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Experts disagree: Can the wind turbine's cast iron main shaft be replaced by carbon fiber?

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fredag 20. april 2012

The main shafts of the large wind turbines are targeted. As the wind turbines grow in power and size, many components are getting close to their physical limitations. This tendency forces designers to direct their attention to new technical solutions, thereby considering new sets of trade-offs.

By Kent Krøyer
An exciting idea is circulating in the wind turbine business. Might it be an advantage to use composite materials for the main shaft instead of the common choice of cast iron? The main shaft is the large, short driveshaft that holds the entire rotor hub and transmits all the violent wind forces to the large front bearing and further on to the gear box inside the nacelle.

Every pound counts
This shaft is heavy, and thus presents an attractive opportunity to save several tons of iron from the already immensely heavy nacelle. The demands for every new generation of wind turbines are increasing, and almost everything in it grows bigger and heavier, such as wings, gearbox and generator. But the mechanical loads on the steel tower tube from weight and wind is already massive, thus every pound counts.

Glass fiber and carbon fiber reinforced composites are already in use for axles in many ships, transmitting large torques to their propeller shafts, thus the technology is already proved with heavy forces. The advantages are not only weight reduction but also vibration damping. Both of these would be beneficial in wind turbines.

The German company Centa produces such axles, and it is already supplying the wind turbine market, e.g. with the fast spinning axle from the gearbox to the generator. But this axle transmits a hundred times less torque than the big, slow axle at the front.

Just a matter of defining forces
Centa CEO Lars Haarup is of the opinion, however, that it is possible to construct a main shaft for a large wind turbine from carbon fiber reinforced polymer (CFRP ). But no customer has ordered one yet.

“It is a matter of defining the forces that the shaft needs to be able to endure. Such a shaft will encounter forces in many directions, so it might need a larger diameter. This would mean that the front bearing should be bigger too,” he says. The vibration damping property would however not make a significant contribution to a wind turbine due to the axle’s relatively short dimension.

Ship designers include a rubber coupling in the ship’s drive train to reduce the vibrations more than the shaft itself is able to. But such a solution would be extremely big and expensive for a wind turbine, according to Lars Haarup.

“The most significant advantage of a composite shaft would probably be the weight reduction. But it would also be a more expensive solution, and that has to be weighed against the technical advantages,” he says.

A detail view of broken carbon fibers in a bar, seen through a microscope. Photo: Michael Bemmerl /Wikipedia

Vestas experts disagree
Nacelle Concept Engineer Niels Christian Olsen, Vestas, is skeptical about the whole idea.
“This would be a very high technology solution, almost space technology, to be used in a place where old fashioned cast steel can be used. It would add new uncertainties to the design. Perhaps it is possible to discard half of the weight, but such a solution would still have to be considered thoroughly. Plus the fact that carbon fiber is very expensive,” he says.

Senior expert, Anton Bech, Vestas Blades, is one the most experienced wing-engineers at Vestas. He thinks even less of the idea.

“Several of our wind turbines have a composite axle on the fast side of the gearbox, towards the generator. But I find it impossible to construct the short main shaft with carbon fibers, because these fibers are only strong in the length direction. It is possible to angle the fibers to make them withstand the torque and the bending forces, but it cannot be made strong enough against the transversal forces which are perpendicular to the axis, i.e. from the weight of the hub. It would take fibers in all directions and I don’t see how this could be produced,” he says.

A composite shaft will be built in Asia
Such a statement from an expert at the world’s biggest producer of wind turbines seems to be almost conclusive. End of story. However, not everyone agrees.

“It certainly is possible to construct the main shaft in carbon fiber composites. We have been involved in such a project until recently. Without mentioning the customer’s name, I can tell you that an Asian producer will market a carbon fiber composite main shaft for a 3 MW wind turbine in less than three years from now. The weight reduction is considerable. The steel axle weighed 13 tonnes, whereas the carbon fiber version weighs 4.5 tonnes. I think European wind turbine producers are too conservative,” says Sales Director Niels Immerkjær, Falck Schmidt ACE , who is an expert in composites.

At Vestas, Anton Bech is still not convinced. He wants to see the Asian axle in a real wind turbine, before he will be ready to change his mind.

“A similar problem can be seen if you would make a wooden axle, because wooden fibers also do not have strength in the cross direction. The sideways pressure from the bearings will damage the axle surface. A carbon fiber axle will need fibers in random directions to achieve the necessary strength and this is a compromise, which will reduce strength in the other directions. Steel has the necessary strength in all directions, because it is a crystalline material,” he says.

The debate continues. A recent announcement from the Chinese producer Envision says that its model E128, a 3.6 MW two-bladed direct drive offshore wind turbine, has a carbon fiber main shaft. The first will be erected this summer on the western coast of Denmark.

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