How does a flywheel turn

Old storage technology: start-up is rediscovering the flywheel

Whether lithium batteries or pumped storage - electricity storage has become an indispensable part of our society. They ensure that the laptop or the exemplary solar region still has electricity in the dark.

The flywheel accumulator, on the other hand, is hardly known - one was already installed in Carl Benz's old three-wheeler that started the engine in 1886.

Flywheels are usually too expensive and require a lot of maintenance. Because the principle is: the heavier the better. In other words: If the weight is doubled, the energy to be stored is doubled. But what drives up the costs.

The young Jülich technology company Stornetic (only founded in 2013) therefore came up with a trick. Instead of making it heavier, they made the flywheel faster. Which has several advantages. Spokesman Tobis Gottwald explains the most important ones with simple physics: "If the flywheel rotates twice as fast, it can store four times as much energy."

750 revolutions per second

The rotor, which weighs around 60 kilograms, can run at up to 45,000 revolutions per minute. Carbon fiber is used for this, which is relatively strong compared to steel, but much lighter. "The high speeds are achieved because the rotor is in a vacuum," says Gottwald, explaining the principle. The case is made of steel again, for safety reasons.

In the flywheel, electrical energy is converted into kinetic energy and stored. If the electricity is to be fed back, the electric motor functions as a generator and thus generates electricity again. In the short term, outputs up to the megawatt range are possible.

The advantage over the conventionally used lithium-ion batteries are the high charging cycles. The flywheel storage system from Jülich is designed for 100,000 such charges without affecting the capacity. "It doesn't run down as quickly as a battery," explains Gottwald. In the case of electric cars, the range drops significantly after 3000 to 5000 times of charging and discharging.

Swing memory for on the go

Car manufacturer Volvo finds the idea of ​​a flywheel so convincing that it started a research project to test the storage system in a mid-range vehicle. An idea that was previously unthinkable if you consider the weight of the systems.

And Stornetic is also initially aiming for stationary use - for example in wind farms. So far, the storage of electricity has played no role, as the EEG does not support this. "The volatility of wind power will present wind farm operators with major challenges in the future," says managing director Rainer vor dem Esche, however, seeing an opportunity for his storage facility.

"We help the wind farm to plan the power output better and to make it more predictable," added Gottwald. This opens up completely new business areas for wind farm operators, such as grid services.

The Stornetic storage has an output of 60 or 200 kilowatts and can be built up modularly as required. A more powerful machine is currently being developed. The storage facility is not only interesting for island networks; from Gottwald's point of view, trams and subways are also a big market. They like to store the energy that is released when braking for the power-guzzling start. Capacitors called supercaps are currently used for this purpose.

Siemens is also researching flywheels

Matthias Gerlich from Siemens Corporate Technology also believes that flywheel-based systems are a sensible alternative to batteries. "They provide a lot of power for a short time, need little space and no air-conditioned room."

Gerlich and his team are currently testing a prototype that can provide 125 kilowatts for 15 seconds. It is based on a steel flywheel that weighs 260 kilograms and rotates magnetically at 9000 revolutions per minute in a full vacuum. "Thanks to the magnetic bearing, the wheel floats and is practically maintenance-free," explains Gerlich.

Inexpensive standard components, low-maintenance design, low operating costs - the flywheel storage system from Siemens could soon compete with the supercaps. Although these can provide power for a very short time, the number of charging cycles is limited.

The scientist sees areas of application as a backup system in the event of power failures, decentralized networks or for electromobility. "A mobile use is currently not planned, but in principle not ruled out."

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