Through energy droughts: Hydropower's ability to sustain a high output

Journal article, 2025

Previous research has highlighted concerns about week-long energy droughts in renewables-based energy systems. Reservoir hydropower could offer a viable solution to mitigate such energy shortfalls. However, current energy systems models often oversimplify hydropower by assuming it can operate continuously at maximum output. This study investigates the ability of reservoir hydropower to sustain a high output and thereby mitigate energy droughts. In contrast to most energy system models, the hydropower model used in this study includes cascading, head dependency, turbine efficiency curves and environmental constraints. We estimate that Swedish hydropower can sustain between 77% and 96% of its installed capacity for one week, with the higher end of this range achievable during spring. This range in sustained output is equivalent to about 3 GW, or about 20% of average demand in Sweden, which underscores the importance of understanding the operational limitations of hydropower. Our findings indicate that river bottlenecks, primarily due to regulations on maximum flows, are the main factor limiting hydropower's ability to sustain higher outputs. With the upcoming renewal of environmental permits for hydropower plants in Sweden, these findings provide valuable insights for policymakers. The importance of analysing hydropower's ability to sustain high outputs is not unique to Sweden; the method proposed in this study can serve as a critical tool for similar assessments in other hydro-rich countries. Moreover, the sustained output capabilities demonstrated in this study challenge the prevalent simplified representations of hydropower in energy models, highlighting the need for more sophisticated modelling approaches.