More Big Green Collapse – Those Rotting Wind Turbine Blades

Jan Liersch, Managing Director of Key Wind Energy and lecturer at the TU Berlin, draws attention to erosion problems on modern rotor blades. Interview below:

You are working on rotor blade erosion. What are you investigating?

Jan Liersch: When it comes to blade erosion on wind turbine rotor blades, we are actually dealing with mechanical wear. This is mainly because the blades rotate at a high speed, at the tip of the blade this is over 80 meters per second, i.e. around 300 km/h.

This means that we are dealing with a very, very high impact energy. This affects the leading edge in the outer area of ​​the rotor blade. We have to distinguish between two different causes: Erosion is caused by dust particles in the air and by raindrops. This particularly affects offshore wind farms. A little more rain is to be expected, less dust.

How does this happen mechanically?

Jan Liersch: A drop like this hits the surface and, so to speak, pushes the paint aside. It literally crumples it if the material can’t handle it. Or the raindrop gives a violent impulse to the surface. You can imagine it like an elastic layer, and pressure shocks are then passed on into the material underneath, almost like sound waves. Drops can therefore damage the leaf in the lower layers without being visible from the outside.

Could a small speck of dust or something like that be the first trigger?

Jan Liersch: It may be that we have a small scratch or a small impact crater caused by a grain of dust, for example. We’re really talking about microscopic initial damage here. And then it would be the case that the drop also hits this irregularity. That is, it is simply a ridge or a dent or an edge where this microscopic tidal wave hits and ultimately causes kinetic energy to act there. So we have excessive wear and tear at this point.

Are the large, new rotor blades more affected because they rotate faster?

Jan Liersch: Yes, but that is not due to the size, but rather to the larger speed number chosen by the sheet designer. For example, the tip of the blade moves not only at 75 m/s, but at 85 m/s. 300 km/h at the tip of the blade is possible. Problems arise primarily in the outer third and on the front edge.

Does this damage have a negative impact on lifespan or yield?

Jan Liersch: Unfortunately, both. If the front edge shows increasing damage, it initially looks like cosmetic damage, but you don’t have to do much about it. But as it progresses, the protective layer of outer coating is gone. Then things go faster and faster, this also affects the load-bearing parts, fiberglass layers can peel off, water can penetrate, and the repair effort ultimately becomes more and more difficult. At the same time, performance drops.

It’s just like changing the aerodynamic profile. This is also a gradual process, so that after four or five years you can begin to notice the performance. Until you finally have a few percent loss in yield. Then it is very important that you do something. But then you can see it very clearly.

Should the coating of the rotor blades be adapted to the respective location?

Jan Liersch: That is exactly the idea we are pursuing. Manufacturers know they will face more erosion at certain locations. We have two types of erosion pollution: dust and water droplets. German offshore wind farms tend to have to contend with drops. It became apparent that some parks needed repairs after a surprisingly short period of time.

How many of the new systems does this affect?

Jan Liersch: At locations with high levels of erosion, all systems are affected. In principle, all offshore systems are affected. But it also depends on whether they received erosion protection before construction or were retrofitted. Too little is known about it on a scientific level. Therefore, together with Fraunhofer IWES, we considered carrying out joint measurements.

We would use specialized sensors that properly measure dust and droplet size to measure the erosion environment at representative locations. We would do this onshore at perhaps a dozen locations in Germany and at two or three offshore locations. If we were to measure this for a few years, we would have an erosion load map. This would allow you to choose the right erosion protection for each location.

Is there erosion protection that can be retrofitted?

Jan Liersch: We receive the most inquiries in our materials laboratory from retrofitters who say: We have developed something that could be particularly good for this type of location. And then we make comparative measurements with standard coatings. We try to find out which location the repair material is best suited for.

There are a number of providers who promise that their material offers particularly good erosion protection at the site. There are two aspects: resistance to erosion and cost. If I want to run my park for another 15 years, but my blade leading edges need to be renovated, I need a material that will last those 15 years. If I have a park that is going to be degraded in three years, it might be more cost effective with an emergency repair.

Would monitoring make sense for prevention?

Jan Liersch: As the only benefit of rotor blade monitoring, it would probably be too expensive. But there are also other reasons why rotor blade monitoring is carried out. Then the erosion aspect would be covered. Or you can reduce the speed of the systems in heavy rain. This would have reduced the erosion load for the period.

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