Effectiveness of Changing Wind Turbine Cut-in Speed to Reduce Bat Fatalities at Wind Facilities 2009


Title: Effectiveness of Changing Wind Turbine Cut-in Speed to Reduce Bat Fatalities at Wind Facilities 2009
Publication Date:
April 01, 2009
Pages: 44

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Arnett, E.; Schirmacher, M.; Huso, M.; Hayes, J. (2009). Effectiveness of Changing Wind Turbine Cut-in Speed to Reduce Bat Fatalities at Wind Facilities 2009. Report by Bat Conservation International, Oregon State University, and University of Florida. pp 44.

We implemented the first U.S.-based experiment on the effectiveness of changing turbine cut-in speed on reducing bat fatality at wind turbines at the Casselman Wind Project in Somerset County, Pennsylvania. Our objectives were to 1) determine the difference in bat fatalities at turbines with different cut-in-speeds relative to fully operational turbines, and 2) determine the economic costs of the experiment and estimated costs for the entire project area under different curtailment prescriptions and timeframes.


Twelve turbines of the 23 turbines at the site were randomly selected for the experiment and we employed three treatments at each turbine with four replicates on each night of the experiment: 1) fully operational, 2) cut-in speed at 5.0 m/s (C5 turbines), and 3) cut-in speed at 6.5 m/s (C6 turbines). We used a completely randomized design and treatments were randomly assigned to turbines each night of the experiment, with the night when treatments were applied being the experimental unit. We conducted daily searches at the 12 turbines from 26 July to 10 October 2008. During this same period, we also conducted daily searches at 10 different turbines that were part of a complementary study to determine if activity data collected prior to construction with acoustic detectors can be used to predict post-construction fatalities, and to meet permitting requirements of the Pennsylvania Game Commission’s (PGC) voluntary agreement for wind energy (herein referred to as “PGC” turbines). These 10 turbines formed an alternative ‘control’ to the curtailed turbines. We performed two different analyses to evaluate the effectiveness of changing turbine cut-in speed to reduce bat fatalities; for one we used 12 turbines to determine differences in fatality between curtailment levels and for another using 22 turbines to determine differences in fatalities between curtailment and fully operational turbines. The experimental unit in the first analysis was the turbine-night and turbines were considered a random blocking factor within which all treatments were applied. In our first analysis, the total number of fatalities estimated to have been killed the previous night, herein referred to as “fresh” fatalities, in each treatment at each turbine was modeled as a Poisson random variable with an offset of the number of days a treatment occurred within a turbine (due to the slight imbalance of the design). For our second analysis, the turbine was the experimental unit, with 12 turbines receiving the curtailment treatment, 10 the control (fully operational at all times). We used all carcasses found at a turbine to estimate the total number of bat fatalities that occurred at each turbine between 26 July and 10 October 2008 and compared fatalities using one-way ANOVA.


A total of 32 fresh bat fatalities were found at the 12 treatment turbines between 26 July and 10 October 2008. Each treatment was implemented at each turbine for at least 25 nights, with one treatment at each turbine implemented for 26 nights. At least one fresh fatality was found at each turbine, and 10 of the 12 turbines had at least 1 fatality during a fully operational night, indicating that fatalities did not occur disproportionately at only some turbines, but were well distributed among all turbines. There was strong evidence that the estimated number of fatalities over 25–26 nights differed among turbine treatments (F2,33 = 8.99, p = 0.008). There was no difference between the number of fatalities for C5 and C6 turbines (χ1 2 = 0.83, p = 0.3625, 95% CI: 0.11, 2.22). Total fatalities at fully operational turbines were estimated to be 5.4 times greater on average than at curtailed turbines (C5 and C6 combined; χ1 2 = 14.63, p = 0.001, 95% CI: 2.28, 12.89); in other words, 73% (95% CI: 53–87%) of all fatalities at curtailment turbines likely occurred when the turbines were fully operational.


Estimated total bat fatalities per turbine (i.e., all carcasses found and corrected for field bias) were 1.23–4.68 times greater (mean = 2.34) at PGC turbines relative to curtailed turbines, further supporting the contention that reducing operational hours during low wind periods reduces bat fatalities. This is a conservative estimate of the difference because treatment turbines were fully operational one-third of the time during the study.


The lost power output resulting from the experiment amounted to approximately 2% of total project output during the 76-day study period for the 12 turbines. Hypothetically, if the experimental changes in cut-in speed had been applied to all 23 turbines at the Casselman site for the study period (0.5 hour before sunset to 0.5 hour after sunrise for the 76 days we studied), the 5.0 m/s curtailment used would have resulted in lost output equaling 3% of output during the study period and only 0.3 % of total annual output. If the 6.5 m/s curtailment were applied to all 23 turbines during the study period, the lost output would have amounted to 11% of total output for the period and 1% of total annual output. In addition to the lost power revenues, the company also incurred costs for staff time to set up the processes and controls and to implement the curtailment from the company’s offsite 24-hour operations center.


Our study is the first U.S.-based experiment of changing cut-in speed to reduce bat fatalities, and only the third we are aware of anywhere in the world. We demonstrated nightly reductions in bat fatality ranging from 53–87% with marginal annual power loss. Given the magnitude and extent of bat fatalities worldwide, the conservation implications of our findings are critically important. However, more studies are needed to test changes in turbine cut-in speed among different sizes and types of turbines, wind regimes, and habitat conditions to fully evaluate the general effectiveness of this mitigation strategy. We plan to initiate a second year of post-construction fatality searches at the PGC turbines beginning 1 April and continuing through 15 November 2009 and will initiate searches for the curtailment study beginning in mid- late July and continuing through the second week of October in 2009 at the Casselman facility.

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