The Bloomsburg WindJET A wildlife sensory impact assessment

John Huckans

Bloomsburg University

of Pennsylvania

Large three-bladed wind turbines are a hazard for bats because the fast and isolated wing tips are undetectable until it is too late for bats to avoid being killed by them (often by barotrauma).

Piorkowski MD, O'Connell TJ, American Midland Naturalist 164:260-269 (2010). Russo D, Jones G, Ecography 26:197-209 (2003). Rydell J, Bach L, Dubourg-Savage MJ, Green M, Rodrigues L, Hedenstrom A, Acta Chiropterologica 12:261-274 (2010a). Rydell J, Bach L, Dubourg-Savage MJ, Green M, Rodrigues L, Hedenstrom A, European Journal of Wildlife Research 56:823-827 (2010b). Baerwald EF, Edworthy J, Holder M, Barclay RMR, The Journal of Wildlife Management 73(7) 1077-1081 (2009). Grodsky SM, Behr MJ, Gendler A, Drake D, Dieterle BD, Rudd RJ, Walrath NL, Journal of Mammalogy 92(5) 917-925 (2011).

It seems reasonable to think that a slower moving turbine with many more blades would give bats more time to detect and avoid danger.

To test this idea, we simulated typical bat echolocation by directing high intensity ultrasound at rotating turbine blades of a novel design (Bloomsburg WindJET) and measuring the reflected wave.

Ultrasonic gated sonar detection of multiple rotating turbine wheels

At 40 kHz, our sonar signal was a sequence of 2 ms square pulses separated by 100 ms intervals (square wave amplitude modulation of a 40 kHz carrier at a 2% duty cycle). This approximates typical acoustic signal of Pipistrellus kuhli during the search phase of a hunt.

Schnitzler HU, Menne D, Kober R, Heblich K, In: Huber F, Markl H (eds) Neuroethology and behavioral physiology. Roots and growing points. Springer, Berlin Heidelberg NewYork 235-250 (1983). Schnitzler HU, Kalko E, Miller L, Surlykke A, J. Comp. Physiol. A 161:267-274 (1987).

Spokes

Directed perpendicularly at blades

Directed at blades at 45o (from left)

Directed at blades at 45o (from right)

Sonar was directed at various parts of the turbine and at various angles. Tests were performed at 10 and 20 RPM. Transducer: MB7076 XL-MaxSonar-WRLA1 (MaxBotix).

WindJET

A typical sonar signal showing both the emitted and reflected pulses

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0  

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0.4  

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0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

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Time (s)

Emitted signal

Reflected signal

Control 1) Sonar directed into free space (where no reflected signal is expected). Control 2) Solid metal wall located the same distance from the emitter as the turbine blade targets.

Results

•  The echo fractions reported in Tables 1 and 2 are all averages. Each average is derived from twenty-five separate echoes.

•  Substantial fractions of the emitted sonar are reflected back to the emitter.

•  Compared to the solid wall control (0.48), very nearly the same fractions were reflected back by the turbine blades and spokes when the directions were perpendicular to the surfaces of these targets.

•  There was greater variability in the reflected signal when the source was directed at the blades at 45o angles, left and right.

•  Weakly reflected sonar signals are common because of the specular nature of high frequency echoes.

•  40 kHz reflections tend to be more specular (have a strong angular dependence) as opposed to diffuse (have a weak angular dependence).

•  The specular nature of high frequency sound reflections is used to advantage in the hunting strategy of bats in a process known as glinting.

Assuming a typical bat such as Pipistrellus pipistrellus were approaching a turbine, at a maximum detectable distance of 15 m (Holdereid) and typical speed of 5 m/s (Griffin), approximately 30 pulses would be emitted before possible impact with a turbine.

Holdereid MW, Korine C, Fenton MB, Parson S, Robson S. Echolocation call intensity in the aerial hawking bat Eptesicus bottae (Vespertillionidae) studied using stereo videogrammetry. J. Exp. Biol. 208:1321-1327. Griffin DR. Listening in the dark: The acoustic orientation of bats and men (Dover, New York) 1-480.

For a standard large three-bladed turbine, on average zero to one pulse would be reflected back to a bat because the total angular coverage of the three blades is approximately 3o out of 360o. This issue was partially explored by Long from the standpoint of Doppler shifts and roughly the same conclusion was reached.

Long CV, Flint JA, Lepper PA. Wind turbines and bat mortality: Doppler shift profiles and ultrasonic bat-like pulse reflection from moving turbine blades. J. Acoust. Soc. Am. 128(4):2238-2245 (2010).

By comparison, nearly every pulse is reflected back by the WindJET.

Conclusions and Outlook •  The WindJET is acoustically

opaque to ultrasonic energy and may have greater detectability by bats than three-bladed wind turbines.

•  We hypothesize that the slow tip speed of this design minimizes air pressure disturbances below the level that produces barotrauma in bats.

•  We should duplicate our test using a conventional turbine.

Other Team Members Dr. John Hranitz

Matthew Hallowell Matthew Tallent