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© Wolfhard Scheer, Fraunhofer IWES
The Competence Center simulates 2 decades at sea
Projektinfo 15/2011
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The time machine for rotor blades

Rotor blades for wind turbines must be capable of withstanding extreme offshore conditions. Permanent loads, high wind and storms test rotor blades to their limits – and they have to withstand 20 years of turbulence. At the Competence Center Rotor Blades in Bremerhaven, blades up to 90 metres in length are tested mechanically. The globally unique tilt block enables the gigantic blades to be tilted while clamped in place and bent vertically by up to 30 metres. This enables blades to be statically loaded with around 180 tonnes.

The rotor blades for megawatt-class wind turbines are becoming increasingly larger. At the alpha ventus offshore wind farm off Lower Saxony’s North Sea coast, half of the wind turbines already have a rotor diameter of 126 metres. By way of comparison, the width of the A380 – the world’s largest passenger jet – measures just 80 metres. Rotor blades are large, complex and highly stressed components that must be capable of withstanding enormous loads for decades while remaining fully functional. High wind, storms and salty air strain all components. Visual inspections have been standard practice until now for identifying damage, whereby the rotor blades are examined in detail every two years. To achieve this, rotor blade inspectors abseil along the downwardly pointing rotor blades and tap them to find any possible weak points. External cracks can be visually recognised and inner damage to the blades can be acoustically determined. However, the weather conditions mean that it is only possible to conduct offshore inspections on just a few days during the year.

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Therefore in order to simulate permanent loads in rapid tests, construction began on the Competence Center Rotor Blades at the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) in 2007. In around 4 to 6 months, surfaces, smaller material samples (coupons) and beams are tested. In addition, entire rotor blades are examined in order to test their vibration characteristics (flexural strength) and the loads. “We’re also developing test and monitoring methods that show manufacturers how their blades behave under realistic conditions,” explains Dr Arno van Wingerde, Head of the Competence Center Rotor Blades at Fraunhofer IWES. The Competence Center has several beam test rigs, material climate chambers and two rotor blade test halls in which 70- and 90-metre-long rotor blades can be tested. A globally unique, 1,000-tonne mounting tilt block stands in the new, giant-sized hall.

The necessary test rigs for the material, component and rotor blade tests were developed and built as part of the InnoBladeTeC project. InnoBladeTeC stands for the “development of new testing methods and test rigs for rotor blades and their components as a substantial element of the Competence Center Rotor Blades”. The 70-metrelong rotor blade test rig has been in operation since 2007. The second rotor blade test rig, which is designed for blades up to a length of 90 metres, has been in use since June 2011 and is booked out until the end of 2012.

When a heavy goods vehicle drives up to the test hall, it is almost always loaded with a covered rotor blade. The blade manufacturers are only prepared to reveal the shape. “Since the test results are top secret and the blades do not have any protective coating, they are delivered in a packaged state,” reveals van Wingerde and adds: “Science is therefore unable to benefit from the test results – only the blade manufacturers and we at Fraunhofer IWES.”

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Addresses

Competence Center Rotor Blades
Fraunhofer IWES