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Test facility for 90 m wind turbine blades

A new facility capable of testing wind turbine blades up to 90 m in length will be inaugurated at the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) in Germany this June.

The central feature of the test facility will be a tiltable 1000 tonne steel and reinforced concrete mounting block. Hydraulic cylinders used to tilt the block, and additional cylinders to load the blade, will allow the IWES engineers to bend the blade tip of 90 m blades through a distance of 25 m.

For testing, the flange of the rotor blade is mounted to the tilt block while the tip of the rotor blade projects upwards at a slight angle. Loading cables are attached at various locations along the blade and to hydraulic cylinders mounted on the floor. When all the cables and hydraulic cylinders are in place, loading and tilting are initiated. As the tilt block is rotated and the tip of the blade begins to move upward, the rotor blade bends as its upward movement is counteracted by the loading cables pulling downwards.

A test facility for rotor blades up to 70 m long was opened at IWES in Bremerhaven in 2009, but it is not equipped with a tiltable mounting block.

“The big advantage of the hydraulic tiltable mounting block is that it allows us to set up the ideal configuration for conducting tests with high precision in a minimum amount of time,” says Falko Bürkner, leader of the rotor blade testing team at IWES.

This is especially important when testing the blades under extreme conditions, which involves subjecting the blades to the maximum theoretical loading it has been designed to withstand, i.e. 100% of the design load.

The new test facility will also be able to simulate the cyclic forces acting on the blades as they rotate due to the force of the wind. The wind causes constantly changing forces to act on the carbon or glass fibre reinforced materials of the blades.

“There is no other engineered rotating system in which the materials must support so many changes in the load parameters,” notes Bürkner.

This fatigue loading is simulated in the new facility by a hydraulic cylinder which alternately pushes and pulls on the blade, causing it to vibrate at its horizontal and/or vertical resonant frequencies.

90 m blades on the way
The IWES explains that the largest wind turbines operating in the world today have rotor blades over 60 m long. Within the next 10 years, manufacturers of high-output wind turbines for offshore wind farms plan to produce blades up to 90 m long. The prototypes of these new blades have to be tested and certified before they can go into production, and that requires equally as large testing facilities.

A full series of extreme load and endurance tests takes around 4 months to complete. During this time, the blade is subjected to the same loads that it would normally have to withstand in 20 years of operation.

“We have a full schedule for the next twelve months – and that is even before the new test hall opens on June 9, 2011,” Bürkner reports.

 

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