Flexible Serrations Help Lower Low-Frequency Noise While Maintaining Turbine Performance
The issue of wind turbine noise has become a pressing topic in recent years in many areas around the world, especially in Europe. The development of new wind power facilities balances the pursuit of open development areas with optimal wind resources with the practicality of grid connection by situating these facilities closer to areas of higher population. These two drivers are often in tension, and this has led to the development of projects with operation in noise-reduced modes, sacrificing power in the process. This is standard practice in the industry today. Aerodynamic noise has continued to increase as rotor sizes and rated power increases. Although serrated trailing edges have been around for many years in the wind industry to combat this increase in noise, innovation has stalled. At present, serrations deliver around 2dB(A) reductions from a bare blade, yet the situation of noise curtailment remains. Further, these technologies do not typically alter aerodynamic performance, although in practice they have been known to reduce it. As such, there is a desire for technologies that can reduce noise even further without sacrificing aerodynamic performance. The FeatherEdge technology offers a promising advancement in this area, delivering both a noise and power benefit, specifically targeting the low-frequency bands.
By Ryan Church, CEO, Biome Renewables, Canada
Traditional serrated blades represent the state-of-the-art, and further reductions in source noise levels were generally only thought possible in practice through the use of noise-reduced modes (SO modes), which reduce the tip speed and limit the output of the generator. For many projects, operating in one or more SO mode can be their only path to realisation. There is, however, a negative financial impact in doing this. Van den Berg et al. looked at 19 different modern wind turbines and noted that ‘the reduction in rated power is 4.8% per dB noise reduction, and for reductions up to 5dB both are strongly correlated’ [1].
A Few Decibels Makes all the Difference
The influence of different noise assessments on the viability, risk and generating capacity of wind farm developments is frequently underestimated. Seemingly small changes to environmental noise limits or assessment methodologies can translate into substantial lost renewable energy generation and development opportunities. Where noise is a constraint because of proximity to noise-sensitive locations, even a difference of 2 or 3dB, which is unlikely to be perceptible in a controlled test, could translate into an up to 40% reduction or enhancement of the energy-generating potential of a scheme. This is because increased separation distance is often the main mitigation measure available and, where the development area is limited, will translate into a reduced project scale. As such, developers will take serrated blades, even if this means that the aerodynamic performance is reduced somewhat, because the power penalties of the noise modes is far greater.
Sound Emissions: A Dominant Problem
With all of these factors at play, there has been a gradual encroachment of wind development around populated areas, which have tighter constraints on land availability, community setback requirements, and noise emission regulations, especially at night. Even geographies that were not previously considered to be noise-sensitive areas, such as India, are now starting to introduce regulations. These factors underscore a compelling need for noise-reducing technologies in wind turbines. Nowhere is this need more acutely felt than in Europe. Noise is now the number one technical factor for onshore wind affecting project viability and underperformance, whereas wake is the dominant factor in offshore wind.
If You Want to Reduce Turbine Noise: Go Low
In technical terms, noise is vibrational waves in the atmosphere. Noise originating from a wind turbine is comprised of a variety of frequencies and wavelengths. When standing 500 metres away from an operating turbine (a common setback distance), not all of these wavelengths will reach you. Many of the high-frequency waves dissipate (or are attenuated, to use the technical term) in the atmosphere. The higher the frequency, the faster the attenuation – the inverse is also true. You can also imagine the low-frequency calls of a whale travelling for tens of kilometres in the water. This is the physics of noise and goes straight to the point of why reducing low-frequency noise is so important. This has also been the hardest to reduce, and traditional serrations struggle to make any impact here. Yet, it is this part of the spectrum that matters most. It does not really matter what the IEC-validated noise level is, it is the spectrum that counts, because projects get built and funded based on what happens at the receiver. In some cases, noise levels can actually increase at the receiver with current serrations, even though the receiver is further away [1]. This paradox has led to many project owners finding themselves in hot water with local permitting authorities.
FeatherEdge: A Flexible Serration Technology
To combat these problems and address market need, Biome Renewables has developed the FeatherEdge serration (Figure 1), a flexible hardware technology inspired by the silent flight of the owl. It is applied to the trailing edge of the outer third of the rotor blade, each section being roughly 500mm wide. A unique collection of up to nine different sizes is specified and tailor-engineered for each blade type. The technology has been tested on a variety of turbine sizes through independent measurements and has been shown to consistently outperform traditional serrated blades. One key aspect of the technology is that it targets the low-frequency part of the noise emission spectra, which, as discussed, is what really matters.
Noise Testing
Recently, independent noise testing campaigns were conducted according to IEC 61400-12-1 on a Senvion MM92 in Canada and an Enercon E-160 EP5 E3 in Germany. In the case of the Enercon test, FeatherEdge reduced the average sound power level by 2.1dB(A) above and beyond the standard serration, but because the low-frequency content was significantly reduced (by as much as 6.8dB at 400Hz), the overall sound power level at the receptor was modelled to be reduced by 3dB(A). To put that into context, that is a 50% reduction. For the Senvion test, the baseline was a bare blade with no serrations, and a longer test campaign was carried out to assess the impact on turbine performance.
The Role of Flexibility
As the technology of FeatherEdge is flexible, it displays passive adaptive characteristics that change the blade shape with altering pitch angle. This is a complex and dynamic phenomenon that needs to be well understood. Through a series of wind tunnel tests at the Technical University of Denmark, WindGuard and OEM tunnels, the aerodynamic impact of FeatherEdge was slowly resolved for a number of blade profile types. This allowed a first prediction of yield improvement, but field testing was required to validate this. In addition, TÜV Nord certification was required to show that this would not hurt the turbine blade and that the technology could last the lifetime of the turbine. Data was collected for several years at the Nergica test site in Canada, where two operational Senvion MM92 turbines exist for testing.
Power Testing
Utilising a side-by-side, power-to-power method, FeatherEdge was applied to one of the machines and a protocol that followed the IEC 61400-11 method was adhered to as closely as possible. In the first phase, the controller was not altered in any way, and the machines were allowed to run as they normally would. This was to assess any native differences in power and loads, as impacted by FeatherEdge. At the time of writing, it is clear that FeatherEdge improved the aerodynamic performance of the turbine in a statistically significant and measurable way, with loads that do not exceed the turbine’s type certificate. Final values are being checked by TÜV Nord, but it is clear that FeatherEdge is delivering both noise and performance benefits, which can be easily accessed through a simple retrofit.
What Benefits Can This Deliver?
The FeatherEdge technology, as demonstrated through independent testing, has shown clear advancement as the state-of-the-art in serration technology. For one, we can now create compliance for turbines which currently exceed permitted noise levels, removing power-reduced noise modes or even shutdowns. We are also able to change from noise- and power-reduced mode(s) to a higher or normal operational mode for turbines currently in compliance by using the additional ‘noise budget’ provided by FeatherEdge. Retrofitting existing wind farms is also possible, even when they are not running in any SO mode, but where the additional ‘noise budget’ provided by FeatherEdge can be used to add additional turbines. Land leases and a grid connection are in place already and there is minimal cost in using/sharing existing infrastructure and grid connections. Repowering of existing wind farms is another topic which provides greater turbine selection flexibility and overall MWh production. Lastly, increased turbine density in greenfield development is possible with operation in higher or normal mode(s), including optimisation of all available modes. A study was completed by Aercoustics Engineering which looked first at a baseline wind farm with two turbines. The addition of FeatherEdge meant that three turbines could now be installed, effectively increasing the yield by 33% and with a lower total noise output of 39.2dB(A) compared with 40.4dB(A) (Figure 2).
What’s Next?
Biome Renewables is currently working with a number of developers and OEMs as this technology is now in market. With increasing implementations, validation grows and the industry can look forward to a future of quieter turbines, greater community acceptance, and higher project profitability.
Further Reading
- Van den Berg, F., Koppen, E., Boon, J. and Ekelschot-Smink, M. 2025. Sound power of onshore wind turbines and its spectral distribution. Sound & Vibration 59(1), 1716.
Biography of the Author
Ryan is an entrepreneur, designer, biologist and inventor. As the founder of Threshold Theory, he uses physics-first principles to understand the world and develop a wide range of products and ideas, from new wind turbine technology to new equations that describe fluidic phenomena. He founded Biome Renewables to commercialise biomimetic energy research and technology and develop novel solutions to today’s renewable energy shortfalls.
