The expansion of renewable energies is progressing: in the first half of 2024, it ac-counted for around 62 percent of the electricity generated in this country. However, periods when wind and sun do not provide enough energy – so-called dark dol-drums – continue to be a challenge, also for research.

To ensure the energy supply even during the dark doldrums, gas and coal-fired power plants are still often used. They can start up quickly and thus compensate for missing elec-tricity supply deficiencies. Additional solutions are needed to deal with dark doldrums more efficiently and to reduce CO2 emissions. Researchers are currently developing various methods to keep the energy system simultaneously stable and environmentally friendly. With its Mission Stromwende (electricity transition mission) in the 8th Energy Research Program of the BMWK, the BMWK is striving to ensure that 80 percent of electricity comes from renewable energy sources by 2030.

In addition to adjustable power plants; storage systems or a management of the energy demand can also keep the energy system flexible – so that it is prepared for times when little energy is being fed into the grid from renewable sources. If it were possible to predict this, the situation would improve for both producers and consumers. This is precisely what the researchers in the LongCast project are aiming for. They are using forecasting models. Their goal: to estimate the electricity generated from wind and photovoltaics for up to six months in advance – and thus be able to identify the risk of dark doldrums.

Predicting energy bottlenecks

Dr. Malte Siefert from the Fraunhofer Institute for Energy Economics and Energy System Technology uses the example of the dark doldrums of December 11, 2024 to illustrate the advantages of forecasts: “On that day, electricity prices were very high at times. One reason: some conventional power plants were carrying out planned maintenance work or had technical problems. They were therefore unavailable. If the dark doldrums had been detect-ed early, the maintenance work could have been postponed. This could have had a price-reducing effect. Large-scale consumers such as steelworks could also have adjusted better to the situation if they had been timeously informed.” Reliable forecasts therefore benefit many players. Although, how do you arrive at a suitable forecasting model?

Hourly values have to be aggregated

The data fed into the model is fundamental to a reliable forecast. At LongCast, the German Weather Service provides such climate forecasts. It already provides the energy industry with hourly values that relate to a radius of five kilometers. “For longer-term forecasts, these data then have to be aggregated to coarser spatial and temporal scales, and the time series have to be averaged,” says Dr. Andreas Paxian of the German Weather Service. In this way, they could help predict the wind or PV (photovoltaic) energy yield for the coming winter or summer.

Stress test for the energy system

When there is a dark doldrum in Germany, electricity – preferably renewable – can be imported from neighboring countries. In the VERMEER project, researchers from the German Aerospace Center (DLR) and the Karlsruhe Institute of Technology (KIT) have analyzed the role that intra-European electricity trading can play in the event of supply bottlenecks. “We are investigating the consequences of large-scale extreme weather events in Europe, such as cold dark doldrums. In doing so, we are trying to assess the extent to which these can be absorbed by electricity trading, taking into account the grid-side transmission capacities,” says project manager Felix Nitsch from the DLR.

To do this, the scientists deliberately push the system to its limits: “We want to find out at what point it tips over. To what extent can a certain balance still be achieved through electricity trading?” says Nitsch. The team examined situations in which Germany cannot fall back on electricity imports from neighboring countries because they themselves are struggling with supply bottlenecks or that insufficient transport capacities are available for balancing.

This is helpful for policymakers when it comes to the issue of long-term security of supply. Furthermore, energy suppliers also benefit from the models developed in the VERMEER project. They can better estimate potential costs and capacities of back-up power plants.

Using hydrogen to compensate for dark doldrums

In addition to theoretical models, specific technologies also make an important contribution to a stable energy supply. These include long-term storage systems. Here, surplus electricity from renewable energy can be stored temporarily and fed into the grid when needed. A surplus arises, for example, when wind turbines produce more electricity than can be ab-sorbed by consumers or the grid when the wind is blowing strongly.

The living lab for the energy transition “RefLau – Referenzkraftwerk Lausitz” in Spremberg, Brandenburg, shows how to prepare for times of low renewable energy supply. In the future, the power plant will use electricity from wind and photovoltaic systems to produce green hydrogen, which will be distributed to various sectors such as gas networks or industrial plants (sector coupling). At the same time, energy from renewable sources can be stored in the green hydrogen produced. These reserves can be drawn on during the dark doldrums.

The reconversion is then carried out by the fuel cell of an innovative grid control power plant. “The power plant also serves to reconvert hydrogen to show that hydrogen can make an essential contribution to ensuring that green electricity is really available 24/7,” says Ben Schüppel, managing director of Referenzkraftwerk Lausitz GmbH. In the future, the plant in Spremberg should be able to operate at full capacity for a two-week period of dark and calm weather. Furthermore, even in less extreme weather conditions, the plant can provide balancing energy and contribute to grid stabilization.

Energy research contributes to a stable energy supply

Whether it's specific technologies such as storage systems and adjustable power plants or measures such as electricity trading, intelligent demand management and forecasting, there is ultimately more than one way to deal with extreme weather events such as dark doldrums. Applied energy research is continuously working to further develop the various solutions and combine them in a meaningful way. In doing so, it is making an important contribution to a stable and resilient energy supply in Germany.