Function

Blades for a 15 MW turbine have a diameter of about 225 m. They form part of the rotor, which also contains a hub casting, blade system, bearings, and pitch system.

Blades are typically made from fibreglass and epoxy resin. There are variations between designs, with some using carbon fibre and others using polyester resins. New resin systems are being developed and tested to enable the recovery and reuse of blade materials at the end of life.

Higher tip speeds typically lead to more efficient energy capture. Tip speeds offshore are higher than those used onshore, but these are limited by design to avoid blade leading edge erosion. Repeated impact by raindrops, particulate matter, lightning strikes, hail, ice, and salt erodes the blades, causing surface roughness and change in aerodynamic shape to the outer part of the blade’s leading edge. This reduces the performance of the blade and increases its sound emissions. If left unrepaired, the damage will continue until it affects the structural integrity of the blade.

An important focus is reducing the risk of leading-edge erosion. One approach is developing better coatings. Another is to incorporate leading edge protection plates like those used in helicopter blades.

Each blade is bolted to a blade bearing that is bolted to a central hub on the main shaft. The blade bearing enables the pitch mechanism to adjust the blade pitch angle which is adjusted almost constantly in medium-to-high winds to regulate rotor speed. Each blade has its own independent pitch system that allows the turbine to be controlled should one pitch system fail.

New turbine designs have larger rotor swept areas compared to their generator rating to achieve a higher capacity factor. Capacity factors of over 50%, are expected for 15 MW offshore turbines, and above that is expected for the next generation of turbines. This compares with capacity factors of about 35% for good onshore sites.

Nacelles are assembled by the wind turbine supplier, using components generally sourced from a range of external suppliers.

Who is involved

Nacelles are assembled by the wind turbine supplier, using components generally sourced from a range of external suppliers.

Key facts

Typical dimensions for a 15 MW turbine are 21 to 25 m long, 9 to 12 m wide and 10 to 12 m high for transport, with masses of 600 to 700 tonnes including the hub.

Key nacelle components include the main bearing, gearbox (where used), generator, yaw bearing and yaw system. The main bearing supports the rotor and transfers the rotor loading to the nacelle bedplate. Particular attention is given to the long-term reliability of the gearbox in the wind turbine drive train.

There has been a move away from high-speed gearboxes for offshore turbines. Most generators use permanent magnets that need no excitation power. This keeps efficiency high, mass low and dimensions small, lowering transport and installation costs but does rely on the supply of rare-earth alloys.

Other nacelle components include the:

  • Yaw bearing which connects the nacelle and tower, enabling the yaw system to turn the nacelle to any wind direction during operation. The yaw system orients the rotor and nacelle to the wind direction during operation.
  • Bedplate which supports the drive train and the rest of the nacelle components and transfers loads from the rotor to the tower.
  • Main shaft which transfers torque from the rotor to the gearbox or, for direct drive designs, the generator.
  • Control system which provides supervisory control (including health monitoring) and active power and load control, and
  • Condition monitoring system which provides additional health checking and failure prediction capability.

Nacelle mass is kept as low as reasonably possible to help with overall system dynamics and minimise logistics costs. To keep nacelle mass down, turbine designs may have the transformer and much of the power electronics in the tower base. Mid-grade steels and cast spheroidal graphite iron are used rather than low-grade materials as they offer the lowest cost per unit fatigue strength.

Before dispatch, the nacelle undergoes a functional test before being prepared for transport and storage. It is also typically tested with its power take-off hardware. New designs of offshore turbines place a high emphasis on maintainability. This is being achieved through modular designs for large components so more subcomponents can be replaced using the nacelle crane. This minimises the need for jack-up vessels, which are expensive and may have lengthy mobilisation periods.