Technology for Wood and Natural Fiber-Based Materials

Research Project

EvalTherm – Efficient inspection of rotor blades on wind turbines by means of passive thermography

Wind energy is a mainstay of the energy revolution. The technical reliability of wind turbines is therefore constantly gaining in importance. In particular, the rotor blades are exposed to high stresses - and damage to these blades can result in serious consequences. Conventional inspection procedures require direct access to the rotor blades. Together with our project partners, we are striving within this project to achieve a simpler solution: inspection by means of a drone and passive thermography. The disadvantage of this method is that not all defects are visible in all weather conditions. A further project goal is therefore software which can be used to calculate the optimum inspection period and the detection limits in accordance with the weather.

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.

How does passive thermography work?

Black-and-white image of the hub of a wind turbine.
© Fraunhofer WKI | Peter Meinlschmidt
Passive thermal image of the hub of a wind turbine. The metal bolts which attach the rotor blades to the hub stand out, even though they are several centimeters below the surface.

Passive thermography exploits temperature changes which occur on the rotor blade as a result of changing meteorological boundary conditions (temperature difference between day and night, solar irradiation). Due to their lower thermal inertia, thin areas of the rotor blade change their temperature faster than thick areas. As a result, the surface temperatures of the blade reflect its internal structure. If the surface temperatures are then visualized using a thermal-imaging camera, valuable information can be obtained regarding the condition of the blade below the surface.

Conventional methods of rotor-blade inspection

Two people abseil down the blade of a wind turbine.
© Fraunhofer WKI | Peter Meinlschmidt
Visual inspection of a rotor blade by industrial climbers.
A large metal framework containing two people and technical equipment is pulled up the tower of a wind turbine using cables.
© Fraunhofer WKI | Peter Meinlschmidt
With the aid of a working platform, it is possible to travel along the vertically positioned rotor blade of a wind turbine, thereby acquiring active thermographic images. The software was developed at the Fraunhofer WKI.
A camera on a tripod stands at the edge of a field. It is directed at a wind turbine which is around 100 meters away.
© Fraunhofer WKI | Jochen Aderhold
Inspection of rotor blades from a distance using passive thermography. The thermal-imaging camera is equipped with a 200mm telephoto lens. The software was developed at the Fraunhofer WKI.

Project partners

  • Bundesanstalt für Materialforschung und -prüfung (German federal institute for materials research and testing)
  • clockworkX GmbH
  • InfraTec GmbH (location: Dresden)

Funding

Official project  title: EvalTherm – Evaluierung der passiven Thermografie für die Zustandsbewertung von Rotorblättern an Windenergieanlagen (Evaluation of passive thermography for the condition assessment of rotor blades on wind turbines) 

Funding body: German Federal Ministry for Economic Affairs and Energy (BMWi)

Funding reference: 03EE3035B

Project management: Project Management Jülich (PtJ)

Duration: 1.9.2020 to 31.8.2023