Height monitoring for wind energy projects

All bat species that occur in Schleswig-Holstein are strictly protected under the Federal Nature Conservation Act and listed in Annex IV of the Habitats Directive.
When wind turbines are operational, bats flying into the rotor swept area are threatened by an increased risk of collision. This has been documented for 18 of the 25 species found in Germany (DÜRR 2023). The risk varies depending on species, season, weather, type of turbine and other factors.

The aim of monitoring is to record and evaluate bat activity in the danger zone of the rotor in a legally secure way. Based on the results, it is examined whether the offence of killing according to § 44 (1) No. 1 BNatSchG (Federal Nature Conservation Act) has been committed during the operation of a wind turbine and how it can be avoided by targeted shutdown measures.
This also allows for the reduction of blanket official shutdown requirements in cases of low bat activity on technical grounds, while maintaining the operation of the turbine in accordance with species protection regulations.

To assess the collision risk, bat activity is recorded directly in the danger zone of the rotor. Since this area is located at heights that cannot be reached using ground-based methods, the recording takes place in the nacelle of the turbine – i.e. in the centre of the danger zone.

For this purpose, a special monitoring system is temporarily installed in the turbine, which records the bats' echolocation calls using an ultrasonic microphone. The audio sequences are then evaluated and the calls are identified down to species level.

At the same time, weather sensors in the nacelle continuously record wind speed and temperature – two environmental parameters whose influence on bat activity has been scientifically proven and which must be taken into account when calculating shutdown specifications.

The analysis is carried out using the well established ProBat software (BEHR et al. 2015, 2018). The software enables a sound estimate of annual collision victims and determines individual operating limits for the respective turbine.

For the investigation period, usually from 1 May to 15 October, ProBat determines individual cut-in wind speeds (the wind speed at which the rotors may start operating) for each turbine, which, if complied with, do not result in the killing of bats. 
These values can be transferred directly to the turbine control system after approval by the nature conservation authority. The calculated cut-in speeds are often significantly lower than the blanket specifications, meaning that the turbine can be operated at maximum capacity without compromising bat protection.