BREMERHAVEN, Germany - Rotor blades are exposed to severe environmental conditions. Rain, hail, sand, salty sea water and dirt all gather on the surfaces and lead, especially on blade-leading edges, to soiling and roughening of paint and coating systems. Temperature changes and UV radiation intensify these effects. This leads to material abrasion and crack formation caused by rain erosion and in turn, results in increased deterioration of the blade’s aerodynamic characteristics. These changes not only influence wind turbine efficiency but also cause increased noise emissions. In order to increase offshore wind turbine efficiency, future rotors are to rotate faster, making the problem worse; at higher velocity, raindrops strike harder against coatings leading to even greater damage.

Researchers at the Fraunhofer IWES are going to develop a test stand that, for the first time and under climatized conditions, will be able to evaluate the resistance of rotor blade coatings to rain erosion. For this purpose and as part of the joint Rotor Blade Rain Erosion project, coordinated by IWES researcher Benjamin Buchholz, a climatized test stand with rotating arms is being created for testing model rotor blades with blade-tip speeds of up to 160 meters per second. “Here, the test conditions are variable – rotational speeds and climatic conditions can be adjusted individually according to the respective real rotor blade operating conditions,” explains Buchholz. “Weather and operational data records provide the necessary bases. In this way we hope to equally ensure test stand quality and the validity of the results obtained.”

Assessing damage mechanisms serves as a basis for the optimization of coating materials such as foils and paint and for the introduction of further measures such as, for example, changes in wind turbine operational management or adjustments to maintenance intervals.

In addition, project partner Automation Dr. Nix GmbH & Co. KG, a coating thickness measurement company based in Cologne, Germany, is developing a mobile system for rotor blade quality control and coating thickness measurements. To date, coating measurements are made in the laboratory using time-domain terahertz spectroscopy. In this way, the contactless testing of multi-layered, non-metallic material coating systems is achieved. Researchers are hoping to be able to develop a portable inspection system for the rain erosion test stand, which is also suitable for use in the field and will also enable less complex, quicker and more cost-effective rotor blade testing.

The project is funded with €1.4 million from the Federal Ministry for the Environment, Nature Conservation and Reactor Safety. It will run until September 2016.