Flywheel storage system reduces material consumption and mechanical loads in wind turbines

Bigger, higher, more powerful – and heavier: the development of wind turbines only knows the upward direction. In 2012, according to an evaluation by Deutsche Windguard, a newly erected average turbine had an output of 2.4 MW and a rotor diameter of 99 meters. Ten years later, the output has almost doubled to 4.4 MW and the turbines have a rotor diameter of 150 meters.

“Rotor blades that are more than 70 meters long have to withstand enormous loads during operation,” says Professor Clemens Jauch from the Institute for Wind Energy Technology at Flensburg University of Applied Sciences. “Their weight is correspondingly high and the loads that have to be dissipated via the tower and the foundation are correspondingly high. And that in turn results in an enormous consumption of concrete and steel for the foundation and tower.” 1,000 cubic meters of concrete can be needed for a foundation, and several hundred tons of steel for the tower, which accounts for between 15 and 25 percent of the cost of an entire wind turbine.

Costs that are likely to be even more significant in future in view of scarce resources and rising CO2 prices. So how can material be saved without sacrificing stability or yield?

A team from the Flensburg University of Applied Sciences asked itself this question and, together with the wind turbine developer Aerovide (formerly Aerodyn) and the hydraulics manufacturer Hydac, developed a flywheel storage system that uses fluid masses in the rotor blades in such a way that the mass inertia of the rotor blades can be varied and mechanical loads on the tower and foundation can be reduced.

Originally, the scientists around Jauch had pursued the idea of ​​being able to provide network services with the help of flywheel storage. But over time, researchers increasingly focused on a secondary aspect: the resource efficiency that could be achieved through the dynamic fluid masses. “With our system, we can significantly reduce the loads in the supporting structure of the wind turbine,” explains Jauch.

It works like this: The flywheel accumulator in each rotor blade consists of at least two hydropneumatic piston accumulators, at least one near the blade tip and one in the blade root. The fluid volume in the system is adapted to the output of the wind turbine and can be around 900 liters of a water-glycol mixture per blade for a four-megawatt system. The storage tanks are connected via a line. Valves and pumps can be controlled by controllers in such a way that the liquid is shifted from piston accumulator to piston accumulator as required.

“Our condition was that we didn’t bring any additional masses into the rotor blades with the installation of the piston accumulators,” says Jauch, explaining the challenge. “That’s why they had to be saved elsewhere.” So the research team replaced some of the material in the belts of the rotor blades with the materials in the piston accumulators, which consist of a thin layer of steel and carbon-fibre-reinforced plastic. “The components of the flywheel storage system become an integral part of the rotor blade’s supporting structure, which means that it hardly becomes any heavier,” says Jauch.

And what is the use of all this in plant operation? “We can implement a whole range of load-reducing functions with our flywheel storage system,” says Jauch. “In this way we can prevent resonances, for example. That means we can prevent the vibrations of a rotor blade building up too much – and it tiring too quickly.” This is achieved by operating the flywheel accumulator depending on the speed. At a given speed, the natural frequency of the rotor blades can be changed by up to 14 percent with the displaceable mass of the flywheel storage device, so that this excitation does not lead to harmful vibrations in the rotor blades.

The advantages of the flywheel storage system become particularly clear when the wind turbine has to perform emergency braking, for example if the grid fails but the machine is running under load, says Jauch: “Emergency braking is an extreme load for a wind turbine.” On the one hand, the drive train accelerates, when the generator can no longer feed power into the grid. “The pitch angle over which the system is braked cannot be moved that quickly. This can lead to overspeed with dangerous centrifugal forces.” The second problem: Adjusting the pitch angle causes a large change in the aerodynamic thrust, up to and including thrust reversal. The flywheel accumulator can mitigate this: “We simulated emergency braking, once with and once without a flywheel accumulator, and were able to reduce the overspeed by eight percent and the change in thrust by 18 percent,” reports the professor.

Rotor imbalances, which in the worst case cause the turbine to oscillate laterally at the tower frequency, can also be avoided with the flywheel storage system. “Further applications include steadying power during operation close to the nominal power and reducing the negative damping of the drive train in full-load operation as well as reducing excitation due to gravitation and wind shear,” says Jauch.

However, the flywheel storage system is not a retrofit solution. The complete wind turbine is designed including the rotor blades and a corresponding control is integrated. “We want to start the second phase of our research project in the summer,” reports Jauch. The scientists then want to adapt the functionality and blade design for different turbine categories, from an offshore high-wind turbine to a typical “bread-and-butter” turbine to a low-wind turbine with a particularly high tower and large rotor. “We want to see which type of flywheel storage system brings the most savings. Then, based on concept designs, we can also say how much material is actually saved,” announces the professor.

However, the reaction of the plant manufacturers has so far been cautious. “Manufacturers are more focused on optimizing their existing system designs for cost savings,” says Jauch. But he is confident that the technology has a chance. The topics of resource efficiency and CO2 footprint are becoming increasingly important, not only for acceptance and profitability calculations, but also as award criteria in public tenders. “With today’s system technologies, there isn’t much potential for savings anymore,” says Jauch.

URINE