The nacelle supports the rotor and converts the rotational energy from the rotor into three-phase AC electrical energy.
What it costs
About £11 million for a 15 MW floating offshore wind turbine.
Who supplies them
Nacelles are assembled by the wind turbine supplier, using components generally sourced from a range of external suppliers.
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 t 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. Several bearing arrangements exist for offshore wind turbines including a single bearing supporting the generator and rotor. Another approach is to support the main shaft with a bearing at each end.
Where used, a gearbox converts rotor torque at a speed of 4 to 8 r/min to a speed of up to about 600 r/min for a medium speed gearbox. The gearbox is a critical item in the wind turbine drive train, with particular attention given to the long-term reliability.
There has been a move away from gearboxes with conventional high-speed generators for offshore turbines. For example, GE Renewable Energy and SGRE have opted for direct drive turbines without a gearbox. Instead, they use a larger and more complex low-speed generator. Vestas has opted for turbines with a gearbox and medium speed generator.
The generator converts mechanical energy to electrical energy. 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.
The yaw bearing 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.
Other nacelle components include the:
- 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 in order to optimise wind turbine life and revenue generation, while meeting externally imposed requirements, 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 (SG) 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. The use of jack-up vessels is not an option at depths suited to FOW.
The nacelle incorporates high levels of remote monitoring, health checking and control.
There are no major differences in the nacelles designed for floating or fixed offshore wind farms. Adjustments are needed to the control system to make the turbine suitable for application in floating.