Dunkelflaute

When I first heard about this phenomenon, I explained it to myself as occurring when there was a cold, cloudy, windless winter's day.  Which isn't too bad a quick descriptor.  But here is a more informative Wikipedia article: 

 

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

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]

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]

Renewables naysayers say that because of dunkelflauten, we can't use wind and solar to power our economies.  But let's have a look at how often they happen:  a maximum of 150 hours a year, or, about 1.7% of the time.  

Because battery storage is getting so cheap, we soon won't need to use gas peaking plants.  The problem with dunkelflauten is that they last much longer than the 4 to 6 hours when electricity demand exceeds supply, the sort of shortage that can easily be covered by batteries.  For now, until cost-effective long-term storage is invented, we will need to use gas to make sure the lights don't go out.  However, we can make synthetic natural gas from green hydrogen using the Sabatier process, or we may store hydrogen, which we can burn in gas plants for the 150 hours a year when we need it.  These won't add net CO2 to the atmosphere. 

Even if we used fossil gas to provide 100% of the power during dunkelflaute events, and renewables/nuclear the rest of the time, we could still cut emissions from power generation by 98%.  

Because we won't be using dunkelflaute gas plants for most of the year, the cost of their electricity per MWh will be high, because the cost of interest payments and depreciation will be spread over only a few hours of usage.  But, by the same token, taken over the whole year, the occasional high cost per MWh will be spread over thousands of hours of electricity generation.

Dunkelflaute is not an insoluble problem.  We can fix it, and still switch to renewables and cut our emissions.