Study shows ultrasonic deterrents significantly reduce wind turbine bat fatalities

The bat deterrent system, inspired by the tiger moth, floods the area with a frequency that makes it difficult for bats to echolocate, so they leave in search of a better place to hunt. (Image credit: NRG Systems)

Wind turbines are a significant cause of bat deaths around the world, but new research conducted in part at Texas State University has shown that ultrasonic acoustics can deter bats from approaching wind turbines and reduce fatalities.

The study was led by Sara Weaver, who earned her doctorate in aquatic resources and integrative biology from Texas State in 2019. Other members of the research team included Thomas Simpson, director of the wildlife biology program at Texas State, Ivan Castro-Arellano, associate professor in the Department of Biology at Texas State, Cris Hein, with the National Renewable Energy Laboratory in Golden, Colorado, and Jonah Evans, with the Texas Parks and Wildlife Department. Their research, “Ultrasonic acoustic deterrents significantly reduce bat fatalities at wind turbines,” is published in the journal Global Ecology and Conservation (https://www.sciencedirect.com/science/article/pii/S2351989420301827).

To earn her doctorate, Weaver led a two-year study that showed a new technology could significantly reduce the number of bat deaths in the Rio Grande Valley at Duke Energy’s Los Vientos Wind Power Project.

"This project was an amazing collaboration between industry, NGOs, government and Texas State University researchers, and shows what we can accomplish when we have a common goal: save the bats," Weaver said. "I am proud of what we achieved during my dissertation efforts, but this isn’t the end. We continue to work towards improving this technology to increase effectiveness for more species, and search for innovative solutions to complex conservation problems."

The Rio Grande Valley has more bats than many parts of the country. An unexpected number were passing through Los Vientos, and many were killed flying into the wind turbines that cover thousands of acres near Brownsville. 

The Bat Deterrent System, an ultrasonic acoustic device developed by NRG Systems, reduced overall bat fatalities by 50 percent with reductions of nearly 80 percent in some species. The study was so successful that Duke Energy will equip 255 wind turbines at Los Vientos with the device. It will be the first commercial-scale installation in the continental United States.

Hundreds of thousands of bats die each year at North American wind farms, and it can be difficult to predict when bats will be affected at a site. Pre-construction surveys at Los Vientos showed little bat habitat, and Duke Energy along with state and federal wildlife agencies expected little impact to bats. However, once the site began generating power, surveys showed higher-than-expected bat fatalities.

This suggests bats may be attracted to the turbines, and researchers have many theories as to why. Bats may think the turbines are tall trees good for roosting. The critters might also think the turbine’s reflective surface is drinking water, or maybe bats are just curious and want to inspect the new structures.

Fatalities at this rate could cause population declines for some species. Bats, unlike birds, are mammals that live for many years and reproduce slowly – most bats have one or two pups per year.

The issues facing bats are complex. Bats are small, they migrate and fly at night, so it’s difficult to learn how many are out there. Some species are struggling with the deadly White Nose Syndrome, while many others face habitat loss, pesticides and the challenges associated with climate change. Wind energy is an additional hurdle.

The Bat Deterrent System is now one of the most promising devices available for preventing bat fatalities. Bats use echolocation to find food, socialize and navigate, Weaver explained. They emit a high-frequency ultrasound that bounces off nearby objects to create a picture of the environment. Bats can determine how far away objects are based on how long it takes the sound to return. Some insects, like the tiger moth, have evolved to emit their own ultrasound as a defense against hungry bats. With the moth’s frequency filling the air, it’s too noisy for the bat to tell where the moth is, so the bat will leave and look for a quieter place to hunt.

“This made scientists wonder,” Weaver said, “’If [moths] can do it, why can’t we?’”

Turns out scientists could. NRG Systems created a device – made of six speakers – that mounts to the top of the wind turbine and floods the area surrounding the blades with a high-frequency ultrasound to discourage bats from approaching. The sound does not hurt bats or nearby wildlife, and humans can’t hear it.

NRG Systems designs and builds instruments like anemometers and sensors that can withstand the strong winds, extreme heat and ice at the top of wind turbines. The rectangular device weighs about as much as a small laptop and five are attached to the turbine’s nacelle (the powerhouse at the top of the turbine).

For Weaver’s study, the team installed the Bat Deterrent System on 16 random turbines. Each night of the study, half of the turbines had devices turned on and half were off. Field technicians, who didn’t know which turbines were on the night before, would search beneath the turbines every morning and record how many and what species of bats they found.

The study showed an overall reduction in bat fatalities of 50%. The technology proved more effective for some species, like the hoary bat, a species suspected to have declining populations nationwide. It showed a 78% reduction, and the Brazilian free-tailed bat, which made up most of the fatalities at Los Vientos, showed a 55% reduction.

“It’s not enough to just say this technology works, we have to take it a step further and actually use it,” Weaver said, “and that’s what Duke is doing.

“We need more companies like this – more people – who want to get ahead of the problem and prevent it from getting to the point where we need regulations,” she said. “We got positive results, but I don’t think this is the limit.”

For more information about Duke Energy’s Los Vientos Wind Power Project, visit www.duke-energy.com/renewable-energy.

For more information about NRG Systems' bat deterrent systems, visit www.nrgsystems.com/products/bat-deterrent-systems.