Status: 08/08/2022 08:12 a.m
Power generation from wind or sun is subject to large fluctuations. But the power grid must be stable – and so far there has been a lack of energy storage. How is the security of supply working?
The energy transition poses challenges for Germany. And at the latest with the throttling of Russian gas supplies, these are still growing. Becoming more independent and producing your own energy is becoming increasingly important.
According to the Federal Ministry of Economics and Technology, renewable energies will account for the largest share in the future, especially wind and solar power. The advantage: they produce hardly any climate-damaging emissions. The disadvantage: They are considered to be so-called fluctuating or volatile energy sources – that is, they never feed the same amount of energy into the grid because they are dependent on the weather, time of day or season.
Production and consumption in balance
However, a certain level of uniformity is very important for security of supply. Because if power consumption and power generation are not in balance, this would “lead to a gradual collapse of the power supply,” explains Ulrike Reimann, spokeswoman for Stadtwerke Saarbrücken.
In order for the network to remain stable, it must always remain on a certain frequency. In Germany, this so-called target frequency is 50 Hertz. Due to the expansion of renewable energies, the grid could get out of balance more often; After all, wind and sun cannot be planned.
Importance of electricity storage
To avoid imbalances, storing electricity is a much-discussed idea. If renewable energies temporarily produce more electricity than is consumed, the surplus could be temporarily stored and called up on days with less energy.
The Federal Network Agency ascribes great importance to such storage systems, but warns against overestimating their potential. Because the possible storage capacities are not sufficient so far. In less than half an hour, all German pump and battery storage would be empty if they had to take over the power supply.
In the overall view, however, they are still important, because there is no single solution to mastering the energy transition. Rather, a “mix of all technological options including electricity storage” is necessary, writes the Federal Network Agency in a report. In order to make power grids fit for the future, research into storage methods should be advanced.
Cushion fluctuations in the network
The so-called balancing power market also plays an important role. This is coordinated by the four transmission system operators in Germany (50 Hertz Transmission, Amprion, TenneT TSO and TransnetBW). They keep a close eye on electricity production and consumption and are responsible for keeping the grid stable. On the control power market, they advertise a reserve power every day, which is intended to cushion fluctuations – both in the case of overproduction and underproduction.
If there is a risk of the power grid being overloaded by too much electricity generated from renewable energies, operators of other flexible power plants can reduce their output and thus compensate for the overproduction. It works the other way around: if the frequency threatens to drop, flexible power plants can increase their output accordingly.
For the Saarbrücken public utility, for example, it looks like this: If more electricity is produced by wind power and fed into the grid than is consumed at a certain point in time, the public utility can shut down its combined heat and power plants and thus relieve the grid, explains spokeswoman Reimann. This allows operators of flexible power plants to generate additional revenue, also by increasing the output of their power plants to compensate for fluctuations.
District heating through excess electricity
In addition, municipal utilities can convert excess electricity into district heating. For this purpose, the energy is fed into so-called “power-to-heat” systems, which transform electrical energy into heat. This works, for example, with an electric boiler. “In this way we make an active contribution to the grid integration of renewable energies and achieve ecological added value,” says Reimann. Because by cushioning the fluctuations, the renewable energies can remain in operation without threatening grid stability.
Numerous other municipal energy suppliers in Germany, for example in Tübingen, Nuremberg, Bielefeld, Augsburg, Bremen and Frankfurt, are proceeding in a similar way.
Amount of excess power fluctuates
How much energy the public utilities can use from excess electricity for such systems is very inconsistent. The Saarbrücker Stadtwerke, for example, obtain “only a few hours a year” excess energy on the control power market for the electric boiler, says Reimann. Between 2018 and 2021, the amount of excess electricity fluctuated between 174,500 and 635,000 kilowatt hours. For comparison: According to the Federal Statistical Office, a single household consumes around 2000 kilowatt hours of electricity per year on average.
Nevertheless, the concept is effective: “Roughly speaking, we can say that with the heat produced in the electric boiler, we save around 1.25 times the amount of gas we use and the associated CO2 emissions,” says Ulrike Reimann.
In Germany, renewable energies were able to cover around 40 percent of electricity consumption last year. The proportion is expected to increase further in the coming years. Mechanisms such as the balancing power market help ensure security of supply even when there is no wind or on gray days.