Wind turbines are a notorious hazard for birds, but less well known is the danger they pose to bats. In 2012 turbines killed more bats than birds, and the numbers of the dead were substantial: about 888,000 bats were found on wind farms, compared with 573,000 birds.

Migrating bats such as the hoary bat, which can travel from as far as northern Canada to Argentina and Chile, make up most of those fatalities because they often navigate through areas dotted with wind farms. Yet researchers have also found carcasses of cave-hibernating bats, including the little brown bat and the northern long-eared myotis—two species that have been devastated by the fungal disease white nose syndrome and that are now being considered for protection under the Endangered Species Act.

Because of white nose syndrome, mounting public pressure and scrutiny from wildlife officials have become a major motivator for wind energy companies to figure out how to prevent bat deaths. Three targeted strategies are in the works.

Operational Changes
Turning turbines off from summer to late fall during low-wind conditions—when bats are most active—is the single most promising option to protect them, according to Ed Arnett, a pioneer of bat and wind energy research efforts. In tests at the Casselman Wind Power Project in Pennsylvania, small changes to turbine operations reduced bat mortality significantly. During nights from July to October in 2008 and 2009, operators shut down the turbines when wind speeds were below 6.5 meters per second. As a result, bat deaths were reduced by 44 to 93 percent, with less than 1 percent annual power loss.

Boom Boxes
Ultrasonic “boom boxes” that emit continuous high-frequency sounds from 20 to 100 kilohertz deter bats from getting too close to turbines by interfering with their echolocation. “We find fewer dead bats at these treatment turbines,” says Cris Hein of Bat Conservation International (BCI). During tests the organization performed in 2009, turbines with the deterrence device killed 21 to 51 percent fewer bats. General Electric Power & Water recently partnered with BCI to develop boom boxes for commercial availability. And new research shows that most of the winged mammals approach turbines from the leeward side, which could provide insight into optimal placement.

Ultraviolet Light
UV light is not discernible to humans, but many bat species are sensitive to it, so several researchers and companies are studying how to use the light to keep bats away from turbines. The National Science Foundation recently awarded a $150,000 grant to Lite Enterprises, based in Nashua, N.H., to test the technical and commercial viability of this technology. “What's promising is that it would extend further than the ultrasonic deterrence and could be cheaper to install,” Hein says. The ultrasonic deterrence's effective range is about 20 to 40 meters from the source, whereas the UV-light-emitting device could extend 50 meters or more.