As wind turbine blades become longer and more flexible, they also become more difficult to test for endurance. At the same time, test methods developed for smaller blades have become more expensive and less effective.
To test the new, larger blades, researchers at the US National Renewable Energy Laboratory (NREL) have developed a novel hydraulic resonance blade test system to replace a conventional system that uses a hydraulic actuator to push the blade up and down in millions of cycles for up to 4 months.
The new system contains a 317.5- to 453.5-kg (700- to 1000-lb), hydraulically actuated weight housed in a fixture attached to the end of the blade. The exact weight used depends on the size of the blade, and the weight is precisely controlled to oscillate up and down and excite the blade at its natural flap frequency.
It uses one-third as much energy as the conventional one does, and the blade oscillates at more than twice the conventional rate. It now takes less than 2 months to apply 3 million cycles to fatigue test a blade.
The system, which will test blades manufactured for giant multimegawatt turbines, is claimed to be the only one of its kind in the world.