CLASSIFICATION OF WIND TURBINE ROTORS BASIC INFORMATION

Wind turbines are usually classified into two categories, according to the orientation of the axis of rotation with respect to the direction of wind.

• Vertical-axis turbines
• Horizontal-axis turbines

Vertical-axis wind turbine (VAWT)
The first windmills were built based on the vertical-axis structure. This type has only been incorporated in small-scale installations. Typical VAWTs include the Darrius rotor, as shown in below.

(a)A typical vertical-axis turbine (the Darrius rotor),19 (b) a horizontal-axis wind

turbine.1

Advantages of the VAWT are:
• Easy maintenance for ground mounted generator and gearbox,
• Receive wind from any direction (no yaw control required), and
• Simple blade design and low cost of fabrication.

Disadvantages of a vertical-axis wind turbine are:
• Not self starting, thus, require generator to run in motor mode at start,
• Lower efficiency (the blades lose energy as they turn out of the wind),
• Difficulty in controlling blade over-speed, and
• Oscillatory component in the aerodynamic torque is high.

Horizontal-axis wind turbines (HAWT)
The most common design of modern turbines is based on the horizontal-axis structure. Horizontal-axis wind turbines are mounted on towers. The tower’s role is to raise the wind turbine above the ground to intercept stronger winds in order to harness more energy.

Advantages of the HAWT:
• Higher efficiency,
• Ability to turn the blades, and
• Lower cost-to-power ratio.

Disadvantages of the horizontal-axis:
• Generator and gearbox should be mounted on a tower, thus restricting servicing, and
• More complex design required due to the need for yaw or tail drive.

The HAWT can be classified as upwind and downwind turbines based on the direction of receiving the wind. In the upwind structure the rotor faces the wind directly, while in downwind structure, the rotor is placed on the lee side of the tower.

The upwind structure does not have the tower shadow problem because the wind stream hits the rotor first. However, the upwind needs a yaw control mechanism to keep the rotor always facing the wind.

On the contrary, the downwind may be built without a yaw mechanism. However, the drawback is the fluctuations due to the tower shadow.