Toward erosion-free wind turbine operation: Physical insights into leading-edge erosion and their application to the erosion-safe mode

Abstract

Many wind turbines experience leading-edge erosion on their blades due to rain and hail impacting at speeds of up to 100 m/s. The impact speed is driven predominantly by the blade tip-speed, which is expected to grow in future turbine generations as they become larger. Erosion can remove substantial amounts of material from the blades. Eventually, the damage can reach deep into the structural layers of the blade, where it then starts to jeopardize its structural integrity. The associated roughening of the blade is accompanied by losses in the annual energy production (AEP). These are estimated to be up to several percent, depending on the severity of the erosion damage. While some leading-edge protection systems have been developed, no satisfying solution has been found, and the mechanisms that lead to erosion have yet to be fully understood. The aim of this thesis is to enhance the understanding of the physical mechanisms that promote erosion, understand which site conditions contribute to erosion and apply the gained insight in the erosion-safe mode.

This thesis starts in Chapter 2 by analyzing the impact of erosion on the AEP loss by using reduced-order modeling and subsequently compares it with the erosion-safe mode (ESM). The ESM is an alternative operational erosion mitigation strategy that aims to mitigate erosion by reducing the tip-speed of the turbine during precipitation events. It is shown that, depending on the mean wind speed and frequency of damaging rain at the site, the erosion-safe mode can lead to a lower AEP loss in comparison to a mildly eroded blade or a blade that was fitted with a leading-edge protection solution that leads to similar flow disturbance. However, it still needs to be sufficiently understood what rain is damaging and what other site conditions might promote erosion.

A step toward resolving this knowledge gap is taken in Chapter 3 by investigating the behavior of rain droplets before impact with the blade. Contrary to prior state-of-the-art, it is shown that droplets deform and break up near an incoming wind turbine blade. This finding contradicts the current approach in erosion research of modeling rain droplets as circular. It is shown that deformation reduces the impact velocity of rain droplets with the blade. This effect depends on the diameter of the rain droplets and can be in the order of 10 m/s. Small droplets experience significantly more slowdown than larger rain droplets. This reduction highly influences the formation of erosion damage since the main driver for erosion is the impact velocity. As droplet deformation and slowdown depend on the rain droplet diameter, the described effect can be termed drop-size-dependent effect.

Chapter 4 continues the investigation of drop-size-dependent effects in leading-edge erosion. An advanced erosion damage model is built that includes several drop-size-dependent effects. It is shown that the significant drop-size-effects all suggest that the erosiveness of rain droplets increases with increasing droplet diameter. This is found to be true on a per-drop basis but also when normalizing for droplet size. Therefore, selecting an appropriate droplet diameter for experiments and numerical studies is essential since not all droplet diameters contribute equally toward forming erosion damage. Drop-size effects have substantial implications for the ESM, as increasing rain intensities shift the composition of precipitation from primarily small droplets to a composition dominated by larger ones. For an equal rain column, high-intensity precipitation events are, hence, more erosive. It is found that, for a coastal site in the Netherlands, 50 % of the erosion damage is produced by the 10 % highest-rain intensity events. Thus, in ESM operation, it is advantageous to reduce the tip-speed mainly during high-intensity precipitation events to maximize lifetime and minimize AEP loss. However, a precise relation between precipitation intensity and tip-speed that optimizes this objective is not yet known in leading-edge erosion research. A novel semi-analytical approach is devised to bridge this gap, taking into account site conditions, turbine type, and drop-size effects. With this approach, it is possible to extend the erosion lifetime of a contemporary blade by a factor of 13 for a moderate AEP loss of 1 %.

A critical component for the successful utilization of the ESM is the accurate forecasting of precipitation events minutes to hours ahead. However, the best approach for obtaining this information is still debated. For the first time, Chapter 5 benchmarks a state-of-the-art weather-radar-based probabilistic rainfall nowcast product by the Royal Netherlands Meteorological Institute (KNMI). The performance of the nowcast is assessed for various lead times for three sample sites in the Netherlands and for two distinct ESM strategies. The results show that the quality of the nowcast degrades with increasing lead times. The 5- and 15-minute lead times exhibit sufficiently good accuracy and response time for the successful utilization of the ESM. Across the sites, for a large 15 MW turbine, a lifetime extension of factor five can be achieved for an AEP loss of about 1 %.

To summarize, this thesis introduced the highly significant effect of droplet slowdown and deformation occurring in the vicinity of wind turbine blades. It investigated drop-size-dependent effects and established their significance for ESM operation. It provided new theoretical insights into the ESM and used these to devise a method for finding optimal ESM strategies that exploit drop-size effects. Finally, it benchmarked the devised strategies using a state-of-the-art (operational) nowcasting product and showed that the ESM could already be a viable erosion-mitigation strategy.