Horizontal axis wind turbine

Horizontal axis machines have emerged as the most successful type of turbines. These are being used for commercial energy generation in many parts of the world. They have low cut-in wind speed, easy furling and, in general, show high power coefficient. However, their design is more complex and expensive as the generator and gearbox are to be placed at the top of the tower. Also, a tail or yaw drive to be installed to orient them in the wind direction.

(1) The main component of horizontal axis wind turbine:

1. Turbine blade:

2. Hub:

3. Nacelle:

4. Power transmission system:

5. Generator:

6. Yaw control:

7. Brakes:

8. Tower:

The constructional details of the most common, three-blade rotor, the horizontal axis wind turbine are shown in Fig. The main components are as follows:

Horizontal axis wind turbine

(a) Turbine blade:

Wind turbine blades need to be lightweight and possess adequate strength and hence require to be fabricated with aircraft industry techniques. The blades are made of glass fibre Reinforced plastic (F.R.P). They have an aerofoil type of cross-section to create lift as the air flows over them. The blades are slightly twisted the outer tip to the root to reduce the tendency to stall. In addition to centrifugal force and fatigue due to continuous vibration, there are many extraneous forces arising from wind turbulence, just gravitational force and directional changes in the wind. All these factors have to be considered at the designing stage. The diameter of a typical, MW range, the modern rotor may be of the order of 100m.

Modern wind turbines have two or three blades. Two or three blade rotor HAWT is also known as propeller type Wind turbine. Three blades are more common in Europe and other developing countries including India. The American practice, however, is in favour of two blades.

The advantages and disadvantages of two and three-blade rotor are as follows:

(l) The three-black machine has smoother power output and balanced gyroscopic force compared to the two-black machine.

(2) The three-blade rotor allows the use of a simple rigid hub. The blades may be cross-linked for greater rigidity.

(3) Adding a third blade increases output by about 5% only, while the weight and cost of a rotor increases by 50%, thus giving a diminished rate of return for an additional 50% weight and cost.

(4) The two-blade rotor is also simpler to erect since it can be assembled on the ground.

(b) Hub:

A hub is the central solid portion of the rotor wheel. All blades are attached to the hub. The pitch angle control mechanism is also provided inside the hub.

(c) Nacelle:

It houses the generator, the gearbox, brakes, hydraulic system, and the yawing mechanism. The nacelle is placed at the top of the tower and is linked with the rotor.

(d) Power transmission system:

Mechanical power generated by rotor blades is transmitted to the generator through a gearbox. From the gearbox, the transmission shaft rotates the generator with a built-in friction clutch. The gearbox is provided to increase the speed to suit the generator. The generated electrical power is conducted to ground terminals through a cable.

(e) Generator:

Generally, the grid-connected large wind turbines have induction generators. They use reactive power from grids and feed the generated power to boost the grid supply. Medium capacity wind turbines use synchronous generators installed to electrify villages and remote places. Small capacity wind turbines use permanent magnet DC generators which supply power to microwave stations and illuminating lighthouse.

(f) Yaw control:

Yaw control continuously tracks and keeps the rotor axis in the wind direction. Yawing is done by two yawing motors, which mesh with a big-toothed wheel mounted on top of the tower. Wind direction sensor used to maintain the orientation. During high-speed wind, i.e. more than the cut-out speed, the machine is stopped by turning the rotor axis at right angles to the wind direction. In small wind turbines, a tail vane is used for passive yaw control.

(g) Brakes:

Brakes are used to stop the motor when power generation is not desired. An emergency stop activates the hydraulic disc brakes fitted to the high-speed shaft of the gearbox.

(h) Tower:

The tower supports the nacelle and rotor. Modem wind turbine generators are installed on tubular towers. Large turbines use lattice towers designed to withstand gravity loads and wind loads. For medium and large-sized turbines, the tower is slightly taller than the rotor diameter. In case of a small-sized turbine, the tower is much larger than the rotor diameter as the air is erratic at lower heights. Both steel and concrete towers are being used.