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NREL Study Finds Barotrauma Not Guilty

March 22, 2013

Photo of a female bat in flight, wings spread and facing the camera.

A female hoary bat in flight. © Merlin D. Tuttle, Bat Conservation International

The contour plot shows the pressure field around a 1.5-MW wind turbine blade profile and the path lines show possible bat flight paths. Enlarge image

Two-dimensional simulation with particle tracks in the rotor plane.
Illustration by Mike Lawson, NREL

For years scientists have speculated that the unexpected number of bat fatalities around wind farms is the result of barotrauma-related injuries. The barotrauma hypothesis suggests that a significant number of bat fatalities are due to internal bleeding caused by rapid changes in atmospheric pressure around operating wind turbine blades. But according to a recent National Renewable Energy Laboratory (NREL) study that was presented at the ninth biennial Wind Wildlife Research Meeting in Denver, Colorado, November 27-30, 2012, it appears unlikely that the pressure changes around operating wind turbine blades are large enough to cause fatal barotrauma.

To study the barotrauma hypothesis, NREL researchers performed two- and three-dimensional computational fluid dynamic simulations to evaluate the extent and magnitude of the pressure field near an operating wind turbine blade. Specifically, researchers simulated the airflow field around a generic 1.5-megawatt wind turbine blade and introduced Langrangian particles with the aerodynamic properties of bats into the flow to approximate the flight paths of bats near operating turbines. The two-dimensional simulations (see figure to the right) were used to estimate the pressure histories experienced as bats fly in the vicinity of a rotating turbine blade. Similarly, researchers used three-dimensional simulations (see figure below) to evaluate the pressure drop experienced when flying through the trailing blade tip vortex. The simulations showed that bats flying very close to wind turbine blades at low wind speeds of about 5 meters per second (m/s) experience a suction pressure of approximately 1.4 kilopascals (kPa), which is less than 2% of atmospheric pressure.

Three-dimensional simulation with particle tracks through the tip vortex. Iso-contours of vorticity are shown to illustrate the location of the blade tip vortex and Lagrangian particle tracks illustrate possible bat flight paths that pass through the tip vortex.

Three-dimensional simulation with particle tracks through the tip vortex.
Illustration by Mike Lawson, NREL

NREL compared the results from the numerical simulations to previous studies on the biological response of mice to overpressure blasts from explosions. Mice were used as a surrogate species for bats, because no data exists for bats. The simulation results showed that the pressure drop around wind turbine blades is an order of magnitude less than the 30 kPa of overpressure, which blast studies indicated was the amount needed to cause mortality in mice. The 1.4-kPa pressure changes experienced by bats around wind turbine blades is significantly lower than the 30 kPa shown to cause fatality in mice, which have a similar body mass as many bats. Furthermore, observations showed that the majority of bat deaths occurred at low wind speeds near 5 m/s, when bats are the most active. Fatalities at higher wind speeds (> 5 m/s) are less common, likely because fewer bats are flying in these conditions. Considering that the pressure changes around wind turbine blades at low wind speeds are insignificant and that there are few bat deaths at higher wind speeds, it seems unlikely that barotrauma is a significant cause of bat fatalities around wind turbines, and that the vast majority of bat fatalities are a result of blade strikes.

2012 Wind Wildlife Research Meeting Presents Latest Findings

Leading scientists on wind energy and wildlife from around the world convened November 27-30, 2012, in Denver, Colorado, for the ninth biennial Wind Wildlife Research Meeting to share their latest findings and evaluate progress in understanding and addressing wind energy's potential impacts on wildlife and wildlife habitat. The meeting, which was co-hosted by the American Wind Wildlife Institute and the National Wind Coordinating Collaborative, featured more than 40 speakers, 70 poster presentations, and 350 attendees. Scientists presented their latest research for onshore and offshore risk assessment, conservation and mitigation opportunities, and more. The meeting also included a full-day training session on the U.S. Fish & Wildlife Service Land-Based Wind Energy Guidelines. Meeting proceedings will be published in early 2013. For more information, contact Christine Real de Azua.