Abstract
The loss of biodiversity and climate change are two existential contemporary crises that can only be solved together due to multiple interactions. The need to produce electricity through a considerable expansion of renewable energies can lead to conflicts with the protection of biodiversity. For example, the operation of wind turbines is associated with the killing of bird and bat species that are at risk of being struck. The risk of impact is in turn not only a significant cause of danger for the bird and bat species concerned, but also a problem under species protection law in accordance with § 44 of the Federal Nature Conservation Act and thus relevant to licensing.
In Germany, the conflict of the risk of bats being struck is currently addressed in the course of planning and approval of wind turbines by means of preliminary investigations and, based on these, restrictions on operating times are included in the operating permit. The extent of the shutdowns is based on a country-specific definition that determines the maximum number of bats that may be struck per year and turbine under the environmental conditions prevailing on site. Below this so-called significance threshold, it is assumed that there is no significantly increased risk of killing bats.
The reason for the present project is to examine the scientific derivation of a uniform significance threshold for Germany. As a result, general threshold values for precautionary operating time restrictions for wind turbines are to be derived depending on the time of day and season as well as wind speed and air temperature.
In Germany, it is mainly the bat species flying and migrating in open space (especially the evening bat, lesser evening bat and rough-skinned bat) and species with pronounced curiosity behaviour (especially the common pipistrelle) that are endangered by the operation of wind turbines. In general, the size of the wind turbines, the length of the rotors and their proximity to the ground, the distance to roosting structures as well as the operating times influence the number of bat victims depending on weather conditions and the time of year. At present, the number of bats killed by wind turbines nationwide is in population-relevant orders of magnitude, especially caused by old turbines that are operated without bat-friendly operating time corrections.
Due to the low reproduction rate of bats, which are classified as K-strategists in terms of population biology, even slightly increased mortality rates lead to an increased risk of extinction. The extent to which increased mortalities at wind turbines affect the population size of bat species at risk of impact cannot be reliably calculated at present, as knowledge about the population size and seasonal density of these spatially and temporally very dynamic bat species is still insufficient. For this reason, it is not possible to seriously calculate a general, population- compatible threshold value for the number of tolerable fatalities at wind turbines. In addition, the protection of individuals under European law applies irrespective of population relevance. In principle, each individual of a species is to be protected pursuant to § 44, and the impact on the population is initially not legally relevant to the killing. The effect on the population is only examined in the course of an exceptional procedure under species protection law. There is a consensus in case law that the facts of the prohibition of killing are only fulfilled if the probability of this significantly increases. Thus, when assessing the significantly increased risk of killing, a project-independent basic risk of killing an individual must be taken into account. With the construction of wind turbines, the probability of a collision-related killing is increased in comparison to the existing life risk in the landscape, unless effective avoidance measures are implemented. With regard to the collision risk, these are above all the operating time corrections. These require general, technically and legally sound threshold values to be defined in advance.
The assessment of the killing risk for the regular operation of a wind turbine is currently handled differently by the federal states. There are very different specifications regarding the threshold value for the tolerable number of dead bats per WT and year (usually up to 2 bat victims per turbine and year). There is also considerable room for interpretation with regard to the specifications for general operating algorithms, especially with regard to the cut-in wind speed and the application of the recording of bat activity at nacelle height. According to current scientific knowledge, a blanket cut-in wind speed of 6 m/s does not guarantee compliance with the significance threshold given in the national guidelines. Since it is not possible to calculate the number of acceptable bat kills from a population biology perspective due to the lack of demographic parameters for bats, while at the same time being highly sensitive to mortality increases, the present document derives a technical recommendation based on the obligation under EU law to protect individuals and the current state of scientific knowledge on the avoidance of bat kills.
In order to minimize the risk of killing during the operation of wind turbines, a nationwide significance threshold of < 1 animal per turbine and year is therefore proposed. Compliance with the proposed significance threshold of <1 must be the threshold value for the blanket cut-in wind speed and should be replaced, at least in the first two years of operation, by a much more differentiated cut-in wind speed that takes into account the rotor blade diameter, the respective month and the respective night tenth as well as the natural area in Germany. This significance threshold should be replaced by a site-specific value after two years of operational monitoring. Monitoring of the blanket cut-in wind speed and temperature threshold (>10 °C) by nacelle monitoring of several wind turbines of a wind farm for at least two years is ideally part of the avoidance measure and the operating time corrections initially apply to the period 15 March to 15 November.
To comply with the significance threshold of < 1 and to determine the regionalised operating time corrections (especially cut-in and temperature), the ProBat tool should be used. This tool calculates the future installation algorithm on the basis of the bat activity measured at the nacelle while complying with the significance threshold and is currently the only scientifically justified method for calculating operating time corrections on the basis of measured bat activity.