Dunkelflaute

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.[6] As of 2022 there is no agreed quantitative definition of dunkelflaute.[7] Li et al. define it as wind and solar both below 20% of capacity during a particular 60-minute period.[8] High albedo of low-level stratocumulus clouds in particular – sometimes the cloud base height is just 400 meters – can reduce solar irradiation by half.[6]

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.[9] There are 2–10 dunkelflaute events per year.[10] 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.[11]

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),[12] while the latter is caused by arrivals of south-coast cyclones.[13]

These periods are a big issue in energy infrastructure if a significant amount of electricity is generated by solar and wind power.[14][15][16] Dunkelflauten can occur simultaneously over a very large region, but are less correlated between geographically distant regions, so multi-national power grid schemes can be helpful.[17] Events that last more than two days over most of Europe happen about every five years.[18] To ensure power during such periods flexible energy sources may be used, energy may be imported, and demand may be adjusted.[19][20]

For alternative energy sources, countries use fossil fuels (coal, oil and natural gas), hydroelectricity or nuclear power and, less often, energy storage to prevent power outages.[21][22][8][23] Long-term solutions include designing electricity markets to incentivise clean flexible power.[24] A group of countries is following on from Mission Innovation to work together to solve the problem in a clean, low-carbon way by 2030, including looking into carbon capture and storage and the hydrogen economy as possible parts of the solution.[25]

• Duck curve
• Variable renewable energy

• Li, Bowen; Basu, Sukanta; Watson, Simon J.; Russchenberg, Herman W. J. (11 October 2021). "A Brief Climatology of Dunkelflaute Events over and Surrounding the North and Baltic Sea Areas" (PDF). Energies. 14 (20): 6508. doi:10.3390/en14206508. eISSN 1996-1073.