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The Magazine

Onshore wind

Less noise – higher payoff

A new blade design, involving serrated blade add-ons with combed teeth, helps the SWT-3.3-130 Low Noise turbine run quietly and efficiently while avoiding acoustic pollution.

In the dead of night, a mouse scurries across a forest clearing. The dark emboldens it to venture into the open. A big mistake, as the small rodent briefly realizes when a sudden, even blacker darkness appears overhead and a silent predator swoops down to carry it away.

One attribute that makes the owl such an efficient hunter is the design of its wing feathers. While the leading edge of the wing is straight against the wind, the trailing fringe has a serrated structure that creates small vortices or swirls of air in flight. As a result, the sound of aerodynamic flows over the feathers’ edges is almost completely suppressed.
For the mouse, silence is deadly, bringing a sudden, even terminal decline in the quality of life. For humans living near wind turbines, the design of the owl’s wing feathers is now bringing noticeable improvements. One of the main challenges with onshore wind farms is the need to reduce the sound of the rotors for nearby residents. The issue is a well-known one: In offshore wind parks, acoustic performance is not an issue, but operation and maintenance costs are considerably higher than on land. And although onshore wind power generally enjoys widespread acceptance, those living close to turbines prefer lower noise levels.

Like the owl’s wings, the SWT-3.3-130 LN blade has a porous trailing edge. This reduces noise by over 10 percent compared to the previous industry standard.

This leads to turbines sometimes running at reduced power due to regulations. Therefore, any measures to bring down noise while maintaining power output are crucial for the operator’s bottom line. Of course it is possible to keep the acoustic signature down with a low-noise operational mode, running the turbine below its full generating capacity. As a rule of thumb, however, a reduction of operational noise by one decibel in noise reduced operation means that annual energy production is lowered by 2 to 4 percent.

Design emulates life
To bring down both the noise and the cost of onshore power, Siemens turbine blade designers have patterned a new design on the model of owl feathers – an approach known as “biomimicry” – that keeps noise levels down and allows greater energy output without exceeding sound thresholds.

This idea is not completely new; the Siemens-designed DinoTails have been in use since 2002. The predecessor of the new model had already featured that serrated edge for increased power output as well as substantially more quiet operation. The combined expertise of several Siemens research groups around the globe was brought together in a “Low Noise” team. Its task: to apply advanced new R&D methods for noise reduction across the entire onshore direct-drive fleet of turbines without impacting energy output.

Siemens turbine blade designers have patterned the new design of low-noise operation on the model of owl feathers – an approach known as “biomimicry.”

With the team’s innovation, the SWT-3.3-130 Low Noise achieves an even lower sound power level of 104.9 decibels at 6 m/s. The aeroelastic tailored blade design sets a new benchmark for use at all wind speeds in noise-restricted conditions. Generally, mechanical noise emanating from the gearbox or generator can be suppressed; the dominant source of noise stems from the aerodynamics of the blades themselves. Specifically, it is the trailing edge noise created by turbulence on the outer 25 percent of the blade that Siemens engineers have been working to eliminate.

Aerodynamic advantages
One way to do that is through the blade’s geometry. Aeroelastically tailored blades have a unique planform design for handling deflection or structural deformation from loads caused by sudden gusts of wind. That aerodynamic advantage helps to keep the noise down. Another innovative approach is the use of advanced turbine controls that can help to optimize blade pitch and rotor speed. The third element, inspired by the silent flight of the owl, is the use of blade add-ons.

The trailing fringe of an owl’s wing has a serrated structure that creates small swirls of air in flight. As a result, the flight of the nocturnal bird is almost silent.

In an ingenious example of industrial design emulating nature, the serration is combined with comb-like teeth to produce a porous trailing edge, reducing noise by over 10 percent compared to the previous industry standard. This had been set by the first-generation SWT-3.3-130, which won gold in the 3MW-Plus category of the “Turbine of the Year” award from Wind Power Monthly magazine in 2015 – based on the criteria of innovative technology, design and manufacturing quality, operational track record, market impact, and ease of installation and maintenance.

Silence please – testing…
During the development phase, wind tunnel testing on two-dimensional airfoil sections showed a measurable acoustic benefit of the trailing-edge serration under real-world conditions, i.e., using varying parameters for flow speed and angle of attack. Subsequent field testing confirmed these results when using downstroke and array methods for individual blades and the IEC standard method for complete rotors, explains Stefan Oerlemans, Key Expert Aeroacoustics at the technology department of Siemens Wind Power. Crucially, both in the wind tunnel and in the field measurements showed that the substantial reduction in noise at all wind speeds doesn’t affect annual energy production: “This structure, inspired by nature, improves the acoustics without compromising the aerodynamic performance,” Oerlemans explains.

This structure, inspired by nature, improves the acoustics without compromising the aerodynamic performance.
Stefan Oerlemans, Key Expert Aeroacoustics, Siemens Wind Power

The new combed-teeth blade add-on will largely replace Siemens’ first generation DinoTail for onshore wind turbines. Serial manufacturing of the Low Noise equipment for the new Siemens wind turbine SWT-3.3-130LN has already begun. The new DinoTail Next Generation will also be applied to the SWT-3.6-130 and the SWT-3.15-142 turbines.

Combed teeth are setting the new standard in the Low Noise series.

While residents in adjacent neighborhoods can thus be spared sound pollution, the new turbine should also give consumers and operators an edge in the market: Beyond the creative engineering of the blade, which allows power output to be preserved in restricted zones such as near residential areas, the optimized generator and a modularized component concept help to bring down the levelized cost of energy (LCoE) for wind power even further.

Christopher Findlay is a business journalist based in Zurich, Switzerland.
Picture credits: Siemens AG