owl in flight

Photo courtesy of: photophilde via Creative Commons

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Scientists at England’s Cambridge University, in collaboration with Virginia Tech, Lehigh and Florida Atlantic Universities in the US, have developed a new coating for wind turbine blades; inspired by owls, the coating reduces noise without adversely affecting performance.

Owls are capable of silent flight, thanks to their intricate feathers, which have what is known as a ‘flexible fringe’. This reduces aerodynamic noise and enables the owl to fly inches from its prey without being detected.

"Many owls – primarily large owls like barn owls or great grey owls – can hunt by stealth, swooping down and capturing their prey undetected," said Professor Nigel Peake, who led the research and works in the university’s Department of Applied Mathematics and Theoretical Physics.

"The structure of an owl’s wing serves to reduce noise by smoothing the passage of air as it passes over the wing, scattering the sound."

But what does this have to do with wind turbines? Approximately two years ago, scientists began to examine owl feathers in fine detail using high-resolution microscopic images. What they found led them to develop a prototype coating that mimics the owl’s wing structure.

By covering a wind turbine blade with a material similar to a wedding veil, the team was able to reduce surface noise by up to 30 decibels during initial tests.

"The structure of an owl’s wing serves to reduce noise by smoothing the passage of air as it passes over the wing, scattering the sound."

"We suspended this wedding fabric above a rough surface and listened to the difference it made to sound," says Dr William Devenport, professor and assistant department head for laboratory facilities at Virginia Tech.

"[However] we figured out there were some undesirable pieces to this fabric that did not exist in the owl, such as fibres that went sideways as well as along the flow."

From this, the team created a coating consisting of 3D-printed plastic.

When put to the test in a wind tunnel, this coating reduced the noise from a blade by ten decibels with no "appreciable impact on aerodynamics".

"We guessed 22 configurations and when we ran the experiment almost all of them worked," adds Devenport. "In some conditions these reduced the noise by a factor of ten, so it was a big surprise.

"Because we had taken the risk on running this full scale right away rather than trying to do something incremental, we were really surprised that it was that successful."

Reducing noise: a necessary challenge

Explaining why he believes noise made from wind turbine blades is a "major issue", Devenport says: "You want a wind turbine to be producing energy close to where it is needed so you don’t lose energy in the transmission, so ideally you would like them to be right on the edge of town… [But] the fact that they make noise – a noise that is irritating – clearly means that you can’t do that. They have to be set back from the residential communities so they are not bothering people. If you can make it quieter, that eliminates one of those constraints."

He adds that the coating could also allow wind turbines to spin faster, which would therefore increase the amount of energy they produce.

Daniele Ragni, assistant professor at the Aerodynamics Wind Energy Flight Performance and Propulsion department at Delft University of Technology in the Netherlands, agrees with this principle.

"Reducing and understanding the noise from wind turbines is indeed a challenge that we need to undertake, especially in view of modern wind turbine rotors, [which are] becoming larger, faster and more widespread," he says. "Lowering their aero acoustic impact might constitute the ultimate advance in their development, promoting a consistent reduction of the cost of energy for all of us."

Measuring the impact

This issue was highlighted in July in a study suggesting that the brain can register low frequency sounds below the level of normal human hearing.

Coordinated by the German National Metrology Institute, the study showed that infrasound produced by turbines as low as 8hz can be picked up by the brain.

According to a GE Reports blog post from last August, a wind turbine at around 300m away from a residential area is likely to have a sound pressure level of 43 decibels.

In the UK, the Department of Energy and Climate Change (DECC) commissioned a review in November 2014 to look at the level of sound from wind farms. The Institute of Acoustics is to support this effort, which could lead to changes in planning guidance for new wind farms and existing operations.

"Current regulations in the UK mean that the ‘swishing’ noise, caused by blades spinning, is limited to 43 decibels at night for the nearest property."

Current regulations in the UK mean that the ‘swishing’ noise, caused by blades spinning, is limited to 43 decibels at night for the nearest property.

A spokesman for the DECC said: "This review should empower local people to stop disruptive wind farms and make sure local authorities have all the information they need before giving a planning application the green light."

"There are currently areas in the world in which we cannot tap [into] their wind potential because of local noise regulations," says Ragni.

"[In addition] to the obvious and dramatic consequences that these effects have on wind farm developers and owners, ultimately the effect is felt as an increased cost of energy for the end user and translates to a poor capacity for wind energy to compete in the utilities market."

Quieter blades: a benefit for all

Following on from this, the benefits of reducing noise from turbines seem rather obvious.

"My guess is that if they are making more energy with the same wind turbine, then [if the coating] could be formed into some add-on product, that is profitable for them," says Devenport.

"Or, if they are in a position where they want to make their wind turbines quieter, this would be a relatively inexpensive way of doing it."

A reduction in noise could also lead to less local resistance. According to Ragni, by understanding the noise production of wind turbine blades, it is possible to find answers that will help wind energy become a "more accepted and widespread source of energy in a more environmentally conscious society".

For that to happen, innovations such as the prototype coating require further development, testing and support from turbine manufacturers and the industry, something Devenport is acutely aware of.

"One thing is that we have a patent application on this and we are talking with various companies about them possibly developing this, applying it to wind turbines and other applications," he explains.

"We’re hopeful of making this happen. There’s nothing specific [in terms of timescale] for these discussions, but if they are successful I think this can be quick [to get to market].

"I think it could be appearing on a wind turbine as soon as six months to a year after an agreement is made."