In the renewable energy sector, a dunkelflaute (German: [ˈdʊŋkəlˌflaʊtə] ^ⓘ, lit. 'dark doldrums' or 'dark wind lull', plural dunkelflauten)^[1] 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.^[2][3][4] In meteorology, this is known as anticyclonic gloom.^[5]

Meteorology[edit]

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^[update] 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]

Renewable energy effects[edit]

These periods are a big issue in energy infrastructure if a significant amount of electricity is generated by solar and wind power.^[14][1][15] 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.^[16] Events that last more than two days over most of Europe happen about every five years.^[17] To ensure power during such periods flexible energy sources may be used, energy may be imported, and demand may be adjusted.^[18][19]

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.^{[20][21][8][22]} Long-term solutions include designing electricity markets to incentivise clean flexible power.^[19] 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.^[23]

See also[edit]

• Duck curve
• Variable renewable energy

References[edit]

Sources[edit]

• 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.