Compounding effects behind renewable energy droughts in Europe

Power systems dominated by renewables are strongly affected by weather variability, affecting electricity demand and electricity generation across countries. In Europe, the interplay between electricity generation from sources such as wind, solar, and run-of-river hydropower, alongside electricity demand, can result in high residual load and associated renewable energy droughts (REDs). Concurrent REDs across multiple regions further challenge the increasingly interconnected European energy system. Understanding the compounding effects between energy sources and European regions is crucial to improving the reliability of power systems and reducing shortfalls. Here, we study such compounding effects during the season that is most affected by REDs by using weekly data of solar, wind, and river hydropower electricity generation and electricity demand derived from the PyPSA-Eur model forced with ERA5 weather data during 1941-2023 under present-day installed capacities. In the first step, by focussing on 129 small-scale areas in Europe, we find that wind electricity and electricity demand, including their interplay, are the primary contributors to REDs. Secondly, we explore spatially compounding effects by considering nine individual macro-regions, each composed of highly interconnected small-scale areas. Within each of the nine macro-regions, we find that anomalies in residual loads of small-scale areas compound to cause regionally aggregated REDs, particularly the most extreme REDs. Thirdly, in view of an increasingly interconnected European energy system, we inspect the interplay between shortfalls across the nine macro-regions, revealing that spatially compounding effects may enhance the risks for the energy system. The dependencies among residual loads of the nine macro-regions increase the probability of the regions simultaneously experiencing REDs. In addition, we find that the tendency of some macro-regions to experience REDs simultaneously increases extreme EU-aggregated shortfalls by 12% on average. This research underscores the need to consider compounding effects between electricity generation technologies and electricity demand across multiple regions in the design and optimization of electricity systems.