Experimental blade research

Wind turbine manufacturers currently use special-designed wind tunnels, high-calculation capacity computer programs and engineers who, almost before the blade has been designed, are able to predict how it will behave down to the smallest detail. However, testing the finished full-scale blades in the most realistic way is still essential in order to develop the most reliable and cost-efficient wind turbine blades.

"In 2008, the research area 'Experimental Blade Research' received a DKK 3 million grant from EUDP for Phase 1 and has now received a total funding from EUDP of DKK 10 million for Phase 2, which will boost the entire research area considerably. Already in its first phase, the Risø-based project has generated excellent results, and ten external partners have chosen to join the project, including eight from the Danish wind turbine industry," says Find Mølholt Jensen who is heading the project.

The overall project objective is to jointly develop an experimental platform for testing the blades of the future as well as to develop new methods that provide designers with reliable tools for achieving the best possible results.

Combined loading

Until now, loads were applied to the blades by casting a clamp around the blade and then subject the clamp to vertical and horizontal pulling. However, this does not reproduce the actual loads to which the blade is subjected when in operation with enormous pulling and distortion on both the edge and the flap. Loads that could not be taken into consideration with the old test method, but with the newly developed method loads can now be applied to the blade in a far more realistic way.

"The new test method is based on more realistic loads being applied to the blade where we rotate/turn the blade and subsequently apply a combined pull to the edge and the flap. In this way, loads are also applied to, e.g. the glued joint between the shells, which is one of the known critical areas in the blade structure," Find Mølholt Jensen explains.

In operation, the wind turbine blade may be subject to distortion, but with the old test method, this blade deformation was prevented by the clamp as it encircled the blade, preventing transverse shear. To avoid using this blade-fixing clamp, the new method involves mounting steel anchor plates directly on the blade, allowing deformation and tilting of the blade when subjected to pulls.

Phase 1 solutions already reviewed

In Risø's Experimental Research Facility for Blade Structure, the 34-metre long blade is mounted on a base and load is applied by pulling the blade towards the floor using forces of up to approx. 10 tons. The method is already in use in the wind turbine industry.

The project also involved examining how to relieve some of the loading points on the blade which the new method has revealed. One of the tests involved making a simple coupling of two blade panels using a 'string'. This coupling, which has been developed at Risø, has proven efficient in preventing out-of-plane deformation of the panels. And by preventing blade deformation, the blade's life can be increased considerably. In Phase 2, more stress-relief methods will be tested.

New measuring techniques

A natural element of the development of the new experimental test method was the development of new measuring methods. Find Mølholt Jensen explains:

"During the project, we developed two new measuring methods. The first involves using an optical digital system to accurately determine even the slightest blade deformation. By making an area of randomly placed spots, you can subsequently compare the images from the two cameras filming the blade during load testing and see whether the spots have been displaced and in which direction. The other method uses acoustic emission to measure deformation and can thus help prevent fatal blade cracks during the actual test."

The optical digital method is also being used in the industry where several Phase 2 partners are either already using it or are planning to integrate the system in the near future. Risø's Materials Research Division has played a critical role in developing the new measuring methods, both in terms of knowledge and resources.

Ten external partners and unique knowledge from the aircraft and helicopter industry Vestas, LM Windpower and SSP Technology A/S were already involved in Phase 1, but no less than seven new stakeholders have chosen to participate in Phase 2 of the project, giving a total of ten external partners. The new partners are Dong, DNV, Blaest, Hexcel, Baumer, DLR and Swerea Sicomp.

Two of the new partners, DLR and Swerea Sicomp, are leading research institutions from Germany and Sweden, respectively, with core competencies within other composite structures than wind turbine blades, i.e. aircraft and helicopter wings.