Two recent articles in trade magazines caught me up on efforts taking place within the wind power industry to reduce the noise levels of wind turbines. While it’s great to know it’s on everyone’s mind, it also appears that so far, noise reductions are modest.

In the July 2011 issue of North American Windpower (back issues not available for online reading, sorry to say), one of the feature articles was “Turbine Manufacturers Focus on Reducing Noise Emissions.”  It included discussions with reps from most of the major turbine manufacturers, and was full of fascinating hints of ongoing research.  As the article noted:

As the so-called “low-hanging fruit” of land with good wind and transmission access gets used up and wind turbines move closer to residential areas, noise concerns are expected to become more prevalent, according to wind turbine manufacturers.

“It’s on the top of the minds for all manufacturers,” said Paul Thompson, commercial director of Mitsubishi’s wind turbine group, “we’re all doing things to reduce the amount of noise that’s generated.”  GE’s Henrik Stiesdel stressed that wind turbines do “have a noise impact.  The main remedy is to ensure that they are not sited to close to dwellings. If that’s not possible because you are in a densely populated area, then we have remedies where we control the power output when conditions are such that noise might be exceeding limits.” The article describes this system:

GE’s sound power management (SPM) works by optimizing control setting based on real-time wind conditions, according to Sean Fitzgerald. The SPM option can be configured for day and night modes, at angular intervals depending on the mode switching and based on wind turbine placement. “These applications enable the customer to specify the appropriate, desired sound emission characteristics by customizing the sound power curve to the precise requirements over the entire wind speed range,” Fitzgerald tells NAW.

Gamesa’s Miguel Angel Gonzalez-Posada notes that there’s a trend of having to keep noise as low as possible near populated areas, especially at night.

Gamesa’s turbines, too, have controls to limit noise output, largely oriented around slowing the turbines as needed. “By doing this with the controls of the turbine, we can meet different criteria in the daytime as opposed to nighttime.  We can slow (turbines) down even more at night than we do in the daytime.  We can also selectively use that quiet mode on the turbines that are closer to receptors.” (Editor’s note: this article didn’t quantify the amount of quieting made possible by the GE or Gamesa operational adjustments; in one place I’m aware of GE experimenting with quiet operation, Fox Islands Wind Farm, the expected sound reduction was in the 2-3dB range, which is just barely perceptible. However, reducing the sound output by 6dB can double the distance at which turbines are heard at any given level, which in some situations could relieve nearby neighbors–for example, instead of homes at 1700 feet hearing 40dB, they would hear 34dB, with sound reaching 40dB only at 3400 feet.  So while current techniques are just starting to be perceptible, this line of research could make a difference if they are able to continue building on it. It’s should be noted as well that even a 3dB reduction would lead to a moderate increase in the distance that the 40dB would be heard at.)

Several manufacturers are working on new blade designs, as well.  Mitsubishi has a new blade prototype that’s currently being tested in Oregon, which both increases power and output and is quieter (I was unable to find details on their website). Siemens sometimes uses a sawtoothed trailing edge on the blade, which changes the frequency spectrum of the noise, reducing high frequency noise so those frequencies aren’t audible to as great a distance (Editor’s note: at times, it’s the lower frequencies – more clearly audible at a third of a mile and beyond and through background foliage sounds – which cause many of the community response issues). GE notes that its ongoing blade optimization efforts, including new tip designs and serrations, have increased annual energy production on its larger turbines by nearly 10% without increasing sound emissions.

This last observation reflects what is, at times, the primary focus: to increase energy output without increasing noise, efforts which are welcome of course, but which may not lead to significant reductions in noise exposure for future wind farm neighbors.  A fascinating interview with Fort Felker, director of the National Wind Testing Center at the National Renewable Energy Lab in Golden, Colorado, recently appeared in the July/August issue of Renewable Energy World.  A section looking at noise also framed the quest in terms of increasing output, more than reducing overall noise:

NWTC is looking to make advances in the aerodynamics of turbines and in reducing the noise associated with turbines. A less noisy turbine can spin a little faster.  That’s because tip speed is a fundamental parameter for turbine noise. “The lower (the tip speed) is, the less noise it makes, the higher, the more noise it makes.” Felker said that you want tip speed to be as high as possible with acceptable noise.

These efforts to increase tip speed pay important dividends: faster tip speeds allow the overall weight of the system to be reduced, which in turn can reduce the cost. “When we are investigating noise, we can actually take dollars out of the capital cost of the turbine so I think that’s a real important opportunity.”

For people living near wind farms and proposed wind farms, all this research effort to reduce noise levels may not make a dramatic impact, at least as long as the noise reductions are in service of larger or faster blades.  A new 2.4MW turbine designed by Nordex for maximum efficiency in light wind conditions (such as may be expected in many areas targeted for development, now that the “low-hanging fruit” has been picked) is also touted as being quiet, though it still has a rated acoustic power level of 105db, “allowing the turbine to be used closer to residential areas and facilitating optimal layout at wind farms,” as noted in the NAW article. Siemens, too, is proud of their 2.3MW turbine that is rated at 105dB, “one of the quietest on the market.”

Again, the “success” here is creating larger turbines that have generally similar noise output as previous-generation 1-1.5MW turbines.  So far, there’s no sign of a large turbine that has overall sound output of below 100dB; noise levels would have to drop well below 100dB to allow current siting standards of quarter to third of a mile setbacks to create minimal noise problems for nearby neighbors in quiet rural communities. Still, a hopeful note is struck by a slew of ongoing research and development efforts on turbine designs very different than the current giant rotors; it remains to be seen whether any of these will become commercially viable, but the potential is there that wind farms in 2021 may look – and sound – quite different than they do in 2011.