A day without sun and/or wind is nothing out of the ordinary, so there's no reason for it to make the headlines. But with the introduction of more and more wind- and solar-powered generation units into electricity systems, things are changing. Enough so that the International Energy Agency has devoted several pages of its Electricity 2025 report to this phenomenon.
The threat of dark doldrum
The move away from fossil fuels to produce electricity is accompanied by massive investment in wind and solar farms. In so doing, we're back to being dependent on the laws of nature: day-night alternation, seasonal cycles, shifting air masses and clouds. This loss of control over electricity production is most severe in countries that do not have the decarbonized energy of nuclear power plants. This is the case in Germany, which closed its last nuclear units on April 15, 2023. It's hardly surprising, then, that the German word Dunkelflaute is applied to adverse weather conditions that reduce, or even cancel, electricity generated from renewables. In English, we speak of "dark doldrums" or “dark wind lull".
These episodes can be identified in two ways: on a local scale, by consulting weather data, and on a regional or national scale, by spotting sharp price rises on wholesale electricity markets. On these markets, generating units are ranked in order of increasing operating cost (known as merit order). So, if there is a shortfall in renewable production, the software that regulates the supply-demand balance will automatically call up plants whose production is controllable, but which are ranked lower in the merit order because they are more expensive to operate. These are often natural gas-fired power plants, whose price is known to be strongly affected by political instability in Eastern Europe. There have always been episodes of dark doldrums, but their impact is all the more keenly felt in Europe because they occur in winter, and their frequency and duration are difficult to predict. The IEA report mentions Germany in early November and mid-December 2024 (megawatt-hour price multiplied by 10), and the UK in early January 2025.
Short- and long-term solutions
Recourse to thermal power plants is obviously not the only way to respond to dark doldrums, especially as these episodes vary in duration and location. A momentary drop in consumption, importing electricity via interconnections and drawing on stockpiles complete the panoply. In all cases, adapted electrical and digital equipment is required, and therefore costly investment. Imports must come from unaffected areas and require high-voltage lines that are not saturated. The temporary drop in consumption is a politically sensitive subject, since it requires a sharp rise in prices during periods of gloomy calm to be effective, if not administered rationing. Rather than relying on fossil-fuel-fired power plants, since the aim is to introduce regularity into intermittent production, the solution that will tend to be favored is the storage of surplus production and its withdrawal during periods of anticyclonic gloom thanks to batteries and green "Power to Gas", i.e. the production (and storage) of hydrogen by electrolysis of water, then the withdrawal and transformation of the hydrogen into electricity in fuel cells. Based on a track record of dark doldrums, notably the European winter of 1996-1997, economists at the University of Berlin estimate that energy storage capacity representing 3% to 7% of annual consumption would be required
The calm before the storm?
In its Electricity 2025 report, the International Energy Agency notes that the recent bouts of dark wind lull have been absorbed without much damage to the power systems affected, and that even if there have been substantial price rises, they have been short-lived and therefore without any noticeable effect on consumer bills. It is with optimism that the Agency sees these falls in renewable energy production as tests of resistance, passed with flying colors. But the future is likely to be gloomier, given the growing share of wind and solar power in the energy mix, the increasing electrification of uses (notably heating and mobility), and the more frequent weather disruptions brought about by climate change. The risk is a lengthening of shortage periods over wider areas. A change of scale is needed to produce credible simulations for the future. All the more so as we are not immune to volcanic doldrums caused by the high-altitude projection of smoke and dust reducing solar radiation on a planetary scale and for periods much longer than a day. What caused agricultural crises, famines and epidemics in the Middle Ages and early 19th century would today trigger the collapse of our economies, such is our dependence on electricity.
