the reservoir by determining the extent of vertical mixing and thereby nutrient redistribution and oxygen availability. Further, in Paper C, a full factorial experimental design is employed to systematically assess the relative influence of initial thermal conditions, pumping characteristics such as flow rate, elevation and temperature and reservoir morphometry.

The results provide new insights into the thermal responses of PHS reservoirs and their potential ecological impacts on aquatic ecosystems. This understanding supports the identification of ecological worst-case scenarios and informs strategies for the design and management of PHS systems to minimize environmental impacts while enhancing energy system resilience.