Wind of change

Researchers claim to have developed a new active damping system to cancel out the noise produced by mechanical components on wind turbines.

The technology, a collaboration between the

in Dresden and German industrial partners, uses piezo actuators and sensors to combat noise and actively adjust to changing frequencies.

The motion of rotor blades and vibrating cogwheels in a turbine's gearbox can emit clattering and whistling noises. These sounds can then be transmitted to the structure's tower, which acts as a loudspeaker and radiates the noise across a wide area.

For residents living near windfarms, this can sound like a constant hum. 'People find these monotone sounds particularly unpleasant, rather like the whining of a mosquito,' said André Illgen, Fraunhofer researcher.

According to the research centre, if the wind energy converters hum too loudly, local councils might demand that turbines only operate under partial load — which means they will not only rotate slower but also generate less electricity.

Despite all measures being taken to quell turbine noise, the team realised the effectiveness of current passive damping systems is limited because they only absorb noise at a certain frequency. Since modern wind energy converters adapt their rotational speed to the wind velocity to generate as much electricity as possible, the frequency of the humming also varies. This means the noise continues to penetrate the surrounding area.

According to Illgen, active damping systems react autonomously to any change in frequency and damp the noise — regardless of how fast the wind generator is turning.

The key components of the system, piezo actuators, convert electric current into mechanical vibrations that precisely counteract those of the turbine and cancel them out. The actuators are mounted on the gearbox bearings that connect the gearbox to the tower. The actuators adjust themselves to the respective noise frequencies by relying on sensors integrated into the system.

'They constantly measure the vibrations in the gearbox, and pass on the results to the actuator control system,' said Illgen. The researchers have already developed a working model of the active vibration dampers, and next they will perform field trials.

In recent years much attention has been focused on how wind turbines affect the aesthetics of a location, but the issue of noise has not gone unnoticed. Energy giant E.ON announced in July that it no longer intended to continue development of a 10MW, eight-turbine farm near Ferndale in Wales because of concerns that the project could potentially pose a noise nuisance to nearby homes.

Danny Shaw, head of new business for E.ON, said: 'We certainly didn't take this decision lightly but, as a responsible developer, we simply wouldn't be willing to build a scheme that we thought had the potential to exceed acceptable noise limits.'

The British Wind Energy Association has produced many reports on wind turbine noise, claiming that the noise produced is very low compared with, for example, road traffic.