Slow rotation of turbine blades at low wind speeds
Overall effectiveness category Likely to be beneficial
Number of studies: 1
Background information and definitions
Most wind turbines operate by a ‘cut-in’ wind speed at which the turbine begins to generate electricity and the blades can move at a maximum rotation speed. However, the blades can still rotate below cut-in speeds when electricity is not being generated. Slowing the rotation of turbine blades below the cut-in speed may reduce bat fatalities, which have been found to be higher at low wind speeds (e.g. Horn et al. 2008, Rydell et al. 2010, Wellig et al. 2018).
For studies that prevent turbine blades from turning below the cut-in speed, see ‘Prevent turbine blades from turning at low wind speeds (‘feathering’)’. Cut-in speeds may also be increased in combination with this intervention. See ‘Increase the wind speed at which turbines become operational (‘cut-in speed’)’.
Horn J.W., Arnett E.B. & Kunz T.H. (2008) Behavioral responses of bats to operating wind turbines. The Journal of Wildlife Management, 72, 123–132.
Rydell J., Bach L., Dubourg-Savage M.-J., Green M., Rodrigues L. & Hedenström A. (2010) Bat mortality at wind turbines in northwestern Europe. Acta Chiropterologica, 12, 261–274.
Wellig S.D., Nusslé S., Miltner D., Kohle O., Glaizot O., Braunisch V., Obrist M.K. & Arlettaz R. (2018) Mitigating the negative impacts of tall wind turbines on bats: vertical activity profiles and relationships to wind speed. PLOS ONE, 13, e0192493.
Supporting evidence from individual studies
A replicated, randomized, controlled study in 2006–2007 at a wind farm in an agricultural area of Alberta, Canada (Baerwald et al 2009) found that slowing the rotation of turbine blades at low wind speeds resulted in fewer bat fatalities than at conventional turbines. Average bat fatality estimates were lower at experimental turbines with altered blade angles (8 bats/turbine) than at conventional control turbines (19 bats/turbine). Average bat fatality estimates did not differ significantly between turbines before the experiment (‘experimental’ turbines: 19 bats/turbine; ‘control’ turbines: 24 bats/turbine). Most bats identified during carcass searches were hoary bats Lasiurus cinerus and silver-haired bats Lasionycteris noctivagans (see original paper for data). In 2006, all of 14 turbines were operated using conventional methods (blades rotated freely at low wind speeds). In 2007, six randomly chosen turbines were altered by changing the pitch angle of the rotor blades to slow rotation at low wind speeds (<4 m/s). Eight control turbines were left unaltered. Carcass searches were conducted weekly along spiral transects up to 52 m around each of the 14 turbines in July–September 2006 and 2007. Carcass counts were corrected to account for searcher efficiency and removal by scavengers.Study and other actions tested