Coping with the Dunkelflaute: Power system implications of variable renewable energy droughts in Europe

Coping with prolonged periods of low availability of wind and solar power, also referred to as "Dunkelflaute", emerges as a key challenge for realizing a decarbonized European energy system fully based on renewable energy sources. Here, we investigate the role of long-duration electricity storage and geographical balancing in dealing with such variable renewable energy droughts. To this end, we combine renewable availability time series analysis and power sector modeling, using 36 historic weather years. We find that extreme drought events define long-duration storage operation and investment. The most extreme event in Europe occurred in the winter of 1996/97. Assuming policy-relevant interconnection, long-duration storage of 351 TWh or 7% of yearly electricity demand would be required to deal with this event. As it affects many countries simultaneously, a storage capacity of 159 TWh or 3% of yearly electricity demand remains required even in the extreme case of unconstrained geographical balancing. Before and during Dunkelflaute events, we find complex interactions of long-duration storage with other flexibility options. Sensitivity analyses illustrate that firm zero-emission generation technologies would only moderately reduce long-duration storage needs. Thus, policymakers and system planners should prepare for a rapid expansion of long-duration storage capacity to safeguard the renewable energy transition in Europe. We further argue that including multiple weather years is required for weather-resilient energy system modeling, particularly those with pronounced renewable energy droughts.