Action: Increase the wind speed at which turbines become operational (‘cut-in speed’) to reduce bat fatalities
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- Four studies evaluated the effects of increasing the wind speed at which turbines become operational (‘cut-in speed’) on bat populations. One study was in Canada and three studies were in the USA.
COMMUNITY RESPONSE (0 STUDIES)
POPULATION RESPONSE (4 STUDIES)
- Survival (4 studies): Three randomized, replicated, controlled studies (including one before-and-after study) in Canada and the USA, and one review in the USA found that bat fatalities were significantly reduced when the wind speed at which turbines became operational (‘cut-in speed’) was increased.
BEHAVIOUR (0 STUDIES)
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. Increasing turbine cut-in speeds may reduce bat fatalities, which have been found to be high at low wind speeds (e.g. Horn et al. 2008, Rydell et al. 2010, Wellig et al. 2018). Wind turbines may also be prevented from turning below the cut-in speed. See ‘Prevent turbine blades from turning at low wind speeds to reduce bat fatalities’.
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 randomized, replicated, controlled before-and-after study in 2006–2007 at a wind farm in an agricultural area of Alberta, Canada (Baerwald et al. 2009) found that increasing the wind speed at which turbines become operational (‘cut-in speed’) resulted in fewer bat fatalities than at conventional turbines. Total bat fatality rates were lower at experimental turbines with increased cut-in speeds (average 8 bats/turbine) than at conventional control turbines (average 19 bats/turbine). Bat fatality rates did not differ significantly between turbines before the experiment (‘experimental’ turbines: average 23 bats/turbine; ‘control’ turbines: average 24 bats/turbine). Two bat species were identified during carcass searches (see original paper for data for individual species). In 2006, all turbines were operated using conventional methods. In 2007, 15 randomly chosen turbines were altered by increasing the cut-in wind speed to 5.5 m/s. Eight control turbines were left unaltered (cut-in speed 4 m/s). Carcass searches were conducted weekly along spiral transects up to 52 m around each of the 29 turbines in July–September 2006 and 2007.
A randomized, replicated, controlled study in 2008–2009 at a wind farm in a forested area of Pennsylvania, USA (Arnett et al. 2010) found that increasing the wind speed at wind turbines become operational (‘cut-in speed’) resulted in fewer bat fatalities than at conventional wind turbines. Average bat fatalities were lower at turbines with increased cut-in speeds (5 m/s: 0.3–0.7 bats/turbine; 6.5 m/s: 0.5–0.6 bats/turbine) than at turbines with conventional cut-in speeds (3.5 m/s: 2 bats/turbine). In July–October 2008 and 2009, two treatments (cut-in speed increased to 5 m/s or 6m/s) and one control (cut-in speed of 3.5 m/s) were randomly assigned to three groups of four turbines for 25 nights/treatment. Daily carcass searches were conducted along transects in 120 x 126 m plots centred on each of 12 turbines. If applied to the entire wind farm (23 turbines), annual power output losses were projected to be 0.3% with cut-in speeds increased to 5 m/s, and 1% with cut-in speeds increased to 6.5 m/s.
A review of 10 studies in 2006–2012 at wind energy facilities in Canada and the USA (Arnett et al. 2013) found that increasing the speed at which wind turbines become operational (‘cut-in speed’), or increasing the cut-in speed along with preventing rotor blades from turning at low wind speeds, resulted in fewer bat fatalities in all 10 studies. In eight studies, average bat fatalities were reduced by 47–82% when cut-in speeds were increased, and by 57–89% when rotor blades were also prevented from turning at low wind speeds, compared to conventionally operated turbines (see original report for more detailed results). Two studies found that bat fatalities were reduced by 20–38% at wind turbines when cut-in speeds were increased, but sample sizes were small and differences were either not statistically signficant or were not tested. In seven of 10 studies, cut-in speeds were increased to 4–6.9 m/s compared to the standard manufacturer’s cut-in speed (3–4 m/s). In three of 10 studies, turbine blades were also prevented from turning at low wind speeds by changing the angle of the blade parallel to the wind or turning the turbine out of the wind. Two of 10 studies reported estimated losses in power generation to be <1% of the total annual output. Three studies in this review have been summarised individually (Baerwald et al. 2009, Arnett et al. 2010, Martin et al. 2017).
A randomized, replicated, controlled study in 2012–2013 at a wind farm in Vermont USA (Martin et al. 2017) found that 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 turbines with conventional cut-in speeds. The total number of bat fatalities was 62% lower at wind turbines with increased cut-in speeds (average 0.5 bats/turbine) than at fully operational turbines (1.4 bats/turbine). At treatment turbines, cut-in wind speeds were increased to 6 m/s when temperatures were >9.5°C. Fully operational control turbines had a cut-in wind speed of 4 m/s. In each year, eight of 16 turbines were randomly assigned the treatment for 60 nights. Daily fatality searches were carried out in June–September 2012 and 2013. Rectangular study plots around each turbine were searched using transects spaced 6 m apart. If applied to all turbines, it was estimated that the operational changes would result in annual energy losses of 1%.
- Baerwald E.F., Edworthy J., Holder M. & Barclay R.M.R. (2009) A large-scale mitigation experiment to reduce bat fatalities at wind energy facilities. The Journal of Wildlife Management, 73, 1077-1081
- Arnett E.B., Huso M.M.P., Schirmacher M.R. & Hayes J.P. (2010) Altering turbine speed reduces bat mortality at wind-energy facilities. Frontiers in Ecology and the Environment, 9, 209-214
- Arnett E.B., Johnson G.D., Erickson W.P. & Hein C.D. (2013) A synthesis of operational mitigation studies to reduce bat fatalities at wind energy facilities in North America. A report submitted to the National Renewable Energy Laboratory. Bat Conservation International report.
- 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