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Experiment paves way for more reliable wind turbine design

Risø DTU has just concluded a large full-scale experiment on a wind turbine taking detailed measurements of the load distribution on a 40 m long wind turbine blade under natural wind conditions, thereby providing a very accurate picture of the wind flow over the wind turbine blade surface.

The project was spearheaded by Helge Aagaard Madsen, Research Specialist, and Christian Bak, Senior Scientist with Risø DTU’s Wind Energy Division, but would not have been possible without the four other partners, Vestas, Siemens, LM Glasfiber and DONG. The four partners have all made considerable contributions in the form of both financial resources and their know-how.

Consortium members

In addition to Risø DTU, members of the consortium behind the DAN-AERO MW wind turbine experiment in Tjæreborg are Vestas, Siemens, LM and DONG Energy which have invested approx. DKK 5 million in the project. Funding has also been granted by the Danish Energy Research Programme EFP-2007.

The wind turbine blade was built by LM Glasfiber and features 350 measuring points in the form of pressure sensors, microphones etc., all connected to a measuring laboratory at the root of the wind turbine blade.

Det Norske Veritas (DNV) reviewed the safety calculations for the wind turbine blade which, with all the equipment, was somewhat heavier than a normal blade, and found that the wind turbine should be allowed to operate at maximum wind speeds of 15 m/sec. And as the measuring points would tolerate absolutely no moisture, the experiment had to take place under perfectly dry weather conditions.

A total of 12 measuring periods were successfully fitted in between the end of June and the middle of September, resulting in a vast volume of data. With a view to processing these data, the consortium is currently applying for funding from the Danish Energy Agency under the Energy Technology Development and Demonstration Program (EUDP).

“Our measurements are by far the most comprehensive to date, and because they were conducted out in the open and on an industrial full-scale wind turbine, they take account of the impact of turbulence and blade rotation as well as elasticity. There is no doubt that they will be valuable for international wind energy research as a whole,” says Helge Aagaard Madsen.

“Moreover, we have - so to speak - been listening to the air flow across the blade using 60 microphones and recording 50,000 measurements a second, thereby obtaining an extremely detailed picture of how the wind is translated into load on the blades, i.e. looking at what lies at the very heart of utilising wind power.”

The experiment aims to provide a basis for designing the optimum wind turbine blade profile, striking the right balance between design robustness and sensitivity and ensuring that as much energy can be generated as possible and in as reliable a manner as possible.

Wind tunnel and analysis to follow

In November, more measurements will be carried out on copies of certain sections of the blade in LM Glasfiber’s wind tunnel in Lunderskov. This will allow the scientists to establish the difference between the properties of a blade profile on a full-scale wind turbine in the open air, and the properties of a similar profile under controlled wind conditions in a wind tunnel. This has been the main purpose of the project, and in future it will provide a more reliable basis for designing new airfoils and blades and ultimately the entire wind turbine.

The project will be concluded at the end of 2009, but hopefully a new project will start up next year.

“It is crucial that we find the funding needed to analyse all our data. Only then will all the unique measurements have any value,” says Christian Bak.

Several experiments rolled into one

As part of an ERP2007-II project, DELTA Akustik carried out noise measurements near the wind turbine, standing on the ground holding microphones to measure the wind turbine blade noise.

A laser-based wind scanner developed by Risø DTU was tested as part of the UPWIND EU project. The scanner makes it possible to carry out three-dimensional measurements of wind speeds, wind direction and turbulence around a wind turbine. On this occasion, the concept of sending a laser beam from the wind turbine and up against the wind underwent further testing.

Finally, as part of the TOPFARM EU project, another laser was used to measure the velocity distribution in the wake of the rotor. The data gathered are important because many wind turbines are erected as part of wind farms. Many wind turbines thus stand behind other wind turbines and are consequently exposed to the irregular wind patterns created by the wind turbine in front.

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