What is a Dunkelflaute?

Are Dunkelflauten an unmanageable obstacle to the energy transition?

In the dark and foggy months of winter, the media traditionally publish articles and reports on the Dunkelflaute. It is clear that these articles are not always based on verifiable energy facts but are also influenced by lobbying efforts, just as it is undeniable that the Dunkelflaute is a real phenomenon. Approximately every two years, there is a prolonged period of darkness that leads to corresponding supply bottlenecks. Flexible conventional power plants are currently used to bridge these gaps. In light of the German government's climate goals of achieving carbon neutrality by 2045, a decision must be made on whether to maintain the current solution or to enable renewable energies to manage Dunkelflauten independently.

The operators of conventional power plants, especially the coal lobby, are unanimous on this question: they believe renewable energies cannot handle Dunkelflauten. According to them, conventional large-scale plants are indispensable for ensuring the security of supply in Germany. Reflecting this line of thinking, the current regulatory framework allows operators, for example, to keep obsolete brown coal power plants on standby under the framework of "security readiness." This policy prevents the closure of brown coal power plants, which are otherwise uneconomical due to low electricity prices. In the event of a dramatic power shortage caused by a blackout, operators have ten days to restart their power plants. However, since blackouts are extremely rare and rarely last for such a long period, the stand-by option is certainly not the only way to bridge a blackout.

How often do blackouts occur?

A 2017 study by Energy Brainpool, commissioned by Greenpeace Energy, examined all blackouts that occurred between 2006 and 2016. The authors concluded that over this ten-year period, approximately every second year, there was a two-week period during which neither wind nor solar energy contributed significantly to electricity generation. During such times, the residual load - the portion of electricity demand not covered by renewable energies - was correspondingly high and had to be met almost entirely by conventional power plants.

Recent findings from a 2023 study conducted by the Karlsruhe Institute of Technology highlight that the frequency of such events can vary both within a year and between years. The researchers analyzed occurrences lasting at least 48 hours and found an average of 4.6 blackouts per year during the period from 1979 to 2018.

The authors also examined the months in which fluctuations in solar and wind power yields - and thus the probability of prolonged blackouts - are highest. Their findings confirmed the well-known seasonal balancing effects of photovoltaic and wind power: wind power generation is, on average, higher in the winter months, while photovoltaic power generation peaks in the summer. Additionally, they highlighted the increased variability of wind power generation during the winter months.

This situation is further exacerbated by generally higher electricity consumption in winter, combined with the growing electrification of heating systems. In summary, there is no doubt that the electricity system is becoming increasingly dependent on weather conditions.

Example: The Dunkelflaute in Winter 2022/23

Between December 2022 and March 2023, there were two extended dark periods lasting 30 and 54 days. It should be noted, however, that the duration of these Dunkelflauten also includes periods when the lull in wind activity was temporarily interrupted. In the second instance, the prolonged lull led to a significant depletion of seasonal reservoirs.

Another example is the Dunkelflaute in January 2017. Between January 16 and January 25, fog and a lull in wind activity prevailed across almost all of Germany. Wind and solar installations, with a combined installed capacity of 91 GW, fed only about 4.6 GW into the grid, while electricity consumption reached approximately 63.1 GW. As a result, conventional power plants had to cover the majority of Germany's electricity demand. On January 24, the share of conventional power plants in electricity supply even exceeded 90 percent.

Even during this extreme darkness, the German power grid was not at risk of a blackout - Germany continued to export electricity. The gas-fired power plants, designed for such emergencies, with a total capacity of 28 GW, were still far from utilizing their full capacity, operating at just 10 GW. Additionally, there is the option of purchasing grid reserves from abroad. However, critics argue that during a blackout in Germany, neighboring countries are often affected as well.

In January 2017, power generation from brown coal power plants showed little response to the blackout, nor did the nuclear power plants that were still operational at the time. The supply gaps were largely closed through the flexible use of gas and coal-fired power plants. Standby brown coal plants, which would have required ten days to begin feeding electricity into the grid, were not activated at all - they were unnecessary to address the blackout.

Renewable Energy Concepts for Bridging the Dark Period

It is widely acknowledged that the supply gap caused by the Dunkelflaute presents a significant challenge for the power grid and that renewable energies alone cannot yet fully address it. However, effective concepts already exist to protect against weather-related gaps in the power supply without relying on conventional power generators. The following list highlights some of these approaches.

By combining the various technical solutions presented, large portions of the energy supply can already be secured today in the event of a Dunkelflaute. It is crucial to note that no single approach can fully bridge every blackout. Only a combination of as many of these methods as possible can provide a comprehensive solution to the challenges posed by a Dunkelflaute.

Pumped Storage Power Plants

Pumped storage power plants are already used to absorb peak loads during regular operation. As the largest existing electricity storage facilities, they play a crucial role in quickly compensating for potential blackouts - a capability already in use today.

In the future, Norwegian and Swedish pumped-storage hydroelectric plants could assume a network balancing function for all of Europe, provided that efficient north-south power transmission lines are established.

Bioenergy

Bioenergy plants, which already contribute to bridging the gap between sunshine and darkness, offer considerable potential. Together, they provide 9.5 GW of electricity generation capacity nationwide (BNetzA, as of November 2024). By focusing on bioenergy's role as a "gap-filler," this highly flexible capacity could be significantly expanded without requiring additional land for cultivating energy crops. This could be achieved through measures such as overbuilding with additional CHP (combined heat and power) capacity, gas storage, and heat buffers.

In this scenario, bioenergy plants would produce electricity only when the grid is undersupplied with solar and wind energy, ensuring an optimized and sustainable contribution to the energy mix.

Run-of-River Power Plants

Run-of-river power plants with a total capacity of approximately 6.4 GW (BNetzA, as of November 2024) are installed across Germany. They continuously contribute to base load power supply by harnessing the flow velocity of rivers to generate electricity. However, due to varying water levels in rivers, there are seasonal fluctuations in power generation. Peak production typically occurs in summer, while electricity generation decreases in winter when water levels are lower.

The German Federal Environment Agency points out that Germany’s potential for hydropower utilization has largely been exhausted. This means that the expansion potential for hydropower has been mostly tapped, and the installed capacity has remained largely unchanged for several years. While run-of-river power plants cannot buffer short-term load peaks or the peaks of a Dunkelflaute, they represent a stable and reliable foundation in the renewable energy mix.

Backup Power Systems

Backup power systems, also known as emergency power systems, are installed in various locations, with output levels varying from region to region. While they are not suitable for permanent or climate-neutral power generation due to their high costs, they can respond quickly to short-term demand peaks.

Currently, 500 to 1,000 MW of backup power plants are already integrated into virtual power plants to help compensate for fluctuations in grid frequency.

Variable Electricity Rates and Demand Side Management

Consumer flexibility offers another approach to minimizing blackouts: variable electricity tariffs create incentives to shift electricity demand from expensive hours, when electricity is scarce, to cheaper hours, when electricity is more abundant. This approach not only benefits consumers economically but also enhances the stability of the entire power grid.

By consuming electricity when it is plentiful and inexpensive, and voluntarily reducing demand during periods of scarcity and higher costs - such as during a Dunkelflaute - consumers contribute to a more balanced and resilient energy system.

Hydrogen Power Plants (Power-to-Gas)

Power-to-gas (P2G) plants, which use flexible electrolysis to convert cheap excess wind or solar energy into hydrogen, offer significant potential. By converting renewable energy into hydrogen, electrolyzers perform two essential functions in power grids with a high share of renewable energy sources.

First, they enable the targeted absorption of excess electricity that cannot be utilized elsewhere. This capability is becoming increasingly important as the steady expansion of solar and wind power plants leads to more situations with favorable weather conditions where electricity generation exceeds immediate demand. As renewable energy sources continue to grow, the frequency of periods with a negative remaining electricity demand - when renewable generation exceeds consumption - is expected to increase. During these periods, electrolyzers can cost-effectively absorb surplus electricity and convert it into hydrogen.

Second, by producing hydrogen, electrolyzers act as a reserve for the entire energy system, similar to today’s natural gas storage facilities. The hydrogen produced can be stored locally, transported via hydrogen pipelines, or injected into the natural gas network. It can later be used to generate electricity in fuel cells or converted gas power plants during periods of low wind and solar generation. This makes hydrogen technology a vital tool for ensuring power supply during prolonged periods of low renewable energy output.

Germany’s energy policy strongly emphasizes hydrogen. According to the National Hydrogen Strategy, ten gigawatts of electrolysis capacity are to be installed by 2030, covering 30 to 50 percent of the nation’s hydrogen demand.

Battery Storage

Battery storage can contribute to mitigating blackouts, although only for a limited duration. Since their capacity is primarily designed for short-term stabilization, acting as "sprinters" to smooth out temporary fluctuations, they are more effective in shortening blackouts rather than bridging them entirely.

However, the total battery storage capacity in Germany has been increasing sharply for several years. This growth suggests that the combined battery network - comprising stationary large-scale storage systems, home storage, and e-mobility battery storage - will play a significant role in the energy economy. In 2023, the total capacity is approximately 1.4 GWh; by 2026, it is expected to rise to 8.6 GWh.

According to Gunnar Wrede of the German Federal Association of Energy and Water Management, this expansion is driven by the decreasing costs of lithium-ion batteries and the ability to amortize these systems without reliance on government subsidies.

Conclusion: The Dunkelflaute Can Be Overcome "Unconventionally"

The outlined strategies demonstrate that a combination of various approaches can eventually make the use of conventional energy sources unnecessary during dark periods. However, achieving this requires the consistent expansion of not only solar and wind power generation but also all flexibility options within the power grid. It is evident that immediately eliminating the use of conventional power plants to address blackouts is not feasible. Similarly, it is unrealistic to expect an entirely renewable energy supply in the very near term. Nevertheless, the ongoing development of renewable energy technologies will enhance the diversity and efficiency of alternative solutions. Additionally, blackouts can be predicted to a certain extent, which significantly aids in managing and securing a sustainable energy supply.

Germany’s power grid is set to phase out conventional power plants by 2045 to meet its climate neutrality goals. The transition away from brown coal and nuclear power is already in progress. While the nuclear phase-out has been completed, the share of inflexible brown coal power plants in electricity generation is rapidly being replaced by renewable energy sources, provided the grid expansion continues at a swift pace. The next step involves replacing coal-fired power plants with gas-fired power plants capable of running on natural gas and eventually green hydrogen. Political support remains crucial for implementing the presented supply security concepts.

Studies, such as those conducted by Energy Brainpool for Greenpeace Energy, the Technical University of Lappeenranta, and the Energy Watch Group, indicate that electricity supply security can be maintained using exclusively climate-neutral technologies. Research from RWTH Aachen University has further demonstrated that a higher share of renewable energy does not compromise supply security during dark periods. The key will be the appropriate design and implementation of the electricity system.

Disclaimer: Next Kraftwerke does not take any responsibility for the completeness, accuracy and actuality of the information provided. This article is for information purposes only and does not replace individual legal advice.