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This is an excerpt from the Fourth Quarter 2012 edition of the Wind Program R&D Newsletter.

For years scientists have hypothesized that the increasing number of bat fatalities in wind farms is due to barotrauma-related injuries. But according to a study presented by the National Renewable Energy Laboratory (NREL) at the ninth biennial Wind Wildlife Research Meeting in Denver, Colorado, November 27-30, 2012, it appears unlikely that barotrauma causes immediate death in bats flying near operating wind turbines.

Barotrauma fatalities are caused by fatal levels of internal bleeding brought on by rapid changes in atmospheric pressure—pressure changes that may occur around the blades of an operating wind turbine. Examination of dead bats collected around wind turbines showed that as many as 90% displayed signs of internal hemorrhaging that could be indicative of barotrauma injuries.

To investigate the hypothesis that bat fatalities are linked to barotraumas, NREL 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. The simulations were performed using a 1.5-megawatt wind turbine airfoil profile and Langrangian particles to represent flying bats. The particles were given approximate aerodynamic properties of a bat and then released into the flow to track their flight close to the turbine blade. Two-dimensional simulations were used to evaluate the pressure drop as the particles (representing bats) flew by the suction side of the blade. In a similar manner, three-dimensional simulations evaluated the pressure drop experienced flying through the trailing tip vortex of the blade. 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% of atmospheric pressure. High bat fatalities at wind facilities have only been observed at lower wind speeds below 6 m/s.

NREL compared the results from the simulation 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 exist for bats. The simulation results show that the pressure drop around wind turbine blades is an order of magnitude less than the 30 kilopascals of overpressure that blast studies showed was needed to cause mortality in mice. It therefore seems likely that barotrauma is not a significant cause of death as previously hypothesized, and that the vast majority of bat fatalities around wind turbines are a result of blade strikes.

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