Wind farms can become a death trap for bats. To determine the risk of such collisions, scientists usually use ultrasonic detectors that record the activity of the animals. This can reveal when shutdown times and other protective measures are needed. However, as has now been shown, these measurements are usually very imprecise, especially in the case of larger, modern wind turbines. A remedy is required here.
Energy production from wind power is an important pillar of the German energy transition in order to reduce dependence on fossil and nuclear energy sources and to achieve climate-neutral power generation. In order to maximize the energy yield, large wind turbines with particularly long rotor blades are being used more and more frequently. A setback, however, is that many rare and protected bats collide at wind turbines - the number is estimated at 15 bats per wind turbine and per year. Switching off the wind turbines at times of high bat activity can help here, because this can reduce the mortality rate of the animals by 80 percent, while the operators of the wind turbines would only make a small monetary loss of 0.5 to two percent.
But when is the activity of the bats greatest and therefore the right time to switch off the wind turbines? This is currently being recorded with the help of ultrasonic detectors, which register at what times and under what conditions, such as ambient temperature and wind strength, the bats come within range of the dangerous rotor blades. How good and reliable these measurements are has now been investigated by Christian Voigt from the Leibniz Institute for Zoo and Wildlife Research (IZW) and his colleagues. In their study, they determined which factors can falsify the number of bats registered by the detectors and to what extent. To do this, they varied both the spatial distribution of the bats and the detection range of the ultrasonic detectors in the risk zone of the rotor blades in a mathematical simulation.
Reliability dependent on bat call and flight patterns
The results of the modeling showed that bat activity predictions are only accurate under certain conditions. This is the case, for example, when the bats fly through the risk zone within reach of the rotor blades with equal frequency and density. However, if the bats pass one part of the risk zone more frequently than another, this irregularity leads to the acoustic activity being overestimated or underestimated. Bat activity was also underestimated when the animals used particularly high-frequency echolocation because these calls are less well recorded by the measuring instruments. Bat species with low-frequency calls, on the other hand, were adequately recorded by the ultrasonic detectors because their calls are carried over relatively long distances.
However, both the overestimation and the underestimation of bat activity are problematic: “If the acoustic activity of bats is underestimated, the wind turbines are switched off too briefly and at the wrong time, and many bats die. If, on the other hand, the acoustic activity is overestimated, the shutdown regulations are too strict and the systems do not produce any energy, although no bats are in danger,” explains Voigt.
More precise predictions needed
In many cases, the ultrasonic detectors used to date can no longer clearly determine bat activity in the larger wind turbines, as the results of Voigt and his colleagues showed. . "The technical solutions for protecting bats could become ineffective in large systems, which is at the expense of the bats and at the expense of energy production," says Voigt.
According to the scientists, however, acoustic detection could be improved, for example with more sensitive and additional ultrasonic detectors that can also cover the area of longer rotor blades. But a better understanding of the spatial distribution of bat passages could also help to predict local bat populations more precisely. Despite all the modeling and predictions, it is still important to search the area around the wind turbines for victims to check whether the recommended shutdown times are actually effective.
Source: Leibniz Institute for Zoo and Wildlife Research Berlin, Article: Conservation Science and Practice; doi: 10.1111/csp2.12841