Dunkelflautes last just a few hours (oops, days)

There was a dunkelflaute at the end of last month. “Dunkelflaute” is a German word, mostly translated as “dark doldrum”. It is a period in which there is no(t much) sunlight and no(t much) wind, resulting in very low electricity production from solar panels and windmills. The media told us that Monday November 29 was such a day.

This is what solar and wind did compared to total load:

Solar and wind indeed did not do well on that day. Electricity production by solar and wind was mostly hugging the x-axis for most of the day. The combined average capacity factor for solar and wind was a measly 1.53% (between 0.06% and 7.89%). It was particularly wind that did poorly. Luckily there was some sun to elevate the capacity factor, but even that was pretty limited because days are short at this time of the year at our latitude.

Looking at the days before and after November 29, solar and wind didn’t do much better on some days. Here an overview of November 27 until December 3:

Date (dd/mm/yyyy)	Demand (MWh)	Solar + Wind		Dispatchable		Capacity factor solar + wind
		Production (MWh)	Share (%)	Production (MWh)	Share (%)	Minimum (%)	Average (%)	maximum (%)
27/11/2022	204,080.53	61,661.33	30.21	142,419.21	69.79	10.48	22.69	30.21
28/11/2022	233,110.95	13,600.10	5.83	219,510.86	94.17	0.35	5.00	10.08
29/11/2022	261,790.41	4,149.07	1.58	257,641.34	98.42	0.06	1.53	7.87
30/11/2022	241,933.62	5,654.00	2.34	236,279.62	97.66	0.12	2.08	6.6
01/12/2022	241,958.15	13,803.95	5.71	228,154.20	94.29	2.02	5.08	10.44
02/12/2022	243,268.10	40,371.48	16.60	202,896.63	83.40	1.19	14.86	26.24
03/12/2022	227,727.42	39,371.59	17.29	188,355.83	82.71	8.43	14.49	24.27

The period of low production went roughly from Sunday November 28 until the morning of Friday December 2:

Depending on the definition one uses, this might even been four days of dunkelflaute.

How long do those periods of low production last? A quick look in the media for reporting on dunkelflautes showed that there was one in November 2021 lasting at least one day. There were also nine days of very low production spread over January 2017.
According to Elia (the Belgian grid manager), dunkelflautes can last between a few days to one or two weeks.

That is waaaaaay more than what our federal Minister of Energy was putting forward not that long ago. In an interview in a Flemish newspaper on November 22, she apparently seems to expect that dunkelflautes are very short-lived and therefor easily solved. She made this claim in her (non-)answer to the question of the reporter “What good are windmills when there is no wind?” (translated from Dutch, my emphasis):

The wind always blows somewhere in Europe, especially at sea. That is why we are establishing electricity connections with Denmark and Norway. There are also other sources, such as the sun, hydropower and batteries. I do not believe in the doom of the Dunkelflaute (a period in which little or no energy can be generated due to a lack of sunlight and wind, ed.). That can happen for a few hours, but you can solve that with battery storage. Don’t forget that we sometimes have too much energy today.

That didn’t age well. It took just one week for this balloon to pop. She apparently grossly underestimates such periods of low production by solar and wind in Belgium.

Sure, dunkelflautes can last a few hours and that could be solved, but they can also last several days to more than a week and that will not be easily solved, especially with an ever increasing share of intermittent power sources in the energy mix.

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