Multi-threshold time series analysis enables characterization of variable renewable energy droughts in Europe

Variable renewable energy droughts, so called Dunkelflaute events, emerge as a challenge for climate-neutral energy systems based on variable renewables. Here we characterize European drought events for on- and offshore wind power, solar photovoltaics, and renewable technology portfolios, using 38 historic weather years and an advanced identification method. Their characteristics heavily depend on the chosen drought threshold, questioning the usefulness of single-threshold analyses. Applying a multi-threshold framework, we quantify how the complementarity of wind and solar power temporally and spatially alleviates drought frequency, return periods, duration, and severity within (portfolio effect) and across countries (balancing effect). We identify the most extreme droughts, which drive major discharging periods of long-duration storage in a fully renewable European energy system, based on a policy-relevant decarbonization scenario. Such events comprise sequences of shorter droughts of varying severity. The most extreme event occurred in winter 1996/97 and lasted 55 days in an idealized, perfectly interconnected setting. The average renewable availability during this period was still 47% of its long-run mean. System planners must consider such events when planning for storage and other flexibility technologies. Methodologically, we conclude that using arbitrary single calendar years is not suitable for modeling weather-resilient energy scenarios.

💡 Research Summary

This paper investigates the occurrence and characteristics of variable renewable energy (VRE) droughts—commonly referred to as “Dunkelflaute”—across Europe, using a novel multi‑threshold time‑series analysis. The authors compile 38 years of high‑resolution weather‑reanalysis data (1979‑2016) for 33 European countries (EU27 plus the United Kingdom, Norway, Switzerland, and the Western Balkans) and compute hourly capacity factors for on‑shore wind, off‑shore wind, and solar photovoltaic (PV). Rather than adopting a single, often arbitrary, drought threshold, they define a set of relative thresholds ranging from 10 % to 100 % of the long‑run mean capacity factor in 5 % increments for each country‑technology pair.

A drought is identified as a consecutive period during which the moving‑average of the capacity factor stays below a given threshold. The detection algorithm iteratively reduces the averaging window, first capturing the longest low‑availability spells and then progressively shorter ones, while allowing brief spikes of higher generation within a drought. This approach overcomes the limitations of earlier methods that either ignored such spikes or produced results highly sensitive to the chosen threshold.

The analysis reveals that drought characteristics (frequency, duration, return period, and severity) vary dramatically with the selected threshold. Low thresholds isolate extreme, short‑lived droughts; higher thresholds capture longer, moderate‑severity low‑availability periods. When wind and solar are considered jointly in a technology portfolio, the “portfolio effect” reduces maximum drought durations by 47 %–64 % compared to each technology alone, reflecting the seasonal complementarity of wind (high in summer) and solar (higher in winter).

Beyond the portfolio effect, the study quantifies a “balancing effect” by constructing an idealized “European copperplate” scenario in which all 33 countries are perfectly interconnected, allowing unlimited spatial power exchange. Under this assumption, the longest portfolio drought shortens by an additional ~65 %, especially for on‑shore wind, because wind deficits tend to be out‑of‑phase across regions. The balancing effect is weaker for offshore wind (limited to coastal nations) and weakest for solar PV, whose seasonal low‑availability is synchronized across most of Europe.

To identify the most consequential events, the authors introduce a “drought mass” indicator that combines duration, severity (how far below the threshold the capacity factor falls), and the number of affected countries. The most severe national event occurred in the winter of 1995/96 in Germany, lasting 109 days. In the fully interconnected European scenario, the worst event was in winter 1996/97, lasting 55 days with an average renewable availability of only 47 % of the long‑run mean. These extreme droughts align with the longest discharge periods for long‑duration storage (e.g., hydrogen, compressed air) in a fully renewable European power system, underscoring the need for substantial storage capacity.

The authors argue that using a single calendar year for scenario analysis is inadequate because extreme droughts often span year boundaries (e.g., winter 1996/97) and may be missed entirely in a limited time window. Multi‑year, multi‑threshold analyses provide a more robust basis for climate‑resilient energy system modeling.

Methodologically, the paper demonstrates that (i) drought identification must account for both short spikes of high generation and longer low‑availability phases, (ii) relative thresholds enable meaningful cross‑regional and cross‑technology comparisons, and (iii) the combination of technology diversification and spatial interconnection can dramatically mitigate VRE drought risk.

In conclusion, the study offers a comprehensive framework for characterizing VRE droughts, quantifies the benefits of renewable portfolios and European‑wide balancing, and highlights the implications for storage sizing, transmission planning, and policy design in the transition to a climate‑neutral, fully renewable European electricity system.