Abstract. A large number of bat fatalities have been reported in wind energy facilities in different regions globally. Wind farm operators are required to monitor bat fatalities by conducting carcass surveys at wind farms. A previous study implemented the ballistics model to characterize the carcass fall zone distributions after a bat is struck by turbine blades. The ballistics model considers the aerodynamic drag force term, which is dependent upon the carcass drag coefficient. The bat carcass drag coefficient is highly uncertain; no measurement of it is available. This paper introduces a methodology for bat carcass drag coefficient estimation. Field investigation at Macksburg wind farm resulted in the discovery of three bat species: the hoary bat (Lasiurus cinereus), eastern red bat (Lasiurus borealis), and evening bat (Nycticeius humeralis). Carcass drop experiments were performed from a dropping platform at finite height, and carcass position time series data were recorded using a high-speed camera. Falling carcasses were subjected to aerodynamic drag and gravitational forces. Carcasses were observed to undergo rotation, often rotating around multiple axes simultaneously, as well as lateral translation. The complex fall dynamics, along with drop from a limited height, prohibit the carcasses from attaining terminal velocity. Under this limitation, the drag coefficient is estimated by fitting a ballistics model to the measured velocity. Multivariable optimization was performed to fit the ballistics model to the measured velocity resulting, in an optimized estimate of the drag coefficient. A sensitivity analysis demonstrated significant variation in the drag coefficient with a small change in initial position, highlighting the chaotic nature of carcass fall dynamics. Based on the limited sample, the bat carcass drag coefficient and terminal velocity were found to be between 0.70–1.23 and 6.63–17.57 m s−1, respectively. The maximum distance carcasses are predicted to fall after impact with a typical utility-scale onshore wind turbine was computed using a 2-D ballistics model. Based on the range of drag coefficients found in this study, hoary and evening bats are estimated to fall within the rotor plane up to a maximum distance of 92 and 62 m, respectively, from the wind turbine tower. The ballistics model of carcasses after being struck by wind turbine blades can be used to obtain fall distributions for bats, guide carcass survey efforts, and correct survey data for limited or unsearched areas.
Bats colliding with spinning wind turbine blades result in bat mortality. Carcass surveys at individual wind turbines vary from daily to once a week and from large cleared plots to only the road and pad area. A physics‐based model is proposed to guide carcass surveys, for designing curtailment studies to detect treatment fatalities and for improving fatality estimates by accounting for unsearched areas. The model considers the effects of carcass size, weight, and drag, and it accounts for the turbine rotor size and rotation rate to simulate the trajectory of a carcass after it is struck by a wind turbine blade. A carcass parameter is defined as the ratio of drag force to body weight, which accounts for the relative effect of bat biophysical and aerodynamic characteristics. By applying restrictions on carcass survey and turbine yaw data, a limited sample of bat fatalities was obtained, and the analysis revealed that bats fall downwind of wind turbines, indicating wind drift significantly influences carcass fall trajectories. The new ballistics model includes the effect of wind drift on fall trajectory of a carcass. The model was used to investigate the sensitivity of carcass fall trajectories to variability of the input parameters. The tests showed that larger values of the carcass parameter, that is, when drag dominates, such as for small carcasses, resulted in larger downwind drift, whereas large carcasses with smaller carcass parameter values resulted in larger distances within the rotor plane. The relationship of wind speed and RPM was found to influence the carcass downwind distance more compared to the within rotor plane distance. Using carcass survey data, turbine operation data, and wind speed records, for seven bats surveyed the day after colliding with a wind turbine, modelled back‐trajectories were used to identify the likely strike location on the rotor. The model can be improved by validating the modelled trajectories with the recorded bat‐blade strikes in thermal videos. It should be noted that the findings of the present study are based on the bat fatalities that met strict criteria leading to small sample size and hence requires further evaluation for testing the robustness of the model.
Abstract. Large number of bat fatalities have been reported in wind energy facilities in different parts of the world. The wind farm regulators are required to monitor the bat fatalities by conducting carcass survey in the wind farms. Previous studies have implemented ballistic model to characterize the carcass fall zone after strike with turbine blades. Ballistic model contains the aerodynamic drag force term which is dependent upon carcass drag coefficient. The bat carcass drag coefficient is highly uncertain and of which no measurement is available. This manuscript introduces a new methodology for bat carcass drag coefficient estimation. Field investigation at Macksburg wind farm resulted in the discovery of three bat species: Eastern Red bat (Lasiurus borealis), Hoary bat (Lasiurus cinereus) and Evening bat (Nycticeius humeralis). Carcass drop experiments were performed from a dropping platform at finite height and carcass position time series data was recorded using a high-speed camera. Falling carcasses were subjected to aerodynamic drag and gravitational force. Carcasses were observed to undergo rotation; often rotating around multiple axes simultaneously and lateral translation. The carcass complex fall dynamics along with drop from limited height prohibits it from attaining the terminal velocity. Under this limitation, drag coefficient can be estimated by fitting ballistic model to the measured data. A new multivariable optimization algorithm was performed to find the best-fit of the ballistic model to the measured data resulting in an optimized drag coefficient estimate. Sensitivity analysis demonstrated significant variation in drag coefficient with small a change in initial position highlighting the chaotic nature of carcass fall dynamics. Based on the limited sampling, the bat carcass drag coefficient range was found to be between 0.70–1.23.
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