Abstract
During the late summer and autumn Nathusius’ pipistrelles migrate from the Baltic States and Russia to western Europe to mate and hibernate. Some of these bats cross the North Sea to the UK. By doing this they can enter offshore wind farms. The moving rotor blades of wind turbines are known to cause mortality among bats. This mortality rate is likely to rise as the total capacity of Dutch offshore wind farms is planned to increase from the current capacity of approximately 1,000 MW to 4,450 MW in 2023 and later. For the planned wind farms in the North Sea it was recognised that the mortality caused by wind turbines could affect the Nathusius’ pipistrelle population. In order to prevent this, wind turbine operations are reduced between 15 August and 1 October during low to moderate wind speeds (at night between one hour after sunset until two hours prior to sunrise, the speed reduction is raised to 5 m/s and rotational speed is low during the idling phase). This curtailment reduces the risk of bat fatalities but also leads to a loss in energy production.
Since 2015 a substantial amount of new information about bat activity in the Dutch offshore wind farms has been generated by acoustic monitoring. This information was used in this study to define a more efficient curtailment strategy. The goal of this study was to reduce both mortality rate and energy production losses.
During the evaluation of the existing curtailment strategy, it became clear that improvements are highly advisable. If the existing curtailment strategy were to be implemented on modern wind turbines, the resulting fatality rate could still be considerable. The main reason for this is the fact that a large proportion of bat activity takes place during wind speeds above 5 m/s when wind speed is measured at wind turbine tower height (100 m). The only way to drastically lower the risk of fatalities is to raise the cut in speed to above 5 m/s. Since energy production significantly increases above 5 m/s, this can only be done during very specific conditions with high bat activity. High temperatures and easterly winds are two significant predictors of bat activity that are currently not being used. A more efficient curtailment strategy is possible by adding these parameters and by shifting the curtailment season to 25 August 10 October.
To determine the settings of an optimal curtailment strategy a theoretical approach was used to estimate a theoretical fatality risk since the actual fatality rate is unknown. To estimate this fatality risk an association between the number of fatalities and both bat activity and the rotational speed of a wind turbine was assumed. Subsequently, this fatality risk was divided by energy production. The curtailment strategy was optimised by stepwise adding and subtracting conditions to the curtailment with small increments, including conditions with a high (above average) ratio of fatalities to energy production and excluding those with a low ratio until an optimal setting was achieved with both the lowest bat fatality risk and the lowest loss of energy production.
The optimal curtailment strategy consists of increasing the cut in speeds to 5.5 6 m/s when there is an easterly wind and an unaltered cut in speed (the turbine default setting) when temperatures are low and the wind westerly. Compared to the existing curtailment strategy, this new strategy results in a 12% lower loss in energy production and a substantially lower risk of fatalities (15%).