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88 Fundamentals of Ocean Renewable Energy

Brake: Stops the rotor in case of emergency.

Controller: Starts the wind turbine at cut-in speeds (generally around 3.5 m/s)
and shuts off the turbine at very high wind speeds (over 25 m/s, cut-out
speed) to protect the device.

Gear box: Transmits the power (torque times angular speed) from the low-
speed shaft to the high-speed shaft. The rotational speed required for a
turbine is very high (≈1000 rpm) compared with the rotational speed of
the rotor (≈20 rpm).

Generator: Produces the AC electricity.

High-speed shaft: Transmits the power from the gearbox to generator, and
rotates/drives the generator.

Low-speed shaft: Transmits the power from the rotor to the gearbox.

Nacelle: Houses the generating components of a wind turbine including the
gearbox, generator, controller, and brake.

Pitch system: Adjusts the angle of attack of wind by turning the blades. The
rotational speed and the generated power can be controlled/optimized by
the pitch system. The pitch system can stop the turbine rotating at cut-in
or cut-out speeds.
Rotor: Consists of blades and the hub.

Tower: Supports the wind turbine. Towers are mainly built using conical
tubular steel.
Wind direction: Upwind wind turbines (shown in Fig. 4.5) face into the wind,
whilst downwind turbines face away from the wind.

Yaw drive: Aligns the turbines towards the wind. This keeps the wind turbine
facing the wind when the wind direction changes for upwind turbines.

Yaw motor: Used to power the yaw drive.

4.2.1 Aerodynamics of Wind Turbines
The aerodynamics of horizontal axis wind turbines is very similar to the
hydrodynamics of horizontal axis tidal turbines, reviewed in Chapter 3. Here,
a brief overview is presented, whilst more details are provided in Chapter 3 and
elsewhere (e.g. [5]).