Research | IZW | 08-10-2021

Acoustics meets wildlife research

How the monitoring of bats around wind turbines can (and should) be improved

Dead bat below a wind turbine. | Foto: Christian Voigt, Leibniz-IZW

Many of the great historical moments in science were made possible by researchers who had extraordinary knowledge and visionary spirit in a wide range of disciplines – both in and beyond science. Counting among names like Gottfried Wilhelm Leibniz and Leonardo da Vinci is another universal scholar by the name of Hermann Ludwig Ferdinand Helmholtz. In addition to his contributions to physiology and pathology, he is particularly known for his works on electro- and thermodynamics as well as optics and acoustics. Today, universal scholars, with their extreme diversification and expertise in the sciences, may be rare – but trans- and interdisciplinary thinking is often of great benefit when trying to understand the world, which is decidedly not compartmentalised into pigeon holes. If Helmholtz, with his acoustic resonator, and Edward O. Wilson, with his holistic view of research and protection of biodiversity, had worked together, it might have culminated in a research paper much like one that was recently published by the Leibniz Institute for Zoo and Wildlife Research. The biologists used acoustic analyses of the orientation and communication of bats in order to improve the methods used to protect these fluttering mammals around wind turbines – a contribution towards reconciling the full energy transition (“German Energiewende”) from conventional energy sources to renewable sources with conservation goals.

In order to transition from conventional to renewable energy sources, more and more wind turbines are being put up everywhere in the world. While this is climate-friendly, it has a tragic side-effect in that the turbines cause huge numbers of bat fatalities. This is a problem that wind power operators and conservationists must solve because all bat species are protected by law due to their rarity. To ensure that the energy transition does not jeopardise conservation efforts, ultrasonic detectors are already being used to record the acoustic activity of bats. To find out when the operation of turbines poses a threat to bats and when it does not, the detectors determine the times and environmental conditions at which bats are particularly active around the turbines. For this purpose, microphones record the echolocation calls of bats as they fly into the risk zones of the rotor blades. From these recordings, threshold values for things such as temperature and wind strength can be derived for bat-safe operation of the turbines. This way, wind turbines will only produce electricity when no or only few bats are active – for example mitigation measures such as curtailing the operation of turbines are practiced at night, at relatively high ambient temperatures, during migration periods, and at low wind speeds.

Yet, despite the existing measures, many animals continue to die. “This could be because the acoustic monitoring, while well thought out, is insufficient in its methodological implementation,” summarises bat expert Dr. Christian Voigt, Head of the Leibniz-IZW Department of Evolutionary Ecology, together with international colleagues in a joint publication. “Each bat species produces echolocation sounds at a pitch and volume typical for the species,” Voigt explains. He and his colleagues simulated sound propagation using the example of the common noctule, with calls of a low frequency (about 20 kHz) and a high sound pressure level (110 dB), and Nathusius’s pipistrelle, with calls at a higher frequency (about 40 kHz) but a lower sound pressure level (104 dB). “Our simulations show that, according to the laws of physics, the calls are attenuated with each metre of distance as they propagate through the air by 0.45 dB per metre for common noctules and by 1.13 dB per metre for Nathusius’s pipistrelle,” says Voigt. Thus, with the commonly used detection threshold of 60 dB, ultrasonic detectors record calls of common noctules at a distance of up to 40 m away. For Nathusius’s pipistrelle, the detection range is on average 17 m.

Given that today’s wind turbines can have rotor blades measuring 50 to 60 m, or even longer, the existing detection range is too short. Also, the area above the nacelle, meaning almost the entire upper half of the blades’ rotation circle, is only poorly captured by the ultrasonic detectors. For bats that don’t happen to fly directly towards the ultrasonic microphone, the detection range is drastically shorter still.

These and many other factors limit the effectiveness of the detectors. They only poorly reflect the risk of collision, and regulations derived from their readings may be inadequate as a result. In order to improve coverage of the risk zones of the rotor blades, the scientists recommend additional detectors at other locations, for example above as well as on the downwind side of the nacelle. In order to detect bats flying at lower altitudes or hunting for insects on the mast surface, it may also be advisable to install ultrasonic detectors directly on the mast. Complementary sensor technology such as radar systems or thermal imaging cameras could provide even more information to ensure that the energy transition does not come at the expense of biodiversity.

Jan Zwilling