Lost flexibility: Hydropower capabilities in large-scale, Pan-European Energy system models

Hydropower is an important provider of flexibility, supporting the integration of intermittent renewable energy sources. Despite the need for accurate representation of these units, large-scale energy system models face challenges when modelling hydropower due to a lack of detailed data and standardised datasets. These issues typically risk overestimating hydropower's capabilities in terms of flexibility. To address this, we analysed historical hydropower operation to develop maximum generation limits and ramping rates, as well as forced minimum generation constraints, enabling the estimation of the amount and value of lost flexibility. At the European level, the lost flexibility could amount to 140 GW/h of lost hourly ramping capability, a reduced maximum generation capacity by 68 GW, and a forced minimum generation of up to 10 GW. When applying the constraints to the North European Backbone model, a pan-European energy system model, they exposed the limitations of aggregated hydropower representations that disregard historically observed behaviour in run-of-river and reservoir hydropower plants. The results showed that ignoring operational constraints could lead to underestimating system costs, electricity prices, and the need for other flexibility providers, such as storage, particularly for areas rich in hydropower. Our study highlights the importance of better representing hydropower in large-scale, aggregated energy system models. Overall, the hydropower constraints are especially valuable for studies on high penetration of variable renewable energy sources, storage adoption, and development of the pan-European transmission network. Energy system modellers can improve their work by applying the constraints or revisiting their existing operational limitations following our findings.