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At first the protective layer erodes, then the material destruction continues at the deeper layers.
© Seilpartner Windkraft GmbH
Wind energy plant rotor blades
Projektinfo 14/2017

Example of the course of damage as a result of a minor production fault with a trapped dust particle, scale 500 μm
© Fraunhofer-Institut für Windenergie und Energiesystemtechnik

Parameters such as speed, drop size, quantity of water and weather and climate conditions can be varied in a test stand for rain erosion. The test stand has a base area of 3.3 x 3.8 m and a height of 4.5 m. The test samples have a length of 25 cm and a height of 3 cm.
© Fraunhofer-Institut für Windenergie und Energiesystemtechnik
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Rain can damage rotor blades

Rain and other weather and environmental factors can have a detrimental effect on the rotor blades of wind turbines in the long term. The leading edges of the blades are particularly susceptible to damage. Here the protective layer erodes first and material degradation subsequently continues deeper into the blade structure. A new test stand is now investigating the mechanisms involved in rain erosion in detail. The aim is to provide better protection for rotor blades in the future so that they can be used longer, thus improving the economic viability of wind turbines.

Many forces other than wind act on wind turbines. For example, raindrops, hailstones or grains of sand that impact on the rotor blades with high speeds and thus also with high energy can cause significant damage. These damage mechanisms are collectively referred to as rain erosion. To counter this, the blades are given a special protective coating and sometimes the leading edge, which is subject to particularly demanding conditions, is also fitted with a protective film. This protection is referred to as the Leading Edge Protection System (LEP). The blade then has to resist the influence of weather, climate and UV light in day-to-day service. These stresses are increasing due to the rising speeds of newly developed blades. The tips of a 60 m long blade achieve speeds of over 300 km/h during operation. At this speed, collisions with water drops are consequential in the long term. The planned investigations aim to identify the factors involved and the relative importance of the various influences at play.

This type of damage has a significant effect on the economic viability of wind turbines. Observations of the average service life of Leading Edge Protection (LEP) systems are between four and six years for onshore wind turbines and between two and four years for offshore turbines. However, these values vary significantly depending on the conditions at the location in question and on the quality of the coating. Offshore wind turbines in particular, have disproportionately high repair and maintenance costs. According to data from the International Energy Agency and from the “Erode” EU research project, a wind farm with a capacity of 500 MW loses 332 million euros due to reduced performance and repairs – assuming the current stateof-the-art technology and a service life of 25 years. The Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) has been operating a test stand for rain erosion since 2015 with the aim of understanding the damage processes and developing recommendations for materials and coatings. In this test stand, water drops of various sizes and in various quantities can collide with test samples under realistic weather and climate conditions.

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Rain erosion slows the blades and creates noise

Rain erosion has multi-factorial causes that include rain, hailstones, sand, ice accretion, temperature fluctuations, UV light, humidity and salt, which is not a linear process. After a phase of uniform removal of the protective layer on a blade, crater-like material failure – generally aided by minor production damage – can form at one location, which then quickly spreads. The course of this type of damage varies considerably depending on the site location. It proceeds approximately twice as fast offshore as it does onshore.

When a rotor blade is being manufactured, the two half-shells are generally produced by hand in moulds, using fibreglass composite materials and epoxy resin. The spar caps and trailing edge of the blade are generally added as separate components, as well as the leading edge in rare cases. The two half-shells are placed on one another and glued together. Any manufacturing tolerances are levelled using filler, after which the blade is smoothed by grinding and then coated with a special paint. Some manufacturers add a tape or film to the leading edge. In wind turbine operation, rotor blades are inspected by industrial climbers approximately every two years and minor damage is repaired on site.

The aerodynamic performance capability of a rotor blade is at its highest if the wind layer sweeps over the rotor profile without air turbulence occurring. To achieve this, the blade surfaces must be as smooth as possible. Even minor damage caused by rain erosion can lead to rough surfaces and turbulence. This worsens the aerodynamics of the blade and thus also the performance, economic viability and service life of the overall wind turbine. In addition, sound emissions are increased.

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Addresses

Project management and test stand
Fraunhofer IWES

Mobile layer thickness measuring system
Automation Dr. Nix GmbH & Co. KG

Links (in German)

Research project HyRoS
University Bremen – Institute for product development
Research project with the aim to develop rotor blades with elastic edges

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