Saturday, February 28, 2009
How noisy are wind turbines and wind farms?
This came up recently in response to a question from a resident involved in the wind power debate currently underway in several local towns.
Industrial wind turbines are large pieces of electrical generating equipment with moving parts, particularly swishing blades and humming gearboxes and generators, and so no-one should ever imagine, or try to get away with saying that there won't be noise.
The question is, how much noise will be created?
It's also important to ask where the noise will occur.
When is irrelevant. Although Maine's Site Location of Development Law allows for higher noise from new developments during the day than at night, you can't turn a wind turbine off at night.
The Site Location of Development law on noise boils down to one requirement, after you strip away a lot of fine print, and apply it to turbines which cannot be shut off at night: wind turbine noise may not regularly exceed 45 dBA at any location where the owners have a reasonable expectation of quietness, ie, any home in the countryside in Maine that wasn't built next to an already noisy industrial plant that existed before the home as built.
The GE 1.5 MW models currently used for all major wind power developments in Maine produce about 100+ decibels (dBA) at the turbine head at full-out operation, and about 54 decibels at the base of the tower. The sounds are reduced the further you get from the tower, until they blend into the background noise and can no longer be detected.
Because wind itself produces noise in trees and around buildings, while the turbine speed doesn't increase much after the peak operating speed of 8 or 9 meters per second is reached, the windier it gets, the more background noise there is, while the noise of the turbine remains the same. On a breezy day, woods and trees can produce 54 dBA quite easily, leading to the paradox that turbines are effectively less noisy at higher wind speeds than medium ones.
However, in Waldo County, Maine, we have primarily medium wind speeds of Class 2 and 3, leading to the paradox that turbines are effectively more noisy here than better or worse wind power sites.
Go figure. But it's true.
How quickly the noise reduces as the distance from the tower increases will also depend on the reflective or absorbent qualities of the terrain. Trees are better than open ground. The graph above is for a wind farm in wooded land in New York state.
If you place one or more wind turbines close together in a wind farm, there will be more noise than there would be from just one turbine. The manufacturer publishes data on the noise of just one turbine. The developer must mathematically model the noise from several turbines to predict the noise level at any given site. If a developer is proposing a wind farm, Maine jurisdictions and communities should look to see that the modeling has been done, so folks can be sure that the developer's noise impact predictions in planning applications are not based on just one turbine.
Based on the New York study above, the likely distance or setback that will meet these requirements is probably somewhere between 300 and 400 feet for GE 1.5 MW wind farms in woody sites. This is not to say that neighbors won't hear the turbines at 400 feet, just that the legal requirements will be met, and so the DEP will likely approve the permit, assuming all other requirements are met.
I have heard a rumor, but not had time to confirm, that GE has recently recommended a much larger setback of 650 feet for their 1.5 MW models, while the European countries tend to use an even higher standard of 500 meters.
The primary reason for these higher setback recommendations, as I understand it, is not noise, but ice throw safety. Turbine blades can accumulate ice during ice storms and other suitable weather conditions, which can get thrown off. The industry standard recommendation for ice throw safety is 1.5 times the turbine height.
Even with the 650 foot setback recommendation, it will be possible to locate turbines on sites in Waldo County and meet the requirements of the Site Location of Development Law. Towns may, however, enact stricter regulations, should they choose to do so. Care should be taken if towns decide to control turbines using noise performance standards because many agricultural and industrial operations will fall foul of the new regulations. It's probably best to require a setback specifically for large turbines instead.
Not all turbines, just the large ones.
The little camp owner or homesteader who wants to run a little Air-X or Skystream 3.7 household turbine shouldn't have to have such a big setback.