Classification of Wind Turbines | Electrical Engineering

Basically, the wind turbines are of two types namely hori­zontal axis wind turbines, such as traditional farm windmills used for pumping water and the vertical axis wind turbines, such as the egg beater-style Darrieus model, named after its French inventor.

Most large modern wind turbines are horizontal-axis turbines.

In horizontal-axis turbines, the axis of rotation is hori­zontal with respect to ground (and roughly parallel to the wind stream) while in vertical-axis turbines, the axis of ro­tation is vertical with respect to ground (and roughly perpen­dicular to the wind stream).

1. Horizontal-Axis Wind Turbine (HAWT):

Horizontal-Axis Wind Turbine (HAWT) has the main rotor shaft and electrical generator at the top of the tower and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gear­box, which turns the slow rotation of the blades into a high speed rotation that is more suitable for driving an electrical generator.

Since a tower produces turbulence behind it, the turbine is usually pointed upwind of the tower. Turbine blades are made stiff to prevent the blades from being pushed into the tower and are sometimes tilted forward into the wind a small amount.

Downwind machines have been built, despite the prob­lem of turbulence (mast wake), because they do not require an additional mechanism for keeping them in line with the wind and because in high winds the blades can be allowed to bend which reduces their swept area and thus their wind resistance. Since cyclic turbulence may lead to fatigue failure most HAWTs are upwind machines.

Turbines used in wind farms for commercial production of electrical energy are usually three-bladed and pointed into the wind by computer-controlled motors. These have high tip speeds of over 320 kmph, high efficiency and low torque ripple, resulting in good reliability.

The blades are usually coloured light gray to blend in with the clouds and range in length from 20 to 40 m or more. The tubular steel towers range from 60 to 90 m tall. The blades rotate at 10-22 rpm. At 22 rpm, the tip speed exceeds 90 m/s. A gear box is commonly used for stepping up the speed of the generator, although designs may also use direct drive of an annular generator.

Some models operate at constant speed, but more energy can be collected by variable-speed turbines which use a solid-state power converter to interface to the transmission system. All turbines are equipped with protective features to avoid damage at high wind speeds, by feathering the blades into the winds which ceases their rotation, supplemented by brakes.

Advantages:

i. Variable blade pitch, which provides the turbine blades the optimum angle of attack. Allowing the angle of attack to be remotely adjusted provides greater control, so the turbine collects the maximum amount of wind energy for the time of day and season.

ii. The tall tower base allows access to stronger wind in sites with wind shear. In some wind shear sites, the wind speed can increase by 20% and the power output by 34% for every 10 m in elevation.

iii. High efficiency due to movement of blades always perpendicular to the wind, receiving power through the whole rotation. In contrast, all vertical-axis wind turbines and most proposed airborne wind turbine designs, involv­ing various types of reciprocating actions, need airfoil surfaces to backtrack against the wind for part of the cycle. Black-tracking against the wind leads to inherently poor efficiency.

iv. The face of a horizontal axis blade is struck by the wind at a consistent angle regardless of the position in its rotation. This leads to a consistent lateral wind loading over the course of a rotation, reducing vibrations and audible noise coupled to the tower or mount.

Disadvantages:

i. Transportation difficulties in carrying tall towers and blades of length up to 45 m. Transport can now amount to 20% of equipment costs.

ii. Tall HAWTs are difficult to install, requiring very tall and expensive cranes and skilled operators.

iii. Requirement of massive tower construction for sup­porting the heavy blades, gearbox and generator.

iv. Reflections from tall HAWTs may affect side lobes of radar installations producing signal clutter, although fil­tering can suppress it.

v. Their height makes them obtrusively visible across large areas disrupting the appearance of the landscape and sometimes causing local opposition.

vi. Downwind variants suffer from fatigue and structural failure caused due to turbulence when a blade passes through the tower’s wind shadow (for this reason, the majority of HAWTs use an upwind design, with the rotor facing the wind in front of the tower).

vii. HAWTs need an additional yaw control mechanism to turn the blades and nacelle toward the wind.

viii. In order to minimize fatigue loads due to wake tur­bulence, wind turbines are usually sited a distance of 5 rotor diameters from each other, but the spacing depends on the manufacturer and turbine model.

2. Vertical-Axis Wind Turbine:

Vertical-axis wind turbines (VAWTs) have the main rotor shaft arranged vertically. The main advantages of such arrangement are that the turbine does not require to be pointed into the wind to be effective. This is an advantage on sites where the wind direction is highly variable. With a vertical axis, the generator and gearbox can be placed near the ground, so the tower does not require support and it is more accessible for maintenance. Drawbacks are that some designs develop pulsating torque.

Because of difficulty in mounting vertical-axis tur­bines on towers, they are usually installed nearer to the base on which they rests, such as the ground or a build­ing rooftop. The wind speed is slower at a lower altitude, so less wind energy is available for a turbine of given size. Air flow near the ground and other objects can cause turbulent flow introducing issues of vibration, including noise and bearing wear that may increase the mainte­nance cost and shorten the service life.

However, when a turbine is mounted on a rooftop, the building generally redirects wind over the roof and this cans double the wind speed at the turbine. If the height of the rooftop mounted turbine tower is around 50% of the building height, this is near the optimum for maximum wind energy and mini­mum wind turbulence.

Eggbeater or Darrieus turbines have good efficiency but develop large torque ripple and cylindrical stress on the tower that results in poor reliability. They also usually need some external power source or an additional Savonius rotor to start turning due to poor starting torque. The torque ripple is re­duced by using 3 or more blades resulting in a higher solid­ity for the rotor. Newer Darrieus type turbines are not held up by guy-wires but have an external superstructure con­nected to the top bearing.

Giromill is subtype of Darrieus turbines with straight, as opposed to curved, blades. The cycloturbine has variable pitch in order to reduce the torque pulsation and is self-starting. The advantages of variable pitch are- high starting torque; a wide, relatively flat torque curve; a lower blade speed ratio, a higher coefficient of performance; more efficient in turbu­lent winds; and a lower blade speed ratio resulting in lower blade bending stresses. Straight Vee or curved blades may be used.

Savonius wind turbines are drag type devices with two or more long helical scoops to provide a smooth torque.

Advantages:

i. VAWTs are usually mounted with the lower bearing mounted near the ground; therefore, massive tower structure is not necessarily required.

ii. Design without yaw mechanisms is possible with fixed pitch rotor designs.

iii. VAWTs may be built at locations where toller struc­tures are prohibited.

iv. VAWTs may have a lower noise figure.

v. VAWTs situated close to the ground can take advan­tage of locations where mesas, hilltops, ridgelines and passes funnel the wind and increase wind velocity.

vi. The generator of a VAWT can be located nearer the ground, making it easier to maintain the moving parts.

vii. VAWTs are lower wind startup speeds than HAWTs. Typically, they start producing electricity at 10 kmph.

Disadvantages:

i. VAWT may not develop as much energy at a given site as a HAWT with the same footprint or height. This is due to location of rotor close to the ground where wind speeds are lower because of the ground’s surface drag.

ii. The stress in each blade due to wind loading changes sign twice during each revolution as the apparent wind di­rection moves through 360 degrees. This reversal of the stress increases the likelihood of blade failures by fatigue.

iii. While VAWTs components are located on the ground, they are also located under the weight of the structure above it, which can make replacement of parts very difficult if the structure is not designed properly.

iv. A VAWT that uses guy-wires to hold it in place puts stress on the bottom bearing as all the weight of the rotor is on the bearing. Guy wires attached to the top bearing in­crease the downward thrust in wind gusts. Solution of this problem needs a superstructure to hold a top bearing in place in order to eliminate the downward thrusts of gust events in guy wired models.