Dunkelflaute

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]

A dunkelflaute of three days in Germany 2023 (wind in light blue and solar in yellow)

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.^[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][failed verification]}

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

These periods are a big issue in energy infrastructure if a significant amount of electricity is generated by variable renewable energy (VRE) sources, mainly 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 once 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 power which is available when needed.^[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]

Droughts

By analogy with hydrological droughts, long used in planning for hydroelectricity, the researchers of the future VRE-intensive power grids in the 2020s started using the term variable renewable energy drought (VRE drought or simply power drought) that is nearly synonymous to the dunkelflaute.^[24][25] Unlike the dunkelflaute, the drought can be a series of isolated adverse events, the most severe effects are forecasted are of this series type, and the planning for resource adequacy thus should span multiple years.^[26] Kittel et al. indicate the years 1996–1997 as particularly bad example of the VRE drought, they call for an additional EU-wide energy storage of 50 to 170 TWh (on top of current projections) to accommodate a series of events of this magnitude.^[27]

See also

• Duck curve
• Variable renewable energy