For 25 years, Franz Trieb has worked in the Energy Systems Analysis Department of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). The department of the same name at the DLR Institute of Engineering Thermodynamics in Stuttgart is investigating energy technologies with the idea of achieving comprehensive sustainability that takes into account ecological, economic and social factors, and has thus made a major contribution to the success of renewable energy sources in Germany and worldwide. Franz Trieb and his colleagues want to create a factual picture that will support discussions and decision-making processes. With its energy research, DLR is committed to developing technologies and scenarios for a sustainable energy system that meets the energy needs of industry and society.
Dr Trieb, how does one come up with the idea of investigating the impact of wind farms on flying insects?
Franz Trieb: A comprehensive assessment of German energy scenarios based on a large number of indicators, which we carried out in 2017, revealed that the compatibility of wind farms and flying insects during the migration of insects to new breeding grounds was still an open question. Initial research has shown that the remains of flying insects on rotor blades can lead to large reductions in the efficiency of wind turbines and have motivated the establishment of a global cleaning industry for rotor blades. For this reason, we conducted a study on the subject, drawing on interdisciplinary expertise in the fields of entomology, atmospheric physics, wind energy, aerodynamics and DNA barcoding – a method of species identification based on DNA sequencing.
What was the starting point for your study?
Trieb: An extensive body of technical literature currently shows that large swarms of flying insects also seek high, fast air currents. They allow themselves to be carried by the wind to distant breeding grounds. Observations and measurements have been able to detect high insect concentrations worldwide at altitudes between 20 and 220 metres above the ground – the very same altitudes occupied by the rotors of wind turbines. The phenomenon of insect strikes can reduce the power output of wind turbines by up to 50 percent – this has been the subject of extensive research in both theory and practice. So far, however, the consequences of insect collisions with wind turbine rotors on the insect population and the ecosystem have not been investigated.
What approach was used in your study? How did you proceed?
Trieb: First, we conducted extensive research, collecting and evaluating existing scientific data. Based on this data, we created our own model calculation. On the one hand, this model calculation is based on an average insect density of around three creatures per 1000 cubic metres of air at the level of the wind turbine rotors. This figure was based on regular insect catches over Schleswig Holstein by entomologists between 1998 and 2004. On the other hand, for our model calculation, we extrapolated the volumetric flow, that is, the ‘air throughput’ of all the wind farms in Germany. Here, there are around 30,000 wind turbines with a total rotor area of around 160 square kilometres, which with a nominal wind speed of 50 kilometres per hour reach an average of 1000 nominal full load hours during the insect flying season from April to October. By simply multiplying these numbers, we calculated a seasonal air flow rate of about eight million cubic kilometres – that is more than 10 times the total German airspace up to a height of two kilometres. If one multiplies the insect density and airflow rate, then around 24,000 billion airborne insects fly through the rotors in Germany each year.
A simple approximation of the resulting damage can be derived from studies by Sandia National Laboratories on the contamination of rotor blades by flying insects. Four factors are multiplied with each other: the ratio (five percent) between the blade surface visible from the wind’s direction and the circular area swept by the rotor blades; the average proportion of the polluted blade area on both sides of the rotor blades totalling around 80 percent relative to the visible blade area; the so-called collection efficiency for winged insects averaging 40 percent; and a ratio between median relative blade speed (45 metres per second) and nominal wind speed (14 metres per second) of about 3.2. According to the figures, on average, about five percent of the creatures flying through a running rotor get hit. This amounts to approximately 1200 billion insects per year. These figures only consider creatures that leave visible residues on the rotor blades.
What conclusions can be drawn from your model calculation?
Trieb: Our model calculation points to an aspect of wind energy that has not yet been comprehensively researched. Approximately 1200 billion flying insects are struck each year as they fly through the rotors of wind farms in Germany. Such a large number of affected insects could be a relevant factor for the stability of the insect population and could thus influence species protection and the food chain.
What conclusions does the model calculation explicitly rule out? In other words, where do you as a researcher need additional data or more investigations?
Trieb: We cannot make any reliable statements as to what contribution the loss of approximately 1200 billion flying insects calculated in the study makes to the reduction of insect numbers. The reason for this is that we simply do not know how big the total population is or the amount of insect population reduction in hard numbers. In addition, there are as yet no absolute figures on other negative impacts on the insect population such as pesticides, intensive agriculture, climate change or urbanisation, so we cannot compare our numbers with other influences.
What further action would you suggest, based on the study results?
Trieb: From the currently available figures and the DLR model calculation, we cannot conclude either that wind energy plays a significant role in the reduction of insect numbers, or that it has no impact. From a scientific point of view, an empirical examination of the losses theoretically calculated in our study would be very useful as a next step. The goal must be to better understand the relationships between insect migration and wind farm operations. With this study we are offering our knowledge and expertise based on a one-year project, so that researchers from different disciplines, together with industry, operators and decision-makers can develop and implement measures to reduce potential environmental damage caused by wind farms in the future.
One idea would be, for example, an automatic swarm detection system that controls the rotors of wind turbines accordingly. A simple way to identify affected species would be a regular DNA analysis of insect residues on rotor blades. One potential source of damage that leaves no visible residue, and one that has not yet been investigated, is flying through the low pressure region on the forward side of the rotor blades. The effects of the corresponding barotrauma on the trachea and other organs should be examined.
 Trieb, Franz and Hess, Dennis: Wege zur regenerativen Stromversorgung II – Auswirkungen und Kosten, in: Energiewirtschaftliche Tagesfragen, no. 12, 2017, pp. 56ff; Trieb, Franz: Wege zur regenerativen Stromversorgung III – Elemente und Ausgestaltung, in: Energiewirtschaftliche Tagesfragen, Issue. 6, 2018, table on page 60.
 Weidel, H.: Die Verteilung des Aeroplanktons über Schleswig-Holstein, Dissertation, Christian-Albrechts-Universität Kiel, 2008, https://d-nb.info/1019553197/34.