A new alternator is due to come online at Moneypoint coal-fired power station on the south-west coast of Ireland later this summer. However, a coal-fired steam turbine is not connected, nor is there any other unit that could be used to generate electricity. Instead, a 120-tonne flywheel, about four meters long and two meters in diameter, rests on the shaft and has to be set in rotation with electricity from the grid.
The 120-ton flywheel from the Siemens plant in Mülheim an der Ruhr is intended to stabilize the power grid in Ireland
The only task of the system is to keep “rotating mass” available and, if necessary, to provide so-called instantaneous reserve – i.e. power that directly counteracts frequency fluctuations in the network. Power grids are only stable if feed-in and consumption are in balance. An imbalance changes the frequency, in extreme cases this leads to a power failure.
Instantaneous reserve goes en masse from the grid
For decades, no one thought about instantaneous reserve. Because conventional power plants, with all the rotating mass in their turbine trains, supplied them in abundance practically free of charge. But now that these power plants are being phased out almost everywhere in Europe – and some other parts of the world – instantaneous reserve is increasingly becoming the focus of grid operators. A critical bottleneck is imminent.
“In particular, countries with smaller, isolated power grids and a high proportion of renewable generation – such as Ireland and Great Britain – are already affected by this and are taking massive countermeasures by installing flywheels,” says Stephan Werkmeister from Siemens Energy. The German company has already delivered flywheels to Great Britain, Australia, Italy and – the largest in the Siemens range – to Moneypoint in Ireland. “In the large European interconnected grid, the compensation options are greater.”
Germany has identified needs
The continental European interconnected grid stretches from Portugal via Denmark to Greece and, more recently, to the Ukraine. But the network operators in Germany already have the problem on their radar: In the “Network Development Plan for Electricity” from 2021, a plan that the four transmission system operators in Germany publish every two years under the supervision of the Federal Network Agency (BNetzA), a compensation requirement of 40 gigawatts to adopted in 2035. This corresponds to the output of 40 large coal or nuclear power blocks.
Due to its size, the continental European grid is more stable than smaller grids with a large proportion of renewable electricity generation, such as Ireland and the UK.
When asked by DW, the BNetzA was unable to determine how much instantaneous reserve has fallen out of the network so far. However, the remaining power has so far been sufficient to prevent a black case.
Bottlenecks can lead to widespread blackout
It could get serious, especially if a power grid splits into two parts. This is what happened on January 8, 2021, when a fault in a connection point in Croatia cut off large parts of south-eastern Europe from the rest of the European interconnected grid. A power failure could be prevented, among other things, because sufficient instantaneous reserve was available in both parts.
“In the middle of Germany, a system split could be less easy,” says Sönke Rogalla, who deals with the matter at the Fraunhofer Institute for Solar Energy Systems ISE. The scenario he is thinking of: If on a windy day the wind turbines in northern Germany supply power to the industrial plants in the south, a system split between them would cause a massive surplus of electricity in northern Germany and an equally massive shortage of electricity in the southern power grid.
Only a large amount of instantaneous reserve could then slow down the frequency change so much that the network operators have enough time to switch generation and consumption systems on and off in such a way that a new equilibrium is established. Otherwise, for example, industrial plants would switch themselves off due to safety mechanisms and thus trigger an uncontrollable chain reaction. “The most likely consequence would be a large-scale power failure, which could also spread to other countries in the European interconnected grid,” says Rogalla.
So far, renewables have not been able to provide an instantaneous reserve
The grid development plan stipulates that renewable generation plants should also provide instantaneous reserves in the future. According to the Federal Network Agency, regulations are currently being drawn up that could oblige operators to do so. Only: As of today, they can’t do that at all.
Even if the approximately 30,000 wind turbines have several thousand tons of rotating mass and theoretically several gigawatts of instantaneous reserve: “As of today, wind turbines do not react instantaneously to frequency fluctuations,” says Wolf Schulze, who is researching solutions at the Karlsruhe Institute of Technology.
Alternating current synchronous generators, as used in conventional power plants to generate electricity, react “instantly”, i.e. in real time, to frequency fluctuations and counteract them: “This happens physically inherently,” says Schulze. “A controller is not necessary for this.”
Improve wind and solar power
The DC generators in wind turbines do not do this. Before wind power can be fed into the grid, it has to pass through various converters and phase shifters that are controlled by software. The same applies to photovoltaic systems. Charging and discharging processes are also software-supported for battery storage systems. This process only takes a few hundredths of a second, but even that can be enough to cause the power grid to spiral dangerously in the event of major incidents.
Direct current from the generators of wind power and photovoltaic systems must first be converted into alternating current
“We want to develop software that simulates the instantaneous reaction and causes wind turbines to react accordingly,” explains Schulze. It is known that this is theoretically possible. Sönke Rogalla at Fraunhofer ISE has already shown that it works in practice – at least for photovoltaic systems: “Photovoltaic systems are particularly suitable for providing negative instantaneous reserves,” explains the researcher. This means: They reduce their feed-in as soon as there is a surplus of electricity in the grid.
It won’t work without a rotating mass
In order to upgrade renewable generation plants to the momentary reserve, they would have to be installed with new control software. For wind turbines, this would also mean additional mechanical stress, but this would only be problematic in exceptional cases, says Boris Fischer, who investigated exactly this at the Fraunhofer Institute for Energy Economics and Energy System Technology.
“Providing grid-stabilizing performance would not represent a critical load with everyday frequency fluctuations,” says Fischer. “Only in the case of rare grid faults, which result in large power deficits or excesses, can excessive mechanical stress on the drive train and tower occur in existing wind turbines.”
The Westfalen coal-fired power plant, which has since been shut down, has already been partially dismantled. One of the generators should remain connected to the grid without a turbine in order to have a momentary reserve
This is one of the reasons why the network operators expect that they will have to keep “real” rotating masses available for such major events in the future. However, conventional power plants are not required for this. In Germany, the synchronous generators of some decommissioned coal and nuclear power plants have already been left standing and are now running on the grid – without turbines. Continued use saves money and resources in the short term, but in the long term vacuum-supported flywheels like the one in Moneypoint should prove to be a more efficient and sustainable solution.