Leading Edge Protection - Is it Needed?
Leading Edge Protection (LEP) is essential for wind turbines with nominal power larger than 2MW. Due to the increased blade length, the tip speed of these turbines can reach up to 90 m/s. That means for sites with annual accumulated precipitation of 0.7 m, the tip area of the blade is hit by an accumulated water column of approximately 10.5 m per year. The introduction of 90+ m diameter rotors coincided with the development of the first large utility scale windfarms in the early 2000s. The combination of high tip speeds and installations in harsher offshore and onshore environments also caused tremendous erosion problems.
Older blades have been produced without factory applied LEP and frequently require large repair campaigns of LE damages. With the industry recognizing erosion as a major issue, the race for developing an effective erosion protection solution started. Newer blade generations are now produced with a factory installed LEP in the erosion zone. However, these solutions are not flawless, and repairs are required throughout the lifetime of the turbine.
What and Why?
To ensure that the most optimal repair/retrofit method is selected, the wind turbine operator should understand the observed damage mechanisms on their site. In the aggressive erosion zone, usually spanning a few meters starting from the tip, the loss of material is driven by environmental conditions. When rain hits the blade surface, a combination from the stress waves propagation, surface fatigue from the repeated droplets impact and the direct deformation at high impact speeds are considered the main reasons for erosion development. In that area, the defect is observed as a continuous stripe of material loss, usually centred around the LE. Outside of the aggressive erosion area, surface defects on the LE develop around pre-existing imperfections in the LEP or the underlying layers. The defects there are characterized by variety in sizes and discontinuity, more uneven depth, random location, and rough edges.
The general recommendation for LEP repair is heavy duty solution in the aggressive erosion zone and easy to apply solution for the remaining LE erosion zone. There are 3 main leading edge protection groups – coatings, tapes, and shells. Each of them has its pros and cons, so a good understanding of the erosion condition on a specific site is vital to select the most suitable LEP.
LEP Solution Overviews
LEP shells are precast with a specific shape for each blade type. They are installed on the blade with an adhesive and the edges are sealed with a sealant to prevent water ingress. This LEP type has the highest level of erosion protection. On the other hand, due to its thickness and shape, it affects the AEP negatively. The solution is suitable for the aggressive erosion zone of wind turbines in harsh environments in their early to mid lifetime. Defects that can be observed on the LEP shells are peeling at edges and overlap zone and delamination at the adhesive interface.
LEP tape is supplied in rolls. The adhesive is pre-applied on the LEP tape, however, adhesion promoters and application solutions are required during installation. Edge sealant shall also be applied to protect the edges from water ingress. This solution provides solid erosion resistance, however, it degrades under the effect of UV radiation. It is relatively hard to apply especially around the tip, due to the change of shape there. Moreover, contamination and imperfections in the underlying surface might cause failure of the LEP tape adhesion. Defects on this LEP type include chipping, peeling and scratches from impacts with sharp external objects.
LEP coating is characterised by higher elasticity and flexibility than the normal paint applied on the other areas of the blade. The solution is suitable for combination with LEP shells, covering the erosion area outside the aggressive zone. The application process requires a well executed surface preparation. Insufficient sanding or contamination would greatly impact the adhesion quality of the erosion resistance. Another defect enabler for the LEP paint is the propagation and interaction of the compressional waves from the raindrop impact, which can cause delamination in the interface of the LEP and the underlying surface, or between two layers of LEP coating if the solution is not single layer.
Conclusion
Leading Edge Protection is a critical component to protect the longevity of wind turbine blades. Without proper care and maintenance, cosmetic issues on the leading edge of wind turbine blades can turn into production robbing problems and eventually structural damages that may cause catastrophic failures. If you are unsure of the status of your blades, options in the market, or need guidance about leading edge protection – reach out to an expert!
Hristo Shkalov, Senior Blade Specialist – Wind Power LAB
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