Wind turbine blades of 120 meters with Dutch technology 'Root bushing'

Wind turbines need to get bigger to make wind power truly competitive. However, the connection of the rotor blade is a limiting factor. This has consequences for the maximum dimensions of the rotor blades, or wind turbine blades. 'Root bushing', a Dutch technology, should change this. With 120-meter rotor blades, we will soon be able to generate power for 26,000 households.

The largest wind turbine in the world is on the Maasvlakte. With blades of 107 meters and a height of 260 meters, the colossus produces 12 to 14 megawatts of electrical power, enough for 16,000 households. The Danish Vestas is currently working on a 15 Mw wind turbine that will generate electricity for 20,000 households.

Can it get even bigger? Yes it can. Thanks to a technical innovation by the Dutch company We4Ce from Almelo, the 20 Mw turbine is within reach. It has rotor blades of 120 meters and can generate power for 26,000 households. Put a few of these together in a wind farm - at sea, for example - and a city like The Hague can be supplied entirely with wind power when the wind blows.

In wind turbines, it is the connections between the blades and the rotor that cause the most problems. After all, gigantic forces are exerted on this connection. "Traditionally, T-bolts have been used to mount the blades to the rotor," says TNO project leader Michiel Hagenbeek.

"To anchor the wind turbine blades to the rotor, 80 to 160 T-bolts are needed per turbine. The larger the blades, the more T-bolts. But there is a limit to that. We have now reached a point where no more T-bolts can be used. And so the blades cannot be any longer than they are now. This also reaches the maximum of the power that wind turbines can generate."

That's where We4Ce, founded in 2007, changed things. The company developed a different way to connect the blades to the rotor: "root bushing. The bushing is a serrated steel bushing that is anchored into the composite root of the rotor blade by vacuum infusion.

Managing director Arnold Timmer of We4Ce explains: "The advantage is that the bushing technology requires less space, allowing for 30 percent more bolts. More joints means that more forces can be transferred and therefore blades in future turbines can be larger. If you apply root bushing in current wind turbines, it means a significant weight and cost savings over the T-bolt connection."

Over the past two years, the bushing technology has been fundamentally developed and then tested in collaboration with TNO. Hagenbeek: "Testing the construction at full size is expensive and only provides knowledge of that specific combination of materials. We opted to test all parts of the structure separately. The advantage of these so-called coupon tests is that you can test one specific aspect of the structure.

Timmer adds: "For example, we looked at how the materials steel and composite relate to each other. To this end we performed shear tests, in which you pull on two sides of materials that are glued together. We also tested the lifespan of the structure. After all, a turbine has to be able to run for 20 to 30 years. In a test rig, we released enormous forces on parts of the structure. In total, more than 100 tests were performed."

Root bushing connects steel to composite, a construction that is not yet used very often and about which much knowledge still needs to be gathered. Timmer: "TNO is one of the few research institutes in the world that can test new materials and connections at a fundamental level. So TNO was an obvious choice. The test results have increased our understanding of the connection between steel and composite. The construction turned out to be even stronger than we had thought, which meant that we could use fewer bushings per rotor blade."

Hagenbeek: "During these tests, special cameras and software were used to accurately record the deformations and sequence of collapse of the steel-composite joints. The shape of the bushing - conical or straight - was also examined. Research showed that a trapezoidal shape gives the best results. This shape has since been patented. The root bushing is actually subjected to cyclic loads as the blades rotate. Various bending and pressure forces are applied to the construction. In order to determine the behavior of the bushing under these constantly changing loads, series of fatigue tests have been conducted. We then test the force at which the structure fails. The fatigue curves were then determined. These turned out to be considerably higher for the selected bushing concepts than for other constructions. In simple terms: they are considerably stronger.

The tests were carried out in such a way that the technology investigated anticipates the development of future wind turbines. Timmer: "We have tested techniques that not only enable larger blades, but are also interesting from a cost-price perspective. In a follow-up project, we will be testing even stronger connections, namely bolts with a diameter of 42 millimeters. It will be a few years before this new 'M42 bushing technology' is used in wind turbines.

The aim of We4Ce is to remain a leader. By testing at a fundamental level, we stay ahead of the competition in terms of knowledge. Foreign competition is another reason to work with TNO. We could also have had root bushing tested in Denmark or Spain. But Vestas and Siemens Gamesa are active there. We obviously don't want to share our knowledge directly with these competitors. So it feels better to work with a Dutch partner.

The maximum length of wind turbine blades of land-based turbines is now around 110 meters. Thanks to bushing technology, rotor blades can therefore become even longer. Why is that so important? Hagenbeek: "The power output of a wind turbine is determined by the surface area that the blades cover while rotating. That surface area increases quadratically with blade length. So a few meters longer blades yields a considerably higher yield.

Timmer: "This may be a sensitive issue socially. Not everyone is enthusiastic about large wind turbines in the landscape. But larger turbines are necessary to make wind energy even more competitive. The cost price must be further reduced. Of course, it's up to politicians to determine the right locations for mega wind turbines and wind farms."

Source: tno.nl