Sound Advice
Volume 12, Issue 3, Fall 2009

The Trouble with Wind Farms

Purported to be a legitimate potential alternate energy source, wind farms obviously have some serious environmental issues to overcome, as illustrated in the following two questions. First: What noise source can cause severe annoyance at half the volume of other major noise sources such as road, rail, air and industry? Answer: Wind farms. Second: What makes bats’ lungs explode? Answer: Flying through wind farms.

Let’s consider the noise issue first. Because the wind turbines are rotating devices, they have a tendency to produce a beat frequency. This is the same type of noise you hear from an approaching helicopter, that regular, distinct “whoop, whoop, whoop, whoop” thumping sound. To people living in the vicinity of a wind farm, this can be especially annoying at night.

Low level winds, that is, winds closest to the ground, often die down at night, causing noise levels at that elevation to drop commensurately. However, the wind up at the height of the turbine hubs, a considerably higher elevation than residences, can speed up at night, causing turbine blades to turn faster and create more noise. For the residents, the resultant noise can then be heard much more clearly; it’s easily identifiable too (a big issue in terms of the annoyance factor) and so it becomes a bigger nuisance at night.

Now for the wildlife problem. Studies have shown that bats are attracted to wind farms, although we’re not sure why. One suggestion is that they consider the wind turbines to be some kind of metal tree or roosting place. Perhaps it’s the profusion of insects in the area; themselves attracted by some aspect of the turbine operation: the acoustical frequency they produce or the vibration they create. Simpler still, maybe it’s the brightness of the turbines against the darker backgrounds that makes them stand out. What we do know is that bats are being attracted to wind farms and many are dying.

Common sense would make you think that they’re dying as a result of colliding with the turbine vanes, since the tips of the vanes can travel at about 125 mph when the wind is blowing. Although lots of birds die this way, the bats are more frequently dying from barotrauma, a physiological phenomenon akin to “the bends” in scuba divers. 

The air movement around the turbines causes low pressure vortices, and as the bats fly through those areas, the sudden rapid reduction in air pressure literally causes their lungs to hemorrhage and explode. The same principle occurs when a scuba diver returns to the water’s surface too quickly. Nitrogen that he previously inhaled from his air tanks was compressed into his fatty tissue during the dive. As the pressure on his body decreases with his ascent, the nitrogen moves out of the tissues and back into the bloodstream in the form of bubbles-just like bubbles rush to the top when you depressurize a beer bottle by removing the cap. A scuba diver can control his decompression rate by slowing his ascent which causes it; bats flying through low pressure areas aren’t able to do this, with fatal results.

So far, the only feasible solution to both the noise and the bat fatality problems has been to turn off the turbines at night when bats, nocturnal creatures, are out feeding. However, the power companies don’t want to do this because any lost electricity creation is a dollar loss to them. What’s the long term answer? Right now, there doesn't appear to be one. 

Wind farms are limited in their ability to provide energy because obviously they are only productive when the wind is blowing, which doesn't happen 24/7. Also, the volume of energy they produce is so small compared to other resources that as a practical matter their best use is as a top-up for energy reserves obtained from other sources. Until there's a way for wind turbines to produce a really significant volume of energy, the adverse noise impacts on the local communities and the physical impact on the wildlife appear to outweigh the benefits.

Next Article: Wayside Horn System Test»