Engineers seeking more efficient wind farms are looking to the wings of birds and insects, hoping millions of years of evolution will help power the clean energy revolution.
"Owls are known for silent flight, owing to their unique wing features,” said Professor Hao Liu of Chiba University, Japan, in a statement. "We wanted to understand how these features affect aerodynamic force production and noise reduction, and whether they could be applied elsewhere."
Owl wings feature leading-edge serrations and fringes on their trailing edges, both of which have evolved so that they can avoid alerting their prey. Liu created models of wings with and without leading-edge serrations and found that when the wing has an angle of attack to the air of more than 15º, the serrations smooth the transition between streamline and turbulent airflow. The result is less noise and better aerodynamic performance. However, below 15º, the serrated edges reduce aerodynamic performance along with the noise.
Owls use high angles of attack often enough to make the trade-off worthwhile. In Bioinspiration and Biomimetics, Liu proposes the same could be true for wind farms. By serrating the edges of their blades, wind turbines could become quieter at all angles and more efficient at high angles of attack. Aircraft rotors might experience similar benefits.
Measurements of noise from modern wind turbines (as opposed to earlier, failed designs) seldom produce readings high enough for serious disturbance. Nevertheless, the fact that onshore wind farms will be noisy is a commonly used argument against them, and sound-reduction technology might reduce local resistance.
The work follows on from a publication in Proceedings of the Royal Society A earlier this year of a comparison of three different blades for small windmills. One of these was a traditional steel blade, a second was highly flexible, while a third fell in between. The idea was inspired by the way insects such as dragonflies have wings that bend to minimize drag.
The most flexible blade didn't work well, but the moderately bendable version beat the rigid version by 35 percent in the power produced. These bending blades adapted the angle of their pitch to wind speeds, with lower angles at higher speeds, maximizing energy capture.
If this could be scaled up to apply to modern giant windmills, it would mean a third extra power per turbine, easily eliminating the price gap between wind and existing fossil fuel plants. As exciting as this sounds, replicating the success for commercial wind farms will be challenging. As first author Vincent Cognet of the Paris-Sorbonne University told Science Magazine: “We have to find a material which is flexible, but not too flexible” and can be used for giant blades.
Application may be easier for the smaller blades of off-grid wind generators.