Dunkelflaute

In the renewable energy sector, a dunkelflaute (German: [ˈdʊŋkəlˌflaʊtə] , lit. 'dark doldrums' or 'dark wind lull', plural dunkelflauten) is a period of time in which little or no energy can be generated with wind and solar power, because there is neither wind nor sunlight. In meteorology, this is known as anticyclonic gloom.

Meteorology

Unlike a typical anticyclone, dunkelflauten are associated not with clear skies, but with very dense cloud cover (0.7–0.9), consisting of stratus, stratocumulus, and fog. As of 2022 there is no agreed quantitative definition of dunkelflaute. Li et al. define it as wind and solar both below 20% of capacity during a particular 60-minute period. High albedo of low-level stratocumulus clouds in particular – sometimes the cloud base height is just 400 meters – can reduce solar irradiation by half.

In the north of Europe, dunkelflauten originate from a static high-pressure system that causes an extremely weak wind combined with overcast weather with stratus or stratocumulus clouds. There are 2–10 dunkelflaute events per year. Most of these events occur from October to February; typically 50 to 150 hours per year, a single event usually lasts up to 24 hours.

In Japan, on the other hand, dunkelflauten are seen in summer and winter. The former is caused by stationary fronts in early summer and autumn rainy seasons (called Baiu and Akisame, respectively), while the latter is caused by arrivals of south-coast cyclones.