The impact of particles such as raindrops and hailstones on the leading edge of wind turbine blades causes progressive material damage. This may lead to a loss of material from the solid surface. The resulting damaged surface of the blade may reduce the aerodynamic performance of the wind turbines significantly, and subsequently, it reduces the annual energy production of the wind turbine.
To maintain acceptable aerodynamic performance, wind turbine and wind farm owners need to repair blades several times during the lifetime of the wind farm. This results in significant operation and maintenance costs and additional loss of energy production due to downtime.
Harsh environmental conditions in combination with high blade tip speeds result in extremely large and intense impact forces between droplets and blade surface. The blade tip speed of a modern offshore wind turbine can exceed up to 325 kilometres per hour (90 m/s). Under these conditions, the tip region of wind turbine blades perceives a seemingly harmless rainfall like a true hail of repeating impact of bullets! With the current trend of larger and larger blades with higher tip speeds, it is expected that damage will become even more severe.
In the WINDCORE project, we have developed wind turbine control strategies to limit the damage of the blades caused by rain droplet impact. More specifically, the project explored the strategy to control the rotor speed during a rain shower.
Given the fact that wind turbine blades erosion occurs predominantly during bad weather, the idea is to reduce damage by lowering the rotor speed when it is raining or hailing heavily. In the end, even in the harshest offshore environments, it is fortunate that these kinds of severe damages happen only during a small fraction of the turbine lifetime. Therefore, reducing the rotor speed during a heavy rain will result in a significant extension of blade lifetime while only a small part of the electricity production is lost.
In the WINDCORE project, the optimal speed during harsh conditions has been related to specific offshore environmental conditions. This has been through several aerodynamical computer simulations, modelling and laboratory testing of the leading edge erosion process and characterization of the critical precipitation condition. In this way, the project determined the optimal balance between maintenance cost reduction and shorter downtime due to fewer repairs needed and power production lost because of reducing rotor speed.
The project developed optimised rotor control strategies, based on reducing the rotor speed during heavy precipitations. In this way, the lifetime of wind turbine blades is significantly extended. Wind turbine manufacturers and wind farm operators may use this information to operate wind turbine rotors to make them live longer and become less damaged.
The project showed that the application of the control strategy to mitigate leading-edge erosion may result in a 1.4% O&M cost reduction and a 0.16% annual energy yield increase due to the control strategy, resulting in a four-yearly minor repair instead of annual repairs. The improvement translates to a total cost reduction of 0.57%.
We analyzed that wind turbine blades at coastal locations erode three times faster than inland and that erosion occurs faster at wind turbines with higher top speeds. This implies that from the blade tip a higher length of protection of the leading edge is required for coastal areas.
The WINDCORE team also concluded that there is limited information available on precipitation characteristics like droplet size distribution, intensity for offshore locations in the Netherlands. The information is needed to use a site-specific strategy for leading-edge erosion mitigation and maintenance.
The project learnt that applying the right control strategy for wind turbines is currently not possible due to a lack of knowledge about the precipitation events that lead to erosion. Currently, no useful information is available about the type of precipitation and related weather conditions at the location of existing and future wind farms in the North Sea. Also, it is not yet possible to link precipitation types and data on erosion with other weather data, such as wind fields, seawater aerosols (salt spray up to the nacelle height is possible in certain conditions), temperature, and UV radiation level.
Shortly after completing the WINDCORE project, the project PROWESS started in which we will measure and monitor the characteristics of the precipitation at different sites in the Dutch North Sea and coast. Based on this information we will develop an optimized weather atlas for precipitation and wind with a high spatial and temporal resolution for the Dutch part of the North Sea.
In a follow-up study to WINDCORE, we aim to have larger blades with higher tip speeds producing optimal electrical energy over their entire lifetime without damage to the leading edge. As already discussed with the consortium partners, field tests will be a natural continuation of this project, and it will be the subject of a future proposal.
The project will make use of TNO's modelling competence and TU Delft's competence of erosion and their material knowledge to predict (or confirm) the progress in erosion.
Harald van der Mijle Meijer
+31 6 10 85 23 14
This project is supported by the Netherlands Enterprise Agency (RVO) and TKI Wind op Zee. Project information at the TKI Wind op Zee websiteMillions of collisions at 300 km/h - Ouch!