Windmill blades are coated with one or more protective coatings to help them withstand the effects of erosion from rain, sand, hail, and dirt. In most cases, the full structure of the laminated reinforced blade is coated first with a standard two-component protective coating.
By Sebastian Paus, Market Manager of Protective Coatings, Sulzer Mixpac USA, Inc.
Windmill blades are coated with one or more protective coatings to help them withstand the effects of erosion from rain, sand, hail, and dirt. In most cases, the full structure of the laminated reinforced blade is coated first with a standard two-component protective coating. The leading edge of the blade tip receives another special protective layer to protect against the extra risk of rain erosion. New cartridge-based dispensing technology improves the quality and reliability of this leading edge protection layer, while making it safer and more efficient for those conducting repairs using rope access.
Importance of protecting windmill blades from rain erosion
Erosion is directly related to the speed that rain droplets, hail stones, and other airborne particles, (also known as rain erosion) hit the blade. Blade tips are subject to the fastest speeds because when it rains, as the blade rotates up, the falling speed of the rain adds to the blade's tip speed, hitting the blade at a higher rate. This increased speed leads to faster rate of erosion, an effect that can be likened to sandblasting. The point of damage differs. It begins at certain speeds; below these speeds the standard topcoat can handle the erosion from rain and special protection is not required.
European windmill blade standards expect blades to last 20-25 years - in reality they last from 10-15 years, and even that lifespan is impossible without a coating or some sort of leading edge protection. In fact, an uncoated blade (also known as a blunt blade) will not last a year. Erosion effects can be seen in as little as a few weeks if there is a strong rain. Protective coatings are absolutely necessary to protect the blade's leading edge. Normally between 15-30 percent of the blade length receives the special leading edge coating. For example, if the blade is 60 meters long, the leading edge coating will be placed on about 7-20 meters of the blade, depending on the manufacturer.
Inspection used to locate damaged windmill blades
Windmills are regularly inspected using a variety of techniques, depending upon whether they are located offshore or on land. Inspection, using drones or other methods, identifies the blades that need to be repaired and determines the type of repair necessary.
If full service of the blade is necessary, technicians remove the entire blade and repair it on the ground. If inspection reveals that the blade structure is damaged, the repair includes placing putty or glass fiber on the damaged surface, then smoothing the surface and grinding it, followed by applying the standard top coating if the damage is not on the leading edge.
Many smaller repairs require simply renewing 3 to 4 meters of the leading edge protection. These repairs can be quite time-consuming. Rope access is the standard method for smaller repairs; a technician belays down with a rope and makes the repair tethered to the rope. For larger repairs, a platform is maneuvered with a rope from the top of the tower.
Options for windmill blade coating - advantages and disadvantages
Several different methods for coating the leading edge are used, including multi-layer coatings and specialized tapes applied over the standard coating.
The predominant method is the multi-layer coating. Several systems are available; one well known option is AkzoNobel's (formerly BASF's) RELEST® Wind LEP ETU (easy to use) cartridge-based system, which is especially designed for protecting the leading edge of rotor blades of wind power plants from erosion damage. The coating consists of an exceptionally light-fast and hard-wearing polyurethane elastomer.
Tapes can also be used to protect the blades' leading edge. Tapes are made up of several layers. The base is a woven material, with an adhesive layer and the protection chemicals placed onto the carrier material. While it resembles a very thick duct tape, the tape is actually a one-piece system cut to order and customized to each blade type by the manufacturer.
The advantage of tape is that it is easy to apply and easy to replace if damaged. On the negative side, once damaged, wind can get into the damaged spots and eventually rip off the tape. The tape is all in one piece, so if it starts to tear off there is a chance it will tear off in full, removing all the leading edge protection.
Cartridge-based windmill blade coatings increases repair quality and reliability
New cartridge-based systems for applying protective coating to windmill blade leading edges improve repair processes by replacing time consuming and inaccurate manual mixing and dosing. Using cartridges can be a huge advantage for workers performing rope repairs that are so prevalent in minor repairs of windmill blades.
Workers either have to premix before going on the rope or mix at the point of installation - a difficult task while hanging by a rope 60 to 80 meters off the ground! These systems also increase the quality of the repairs, because in general 90 percent of coating failures (apart from insufficient surface preparation) are due to inaccurate mixing or dosing, a problem eliminated by using a cartridge.
For example, one system that provides extra protection and abrasion/erosion resistance on highly stressed areas like leading edges of wind turbine rotor blades is the Mankiewicz ALEXIT® BladeRep LEP 10DM. The two-component, solvent-free polyurethane product is usually applied using the cartridge-based Sulzer MIXPAC™ MixCoat™ Flex Hose Brush Protective Coating Dispensing System.
Sulzer cartridge-based dispensing systems are also used in the RELEST® Wind LEP ETU system, as well as in products made by several manufacturers of leading edge coatings.
Add reliability and quality to wind turbine blade leading edge protective coatings
Cartridge-based dispensing technologies reduce labor, waste, and disposal costs, while adding reliability and improving safety. Use of new applicator friendly cartridge-based dispensing technologies reduces manual mixing and dosing, improving repair quality and making tricky rope work easier. For windmill blade repairs that require application of leading edge protective coatings, these systems are an excellent option.