Active shape control -- Blade theory

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As part of a EUR 20 million EU project to speed up development of huge offshore wind turbines with rated capacities of 15-20 MW, Finland's technical research institute is reporting good results with a new composite material for rotor blades. Valtio Teknologia Tutkimus (VTT) in Helsinki, a state funded body, is charged with developing stronger, more durable and flexible materials for rotor blades without adding significant weight to those in use today. The aim is to increase the stability of large rotor performance to allow a fourfold increase in generating capacity.

VTT is experimenting with changing the shape of composite reinforced plastic by use of an electrical current. "Blades that change their shape reduce the vibration stress imposed on them which in turn makes it possible to use longer blades and increase the annual capacity of the plant," says VTT's project manager, Esa Peltola.

Reducing vibration stress and ensuring the lifting force of blades can be kept constant in different wind speeds is a key element for success, VTT says. The main focus of the research is in applying what it calls active shape control techniques, where the shape of a blade is altered when the wind speed changes or the blade rotates past the tower. Essentially, this involves embedding shape memory metal wires in the blades. When the temperature of these wires is increased to approximately 60 degrees Centigrade using an electricity current the shape memory metal alloy transforms the blade into the required optimum shape.

"This project is the largest-ever development project in the field of European wind power technology," says Peltola of the EU initiative. "Such technology can make it possible to operate wind power plants in stronger wind conditions which in turn can increase annual production time." The design and implementation of a scale-model two metre rotor blade made of the new material is underway.

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