Complementarity between Solar, Run-of-River Hydro, and Wind Power in European Countries: An assessment from ENTSO-E data for 2016-2024

Effectively integrating variable renewable energy sources (VRES) into energy systems requires understanding their patterns and dynamics. Combining multiple VRES can take advantage of their complementary features, reducing energy storage needs, optimizing transmission infrastructure, and enhancing system reliability to meet energy demands.

This study assesses the evolution of complementarity at the country level between solar, run-of-river hydro (RoR), and wind (onshore plus offshore) power generation in seven European Union countries (Spain, France, Germany, Croatia, Italy, Poland, and Portugal) and the contribution of these VRES to load service during the period from 2016 to 2024. The metrics employed correspond to the stability coefficient (C_stab) proposed by Sterl et al. in 2018 and the total temporal complementarity index (k_t) presented by Canales et al. in 2020 since both allow the simultaneous evaluation of complementarity between three VRES. The study used public net electricity generation data from the European Network of Transmission System Operators for Electricity (ENTSO-E). Countries were selected based on the availability of information for the three VRES during the aforementioned period, and a heterogeneous spatial distribution. No Nordic country had enough information available for at least one of the 3 VRES; consequently, they were omitted. For C_stab estimation by country and year, the base VRES is defined as the one with the highest average production. The maximum annual generation serves as the basis for calculating capacity factors, but this is a limitation to acknowledge since generating units may be added or removed throughout the year.

Under these considerations, the C_stab and k_t metrics indicate that systems whose main VRES is wind power benefit from complementarity, improving their load-serving capacity. In 2024, Croatia and Portugal achieved a k_t value of up to 0.65 and mean C_stab values >0.40, suggesting strong complementarity. Croatia’s highest mean C_stab was 0.48 in spring, while Portugal’s peak was 0.44 in winter. Both countries had their lowest values in summer, with Croatia at 0.41 and Portugal at 0.33. For comparison, Portugal had a mean C_stab of 0.26 in 2016, the only figure above 0.20 in the dataset. Due to the lower daily-scale variation in RoR generation, the mean C_stab averaged >0.10 for France and Italy (where RoR predominates) during the study period. However, in Italy, wind and solar energy exhibit complementarity (C_stab >0.40), which is noteworthy as these sources are more viable options than RoR for increasing installed capacity in these countries. This trend is further supported by the evolution of the contribution of these VRES to meeting the load, which increased from a simple average of around 27% in 2016 to 37% in 2024. This growth was primarily driven by an over 500% increase in installed solar capacity in some countries like Poland and Portugal, while RoR has remained constant or declined.

This study contributes to better comprehending the integration of multiple VRES and how complementarity metrics can be included in practical applications and assessments, supporting global efforts toward decarbonization and sustainable energy development.

Acknowledgements:

• Canales:            ULAM NAWA programme Agreement: BPN/ULM/2022/1/00092/U/00001.
• Jurasz:                Narodowe Centrum Nauki Project: 2022/47/B/ST8/01113.