Over the past several years, wind farm developers have been facing increasing complaints about wind turbine noise. After many years of successfully placing turbines in proximity to homes in farm and ranch country, where typical setbacks of 800 to 1,200 feet, and noise levels of 50dB or more, were well tolerated, wind energy companies are finding that residents in rural areas in the upper Midwest and Northeast are far more likely than farmers and ranchers to respond negatively to turbine sound of 40dB or even less. For some in areas where nighttime ambient noise levels are low, any audible noise is found to be intrusive, creating a challenging new reality for wind energy to come to grips with.

A variety of siting, community engagement, operational and technological approaches are being brought into play in response to this new challenge.

On the siting and community engagement front, after initially considering the new wave of noise complaints to be mostly a surrogate for anti-wind sentiment, most developers have moved to a more open stance, in which they acknowledge that turbine sound will be audible at times and may cause annoyance among some residents. As complaints have spurred more comprehensive sound assessments and more complex sound modeling and prediction techniques, there appears to be a gradual move toward recommending somewhat lower sound exposures at residences.

While 45-50dB has long been considered an acceptable criteria, well-regarded mainstream acoustical consultants have moved toward a 40dB recommendation (Ldn: averaged over the full day and night); while some more cautionary acousticians recommend 30-35dB, at least at night. A 40dB, Ldn criterion tends to require setbacks of a half mile or more, still problematic for many proposed wind farm sites.

In concert with more proactive engagement with communities during the planning process, and somewhat larger standard setbacks of 1,500-1,700 feet, some wind energy developers have made efforts to enhance a sense of local involvement by offering financial benefits to neighbors who are not hosting turbines themselves. Many projects offer "good neighbor" payments that can be seen as fair compensation for a moderate noise or visual impacts, or as simply a way to create shared benefits among a wider segment of the local community.

The Record Hill Wind Farm in Roxbury, Maine, took the "good neighbor" approach one step further. This 22-turbine wind farm was sited over 3,000 feet away from any homes, putting it farther away than any of the homes spurring complaints at Mars Hill, a noise issue hotspot. In addition, the wind farm is providing all homeowners in town with quarterly checks as reimbursement for their home electricity costs. Since most wind farms sell their output to utilities or commercial buyers, local citizens rarely benefit directly from the turbines they live with.

Operational choices can also reduce noise impacts on nearby neighbors. Increasingly, modern turbines feature adaptive controls that allow rotation speed and blade pitch angles to be adjusted during wind or atmospheric conditions that trigger more widespread complaints. Most manufacturers, including Mitsubishi, Gamesa, and GE, are integrating systems to manage sound power output, often with quieter nighttime modes that can also be routinely employed in turbines sited closer to receptors. These techniques offer modest but perceptible decreases in sound levels at nearby homes.

More drastic measures, such as night-time shut downs, have been adopted temporarily in some towns where complaints are widespread (including recently in Falmouth, Mass.), but are rarely considered to be a viable long-term solution, since few projects can work with significantly reduced revenues. Slight increases in cut-in speeds can offer some relief to immediate neighbors, with less economic impact, but since most noise complaints occur when turbines are turning at full speed, the benefits are also moderate.

The move toward building closer to more noise sensitive communities is also spurring technological advancements. One promising technique is retrofitting (or designing) blades with serrated trailing edges, which can reduce high and mid-frequency sound output by a few decibels, potentially reducing audibility of the bothersome pulsing sound for nearby neighbors. Meanwhile, researchers at the National Renewable Energy Laboratory (NREL) and most major turbine manufacturers are addressing the challenges of siting near more noise sensitive communities by working to tweak blade designs in order to allow turbines to increase their output without increasing blade tip speed and thus noise levels.

As commercial wind energy moves into closer proximity with rural communities, communication, flexibility and innovation are the keys to successful project development.

This article was reprinted with permission from Power Engineering as part of the PennWell Corporation Renewable Energy World Network and may not be reproduced without express written permission from the publisher.

Image: Wind turbines via Shutterstock