Prevent turbine blades from turning at low wind speeds ('feathering')

Supporting evidence from individual studies

1. A replicated, controlled study in 2005 at a wind farm in an agricultural area of Alberta, Canada (Brown & Hamilton 2006) found that preventing turbine blades from turning at low wind speeds (‘feathering’) resulted in fewer bat fatalities than at conventional turbines. The total number of bat carcasses recovered by searchers was lower at experimental turbines shut down at low wind speeds (64 bats, 40% of total) than at conventional control turbines (95 bats, 60% of total). The number of bat carcasses did not differ significantly between turbines before the experiment (‘experimental’ turbines: 157 bats, 49% of total; ‘control’ turbines: 164 bats, 51% of total). Five bat species were found, although 97% of bat carcasses were hoary bats Lasiurus cinereus and silver-haired bats Lasionycteris noctivagans (see original report for data). In August 2005, all of 39 turbines were operated using conventional methods (blades rotated freely at wind speeds <4 m/s). In September 2005, odd numbered turbines (20 of 39) were braked and locked to prevent them from turning at wind speeds <4 m/s. Nineteen control turbines were left unaltered. Carcass searches were conducted weekly along transects in circular plots (40-m radius) around each turbine in August–September 2005.

   Study and other actions tested

   Referenced paper

   Brown W.K. & Hamilton B.L. (2006) Monitoring of bird and bat collisions with wind turbines at the Summerview Wind Power Project, Alberta, 2005–2006. Vision Quest Windelectric report.

2. A replicated, randomized, controlled study in 2010 at a wind energy facility in a forested area of West Virginia, USA (Young et al 2011; same site as Young et al 2012) found that preventing turbine blades from turning at low wind speeds (‘feathering’) in the first or second half of the night resulted in fewer bat fatalities than at conventional turbines. Average bat fatality estimates were lower when turbine blades were feathered in the first half of the night (0.05 bats/turbine) or the second half (0.09 bats/turbine) compared to conventional control turbines (0.18 bats/turbine). Fatality estimates for turbines feathered in the first vs second half of the night did not differ significantly. Six bat species were found, although 86% of bat carcasses were hoary bats Lasiurus cinereus and eastern red bats Lasiurus borealis (see original report for data). On nights when wind speeds were forecasted to be low, two treatments (blades feathered at wind speeds <4 m/s for 5 h after sunset or 5 h before sunrise) and a control (blades rotated freely at wind speeds <4 m/s) were each randomly assigned to three groups of eight turbines. Treatments were rotated between turbine groups weekly over 12 weeks in July–October 2010. Daily carcass searches were conducted along transects in plots up to 100 m around each of the 24 turbines. Carcass counts were corrected to account for searcher efficiency, removal by scavengers, and unsearchable areas within plots.

   Study and other actions tested

   Referenced paper

   Young D.P.Jr., Nomani S., Tidhar W.L. & Bay K. (2011) NedPower Mount Storm Wind Energy Facility post-construction avian and bat monitoring: July–October 2010. Western EcoSystems Technology, Inc. (WEST) report.

3. A replicated, randomized, controlled study in 2011 at a wind farm in an agricultural area of Indiana, USA (Good et al 2012) found that preventing turbine blades from turning at low wind speeds (‘feathering’), and feathering along with increasing the speed at which turbines become operational (‘cut-in speed’), resulted in fewer bat fatalities than at conventional control turbines. Total bat fatalities were 36% lower when turbine blades were feathered below the conventional cut-in speed (66 fatalities) compared to control turbines without feathering (105 fatalities). Total bat fatalities were 59% and 75% lower when blades were feathered and cut-in speeds increased to 4.5 and 5.5 m/s respectively (42 and 25 fatalities). Differences in total bat fatalities between treatments were significant. Seven bat species were found, although 81% of bat carcasses were eastern red bats Lasiurus borealis and hoary bats Lasiurus cinereus (see original report for data). Three treatments (turbine blades feathered below cut-in speeds of 3.5, 4.5 and 5.5 m/s) were each randomly assigned to a group of 42 turbines. Two control groups of nine and 42 turbines were left unaltered (blades rotated freely below cut-in speed of 3.5 m/s). Treatments were rotated between turbine groups nightly in July–October 2011. Carcass searches were conducted every 1–2 days along transects in circular plots (80-m radius) around each of the 177 turbines.

   Study and other actions tested

   Referenced paper

   Good R.E., Erickson W., Merrill A., Simon S., Murray K., Bay K. & Fritchman C. (2012) Bat monitoring studies at the Fowler Ridge Wind Energy Facility, Benton County, Indiana, April 1 – October 31, 2011. Western EcoSystems Technology, Inc. (WEST) report.

4. A replicated, randomized, controlled study in 2011 at a wind energy facility in a forested area of West Virginia, USA (Young et al 2012; same site as Young et al 2011) found that automatically preventing turbine blades from turning at low wind speeds (‘feathering’) did not result in fewer bat fatalities than at conventional turbines. Average bat fatality estimates did not differ significantly between turbines with automatically feathered blades (6.5 bats/turbine) and conventional control turbines (7.4 bats/turbine). Five bat species were found across the site (see original report for details). Twelve turbines were assigned with the treatment (blades automatically feathered when wind speeds dropped below 4 m/s for at least 6 minutes). Twelve control turbines were left unaltered (blades rotated freely at wind speeds <4 m/s). The treatment was rotated between turbines weekly for 12 weeks in July–October 2011. Daily carcass searches were conducted along transects in plots up to 100 m around each of the 24 turbines. Carcass counts were corrected to account for searcher efficiency, removal by scavengers, and unsearchable areas within plots.

   Study and other actions tested

   Referenced paper

   Young D., Nomani S., Courage Z. & Bay K. (2012) NedPower Mount Storm Wind Energy Facility post-construction avian and bat monitoring: July–October 2011. Western EcoSystems Technology, Inc. (WEST) report.

5. A before-and-after study in 2011–2012 at a wind energy facility in a forested area of Maryland, USA (Young et al 2013) found that preventing turbine blades from turning at low wind speeds (‘feathering’), along with increasing the speed at which turbines become operational (‘cut-in speed’), resulted in fewer bat fatalities than before the operational changes. Average bat fatality estimates were 62% lower after turbine blades were feathered below an increased cut-in speed of 5 m/s (11 bats/turbine) compared to the previous year without operational changes (29 bats/turbine). The difference was not tested for statistical significance. Five bat species were found across the site (see original report for details). In July–October 2012, all of 28 turbines at the facility were operated with feathering below an increased cut-in speed of 5 m/s. Weekly carcass searches were conducted along transects in circular plots (40-m radius) around 14 of the 28 turbines. Data for before the operational changes (blades rotated freely below a cut-in speed of 4 m/s) were collected in a previous study in July–October 2011. Carcass counts in both years were corrected to account for searcher efficiency and removal by scavengers.

   Study and other actions tested

   Referenced paper

   Young D.P.Jr, Nations C., Lout M. & Bay K. (2013) Post-construction monitoring study, Criterion Wind Project, Garrett County, Maryland: April–November 2012. Western EcoSystems Technology, Inc. (WEST) report.

6. A replicated, randomized, controlled study in 2012–2013 at a wind farm in a forested area in Vermont, USA (Martin et al 2017) found that preventing turbine blades from turning at low wind speeds (‘feathering’), along with increasing the wind speed at which turbines become operational (‘cut-in speed’) at temperatures above 9.5°C, resulted in fewer bat fatalities than at conventional turbines. The average number of bat fatalities was 62% lower at wind turbines when cut-in speeds were increased to 6 m/s at temperatures >9.5°C and the blades were feathered below this speed (0.5 bats/turbine) compared to conventional control turbines (1.4 bats/turbine). Three bat species were found (see original paper for details). In June–September 2012 and 2013, eight of 16 turbines were randomly assigned the treatment (cut-in speed increased to 6 m/s at temperatures >9.5°C and blades feathered below this speed) for a total of 60 nights. The other eight turbines were unaltered (cut-in speed of 4 m/s without feathering). Daily carcass searches were conducted along transects in rectangular plots (3,629–5,746 m²) centred on each of the 16 turbines. If applied to all turbines, it was estimated that the operational changes would result in annual energy losses of 1%.

   Study and other actions tested

   Referenced paper

   Martin C.M., Arnett E.B., Stevens R.D. & Wallace M. (2017) Reducing bat fatalities at wind facilities while improving the economic efficiency of operational mitigation. Journal of Mammalogy, 98, 378-385.