At the start of the millennium the longest commercially available rotor blades were fitted on 2-2.5MW turbines with the maximum length being around 39 metres. Today the longest single-piece onshore turbine blades are in the 60-metre range. And as transportation issues ramp up with the longer blades, there is more interest in segmented blades. Both German turbine manufacturer Enercon and Spain's Gamesa use segmented blades for their largest flagship models, but currently only in small numbers.
Most modern rotor blades consist of an upper and lower shell with inner reinforcement, plus a circular foot section for mounting on the pitch-bearing. Made from composite material, blades are sensitive to mechanical damage during transport and installation. And, during their 20- to 25-year lifespan, they are exposed to dynamic loads and frequent load fluctuations under demanding environmental conditions, including extreme weather and surface "bombardment" with sand and dirt particles.
Visual blade inspections should be carried out regularly and in some cases regular surface cleaning may be necessary to maintain optimal performance.
The rounded airfoil part of the blade called the leading edge directly faces the approaching wind. Common blade designs with separate upper and lower shells have a glue joint at the centre-line of the leading edge. During operation the leading edge is exposed to abrasive wind erosion as well as the risk of mechanical surface damage. Dynamic load cycles can introduce cracks in the filler material and surroundings and cause filler particles to break out completely. Increasingly, the leading edge is covered with a wearand impact-resistant polyurethane protection tape.
The airfoil rear section known as the trailing edge is only a few millimetres thick. This shape helps minimise aerodynamic noise, but it also makes it vulnerable to mechanical damage during transport, storage and installation. Any damages have to be repaired by filler or fibre reinforced material whereby the introduction of structural weak spots must be prevented.
Care should be taken of the bolts at the root section during assembly, handling, transportation and installation, as the vulnerable threads can suffer damage. For maintenance, it may be necessary to check the bolt tensioning occasionally.
Be vigilant
The blade tip is specially shaped for low-noise performance. Like the trailing edge it is rather fragile. Dutch wind technology consultant Jan van Egmond explains that tip sections have to be regularly inspected for damages around the lightning receptors that are placed near the tip - as lightning strikes might erode the blade's composite material around these receptors.
He also draws attention to the water-draining bore holes located near the blade tip. "If these holes are totally blocked or drain insufficiently, condensed water will collect inside the hollow tip section," says Van Egmond. "When this is the case and a lightning strike occurs, the water inside heats up instantly into steam requiring 1,700 times more volume. I have seen a number of damaged blades where a tip was completely ripped open."
Modern blades enable internal inspections for cracks in the laminate close to the stiffening reinforcements, and for checking whether reinforcement-shell bonding is still intact. Van Egmond stresses the importance of an external blade surface inspection. This visual check focuses on cracks in those areas of the blade airfoil immediately above the internal reinforcement structural parts. Additional advanced technology such as thermal scanning is also increasingly employed, he concludes.
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