The rotor blades of modern wind turbines can be up to one hundred meters long. During operation, they are stressed not only by their own weight but also by the forces of the wind, precipitation and temperature changes. Offshore turbines are additionally exposed to attack through salt water. As a result of these influences, damage can develop which impairs the output or can even lead to the total failure of the entire plant. Torn-off rotor blades can be very dangerous for humans and animals and can cause extensive damage to property. Rotor blades must therefore be regularly inspected for cracks, erosion damage and structural defects. These are, however, not always visible to the naked eye.
With active thermography (excitation through technical means), damage below the surface can be made visible. This technique has been used successfully by the wind-power industry for a number of years, thanks in part to research conducted at the Fraunhofer WKI. The procedure is, however, relatively complex: Direct access to the rotor blade is required, for example by means of a cable-bound working platform.
A particularly elegant procedure for the inspection of rotor blades is passive thermography (excitation through meteorological processes). An image can be taken from a distance of several hundred meters using a thermal-imaging camera with a telephoto lens, which is set up on the ground. In many cases, however, the optimal camera location is not actually accessible. Furthermore, if the wind direction changes, the location would have to be changed again and again, entailing the relocation of the extensive equipment. In addition, this technology cannot be used under any circumstances for offshore wind turbines.
Aerial inspection using drones could be the optimal solution. Currently available thermal-imaging cameras suitable for drones are not yet powerful enough for this purpose. Our project partner InfraTec GmbH is therefore currently developing a high-performance thermal-imaging camera which is light and compact enough to be carried by standard drones.
Some weather conditions impair the measurement results of passive thermography. Together with the Bundesanstalt für Materialforschung und -prüfung (German federal institute for materials research and testing ), we are therefore developing forecasting software at the Fraunhofer WKI which enables the weather-dependent calculation of the optimum inspection period and the determination of the detection limits of the thermography inspection. As a result, all defects can be very reliably detected during the inspection with drone and thermal-imaging camera. As consulting engineer, the Sachverständigenbüro Otto Lutz provides us with advice concerning the planning, interpretation and evaluation of the test measurements. The project partner clockworkX supports the project in data structuring, links measurement data with weather data by means of the latest technologies, and is developing an application for the open market.
Fraunhofer Institute for Wood Research 
