Quantifying the Dunkelflaute: An analysis of variable renewable energy droughts in Europe

Variable renewable energy droughts, also referred to as "Dunkelflaute'', emerge as a challenge for realizing climate-neutral energy systems based on variable wind and solar power. Using data on 38 historic weather years and an advanced identification method, we characterize European drought events for on- and offshore wind power, solar photovoltaics, and policy-relevant renewable technology portfolios. We show that drought characteristics heavily depend on the chosen threshold. Additionally, single-threshold analysis fails to detect heterogeneous drought patterns and therefore may lead to incomplete drought characterization. Using single thresholds, as common in the literature, is thus not advisable. Applying a multi-threshold framework, we quantify how the complementarity of wind and solar power temporally and spatially alleviates drought frequency, duration, and severity within (portfolio effect) and across countries (balancing effect). We further identify the most extreme droughts and show how these drive major discharging periods of long-duration storage in a fully renewable European energy system. Such events comprise sequences of shorter, contiguous droughts of varying severity. In a perfectly interconnected Europe, the most extreme drought event occurred in the winter of 1996/97 and lasted 55 days. Yet, the average renewable portfolio availability during this event was still 47% of its long-run mean. In individual countries, such events may last substantially longer and exhibit even lower average availability. For example, the most extreme storage-defining drought in Germany lasted 109 days and occurred in the winter of 1995/96. As extreme droughts may span across the turn of years, single calendar-year planning horizons are not suitable for modeling weather-resilient future energy scenarios.