Horizontal Axis wind Turbine – Advantages and Disadvantages
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 of Horizontal Wind Turbine :
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 Of Vertical Wind Turbine :
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.
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