Wind turbines can rotate about a horizontal or a vertical axis. The horizontal being both older and more common.
Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a 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 quicker rotation that is more suitable to drive an electrical generator.
Since a tower produces turbulence behind it, the turbine is usually positioned upwind of its supporting tower. Turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of 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 don’t need 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 cyclical (that is repetitive) turbulence may lead to fatigue failures, most HAWTs are of upwind design.
Types of wind turbines
In the renewable wind industry, there are two types of design – the horizontal axis and the vertical axis.
Both have their benefits and drawbacks so read on to find out more.
The horizontal axis wind turbine has its rotor and electrical generator situated at the top of the tower.
To get the most energy efficiency, it must be pointed in the direction of the wind. It also needs to have a gearbox which makes the slow rotating blades quicker over time.
The blades themselves are built to stay stiff, which prevents them bending in heavy gusts of wind or turbulence.
They are also fitted quite a distance from the tower and are tilted slightly into the wind’s path.
As given away by the name, this form of wind turbine has its rotor shaft fitted vertically.
This gives it an advantage over the horizontal axis, because it needn’t face the wind to be effective.
In fact it operates perfectly well, even in variable wind loads and changes in direction.
The low rotation speed and higher torque means a tower isn’t necessary and the generator can be placed nearer to the ground. This also cuts maintenance hassle, as it’s easier to reach.
However, this added benefit is also its biggest downfall. Wind speeds are generally lower nearer the ground, which effectively means less energy for the turbine to absorb.
The closer to ground you get, the more severe wind shear is also. This is where the wind speed and direction can change quite dramatically over short distances.
Darrieus wind turbine
The Darrieus turbine, also known as an ‘eggbeater’ turbine for its look, was invented by Frenchman, Georges Darrieus.
It provides good energy efficiency but struggles to maintain reliability. This is because it produces a large torque ripple and causes cyclical stress on the tower.
Torque is the force needed to rotate an object through a twisting movement. The ripple is the difference between the weakest and strongest turn.
However, this torque ripple can actually be reduced with three or more blades, providing more solidity in the rotary function.
The Giromill was developed from the Darrieus, but it has straight blades as opposed to curved.
With a greater torque, it gives the turbine better energy efficiency and is fantastic in turbulent winds.