High-tech remedies

18.05.2015
Camera-equipped drones provide high-end display photos. (Photo: Batcam)
Camera-equipped drones provide high-end display photos. (Photo: Batcam)

Now, an international commission of experts is investigating the exact root causes for the only somewhat suitable protective coating. However, it is already clear that weather conditions and blades’ rotational speeds are partly to blame. For some turbine types, those speeds can reach 300 to 330 km/h offshore, while onshore they are a bit lower due to sound emissions.

At these speeds, rain drops and small particles have the effect of a high-pressure cleaner and eat away at the erosion protection applied to the front third of the blade. Then, the rain wears down the remaining coating layers, exposes the glass fibre reinforced plastics (GFRP) and, in the worst case, ­damages the blade structure.  UV radiation, temperature ­changes and blade vibrations also play a role. ­Because the process is very detrimental to ­aerodynamics, most players in the industry estimate yield losses to be ­between 4 and 8 %.

Imitating reality

Trials on test rigs demonstrate that factory-made coatings actually do not last very long. In this respect, the Poly Tech A/S helicopter test rig in Denmark is regarded as a Mecca, because its results are generally accepted in the industry. The models are accelerated to as much as 150 m/s and simultaneously being ­exposed to rain drops. While some coatings wear off after just two hours, one product from BASF was able to withstand the constant stress for 18 hours.
Although major companies such as BASF and LM Wind Power have their own test stands, uniform standards are lacking for replicable test runs and the procedures to be applied on the test rigs. The latter are to be defined by an international commission. One option is helicopter test rigs, on which the blades to be tested are accelerated to extreme speeds and shot at with rain drops. This type of test rig is avail­able in the US, the UK, Germany and, as previously mentioned, Denmark.

Another possibility is high-pressure jets of water that are shot at the blades. In both cases, the ­methods used are intended to come very close to real conditions and quickly provide data on the material’s ­durability. “What the coating industry has been lacking for this particular field are test procedures and test rigs, specially developed for simulating damage from rain erosion in the lab in a way that is replic­able,” says Susanne Bender of the German Institut für Lacke und Farben in Magdeburg. 

New repair solutions

Until then, manufacturers must rely on their own efforts. Production has already started for Relest, the product developed by BASF for better protecting ­edges and blade tips. GEV was one of the first partners to use this coating. What works well in the lab or during manufacturing, however, also needs to do so in the field. According to experts, repair kits are ­rather cumbersome or even nonexistent. “They should be able to be applied directly to the blade, even in incle­ment weather, without too much effort,” blade expert Brassel insists. Because these solutions do not yet exist, Windtechnik teamed up with einzA Colorvision to launch their own product after rigorous testing. ­Although the Leading Edge Protector performed well, the market still remains reticent.

Sika AG of Switzerland also has two new products on the market that are suitable for repairing edges and damage at the blades. Products differ in terms of their areas of application and, in particular, possible environmental conditions and hardening timeframes. The Sika repair kits can be used at temperatures as low as 5 or 15 degrees, respectively.

Rotos360 Ltd., however, is banking on Gurit, another Swiss company. The service provider patches defective blades both onshore and offshore. For the latter, the blade specialist has a fully equipped ­specialist vessel available. Gurit’s coating, Renuvo, can be used both during production and as a repair kit in the field. From a user’s perspective, the ­advantage is that the coating can be used at temperatures below five degrees Celsius and hardening ­only takes a few minutes even without additional heating systems. “Blade inspections and repairs can be ­carried out 24 hours a day using 12-hour shifts, which double productivity,” says Simon Sanderson, Technical Director of Rotos360.

Torsten Thomas

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