Dunkelflaute

Dunkelflaute is a German term that is used in the energy industry to describe a period of multiple consecutive days in which low or minimal energy can be generated by renewable energy sources, such as solar or wind.

What is dunkelflaute?

‍Dunkelflaute is a phenomenon that can occur in late autumn or during the winter months for several days or weeks due to a decrease in wind and increasingly cloudy skies. This can lead to a decrease in the overall contribution of renewable energy to the electricity grid, posing challenges for maintaining a stable and reliable energy supply for an energy system that is powered solely by renewable resources. 

A study conducted in the Netherlands in 2021 found that almost all periods tagged as dunkelflaute events (with a length of more than 24 hours) in the North and Baltic countries occur in November, December, and January. On average, there are 50–100 hours of such events happening in this three-month period per year. The study found, however, that with a more interconnected power system and greater integration of renewable energy generation, the average frequency of dunkelflaute drops by up to 9%.

As the share of renewables in the energy mix increases, the need for such integration increases. Solar and wind accounted for 27% of the total public net electricity load in the European Union in 2023, up from 22% in 2022. This makes these two energy sources crucial to maintaining Europe's energy security. Even in unfavorable weather conditions, there are still ways to counteract a dunkelflaute, meaning its potential effects should not be exaggerated. 

Challenges in overcoming a Dunkelflaute

Grid stability

The reduced load on the transmission grid during a Dunkelflaute can compromise grid reliability, as conventional power plants have to compensate for the lower supply of renewable energy. The imbalance between supply and demand makes regulating frequency much more difficult.

Economic and environmental impact

Dunkelflaute also arouses environmental and economic challenges. Since renewable resources don’t perform at their full potential, conventional fossil-fueled power plants must often compensate for the loss in supply, which increases carbon emissions.

The increased reliance on fossil fuels can also negatively affect prices, since natural gas and coal are more expensive due to their higher production costs. 

As mentioned before, during a dunkelflaute, demand and supply must be balanced in order to maintain frequency regulation. In order to do that, grid operators need to invest in redundant capacity from conventional sources, which can increase operational costs. 

But there are ways to prevent these higher costs and emissions, making dunkelflaute less terrifying than one might initially think. 

Renewable Energy: Bringing light into the dark  

Alternative power plants 

Instead of relying on conventional power plants, there are also alternative plants that are powered by renewable resources:

Pumped storage power plants already absorb peak loads during regular operation. By pumping water to be stored when there is surplus power and releasing it to generate power in times of need, they are particularly important for abrupt balancing during an impending dunkelflaute.

Bioenergy power plants currently provide 6.7 GW of electricity generation capacity in Germany. By consistently focusing bioenergy on its role as a "gap filler", this flexible output could be massively increased. Bioenergy plants would then only produce electricity at times when the electricity grid is underfed with solar and wind energy.

To prevent supply shortages during a dunkelflaute, large-scale grid extensions that enable the transmission of renewable electricity across national borders would be optimal. This way energy can be traded over large distances without significant losses. However, this is incredibly resource-intensive in terms of time and money, meaning it is neither a viable short-term nor an economically reasonable solution to overcome dunkelflaute. This must, however, be considered in long-term grid planning.

Sector coupling refers to the integration and coordination of different energy sectors into one interconnected system. This way renewable resources are used more efficiently. It also allows more flexibility and ensures that the necessary share of renewables into the power mix. In the case of a dunkelflaute, energy from other interconnected sectors can be accessed without relying on conventional power plants and high emission technologies.

Power-to-gas is the conversion of electrical power into gases, such as hydrogen (power-to-hydrogen). This can be stored for short-term or long-term flexibility and used as a fuel to power industry, heat or transport. Power-to-hydrogen is especially effective in cutting fossil fuels in industrial applications, where temperatures above 500°C are required and makes it very useful in scenarios such as a dunkelflaute. It is also a key component of sector coupling. 

Batteries can be used to store excess energy from solar or wind power that can then be drawn upon when needed. Residential batteries and large-scale batteries are crucial to provide more flexibility to reduce solar curtailment and constant technological advancements, combined with increasing subsidies and government support, is making batteries more economical and efficient.

Demand-side-flexibilty, or demand-side management, is a collection of programs, policies and technologies that all aim to reduce the amount of electricity that is used by consumers during times of peak demand. This can take the form of financial incentives or load shifting. It is an important strategy that can be used to balance the supply and demand in times of dunkelflaute.

Variable tariffs, such as time of use tariffs, optimize power flows and minimize energy costs. Time of use tariffs pass on price signals to consumers, providing a strong incentive to shift consumption to cheaper and cleaner hours and feed into the grid when supply is lower. 

Beyond the hype

The name dunkelflaute might seem frightening but the situation is not as bad as one might think. Especially not when relying on smart decentralized energy assets. The way out of the dunkelflaute lies in intelligently integrating assets and technologies within a flexible and dynamic  power generation ecosystem. This approach mitigates the impact of poor weather conditions and establishes a more holistic energy infrastructure. 

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