Ensuring Safer Wind Turbine Blades

Thursday, May 5, 2011 @ 08:05 AM gHale

Wind turbine blades are big right now and they are getting bigger. The problem is before they go into service they need testing to ensure they are safe.

The largest wind turbines operating in the world today have rotor blades over 60 meters long – roughly equivalent to three truck-and-trailer units end-to-end. Within the next ten years, manufacturers of high-output wind turbines for offshore wind farms plan to produce blades up to 90 meters long.

The new facility will enable engineers to perform load tests on rotor blades measuring up to 90 meters.

The new facility will enable engineers to perform load tests on rotor blades measuring up to 90 meters.

Prototypes of these new blades need testing and certification before they can go into production, and that requires equally as large testing facilities.

That is where the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) in Bremerhaven comes in. In early summer IWES will launch a new facility with the capability of testing rotor blades up to 90 meters long.

This facility comes on the heels of an incident in North Dakota where two wind turbines sustained damage last week and remain idle until repair crews can reach them.

A turbine blade bent and needs replacement, said Nextera Energy of Florida spokesman Steve Stengel. That damage occurred late Thursday night or early Friday morning, Stengel said.

“We’re investigating its cause. We’ve got to get a crane up there. We’re in the process of finding a new blade to replace there,” he said.

Stengel could not say when a replacement blade could be found, but said the remaining 32 towers on the wind farm in Burleigh County would keep operating. The damaged tower feeds Basin Electric customers, but Nextera owns, operates and is liable for them.

The damaged blade was about .2 a mile from the nearest county road and .7 mile away from the nearest home, and does not pose a fall risk to residents or motorists nearby, Stengel said.

A separate tower’s blade about 15 miles south of Minot, owned by Basin Electric, also malfunctioned and bent over itself, said Basin Electric spokesman Daryl Hill. He said it happened Saturday afternoon during the blizzard and windstorm. They are still investigating the cause.

He said the blades should adjust to their angles according to wind speeds. Towers will stop rotating if wind speeds exceed 55 mph. “We don’t know why,” Hill said. “We’ll give it a new blade and determine the cause of the blade failure.”

He noted one blade measures 121 feet long and weighs 7 tons.

At the new test facility in Germany, the central feature will be a tiltable 1000 metric tons steel and reinforced concrete mounting block. Hydraulic cylinders used to tilt the block and additional cylinders to load the blade will allow the engineers to bend a rotor blade as easily as a finger to a blade of grass.

Preparations for testing begin by firmly mounting the flange of the rotor blade to the tilt block while the tip of the rotor blade projects upward at a slight angle. Loading cables attach at various locations along the blade and to hydraulic cylinders mounted on the floor. When all the cables and hydraulic cylinders are in place, the loading and tilting initiate. As the tilt block rotates 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 downward. This testing setup allows for the blade tip of 90 meter blades to bend through a distance of 25 meters.

Researchers and engineers will be building upon their prior rotor blade testing experience with the new 90 meter testing facility. A test facility for rotor blades up to 70 meters long opened at in Bremerhaven in 2009, but it does not have 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,” said Falko Bürkner, who leads 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 should be able to withstand, i.e. 100% of the design load.

The new test facility can also simulate the cyclic forces acting on the rotor blades as they rotate due to the force of the wind. The wind and in turn rotation of the turbine causes constantly changing forces to act on the carbon or glass fiber reinforced materials of the rotor blades.

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

They simulate this fatigue loading 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. A full series of extreme load and endurance tests takes around four months to complete. During this time, the blade undergoes the same loads that it would normally have to withstand in 20 years of operation.

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