Coping with prolonged periods of low availability of wind and solar power, also referred to as renewable energy droughts or "Dunkelflaute'', emerges as a key challenge for realizing decarbonized energy systems based on renewable energy sources. Here we characterize extreme European droughts and investigate the role of long-duration electricity storage and geographical balancing in dealing with such events, combining a time series analysis of renewable availability with power sector modeling of 35 historical weather years. We find that extreme droughts define long-duration storage operation and investment. Assuming policy-relevant interconnection, we find 351 TWh long-duration storage capacity or 7 percent of yearly electricity demand in the least-cost system that can cope with the most extreme event in Europe in our model, which occurred in the winter of 1996/97.
As countries are simultaneously affected, a storage capacity of 159 TWh or 3 percent 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. While nuclear power can partially reduce storage needs, the storage-mitigating effect of fossil backup plants in combination with carbon removal is limited. Policymakers and system planners should prepare for a rapid expansion of long-duration storage to safeguard the renewable energy transition in Europe.