Vertical Axis Wind Turbine Design
Vertical Axis Wind Turbine Design – VAWT Design
A Wind Turbine Generator (WTG) is a device that extracts the kinetic energy from the wind using a rotor consisting of two or more blades which are mechanically coupled to an electrical generator. Therefore wind turbine power production depends on the interaction between the rotor and the wind. But there is an upper limit on the amount of energy that can be derived from the movement of the wind. No wind turbine can produce more electrical power than the amount of power in the wind itself. To do that the wind turbine would have to stop the wind from blowing by extracting 100% of its kinetic energy.
The amount of power converted by a horizontal wind turbine is proportional to the area swept out by the rotor (rotor swept area). So in order to capture as much of the wind’s kinetic energy as possible, the wind turbine blades should be the longest possible. Also to increase this further, wind turbines are mounted on towers that are as high as practicable and most often are mounted on towers that exceed 80 metres (260 feet) in height. This is because in an open unobstructed landscape, the wind speed relative to the ground can increase by a considerable amount as the elevation above the ground increases.
Darrieus Turbines Combined
Most modern wind turbines use a horizontal axis wind turbine or HAWT design in which the turbine blades are mounted on a horizontal shaft with the rotor blades of a HAWT kept perpendicular to the flow of the wind to capture the maximum energy.
This shaft is connected to a gearbox or transmission, and the gearbox is placed between the wind shaft (the axle to which the blades are affixed) and the electrical generator to ensure that the generator turns at an optimal speed in order to produce electricity suitable for the electric grid, typically in the 1,200–1,800 rpm range.
One disadvantage of the horizontal wind turbine generator is that in order for the rotor blades to rotate, the wind turbine body has to continuously rotate to orient the rotor in the direction of the oncoming wind.
This “yaw” control can be as simple as the tail vane on small wind turbines, or more complex motor control on modern towers. However, there is another type of wind turbine design called the Vertical Axis Wind Turbine, of VAWT Wind Turbine for short, which has the advantage of accepting the wind from any direction.
The Vertical Axis Wind Turbine
The “vertical axis wind turbine” has turbine blade designs either vertical or in the shape of an egg beater, and similar in many ways to the wind powered centrifugal ventilators seen on chimney’s and flue’s. The vertical axis wind turbine blades are attached to a central vertical shaft and which has specific advantages over the horizontal wind turbine designs.
When the blades spin, the shaft spins which is attached to an alternator generally located at the bottom of the shaft, often at ground level. Except for the shape of its rotor blades, all other components used in the VAWT wind turbine are the same in both designs, with some minor differences in their placement.
Vertical axis wind turbine blades may be of either a drag-driven or lift-driven rotor design. The most common drag-driven vertical axis wind turbine design is that of the Savonius rotor, which has been used for hundreds of years for water pumping and other such mechanical applications. The Savonius VAWT wind turbine has an S-shaped rotor when viewed from above and operates as a drag device, and hence, the angular velocity of the turbine cannot exceed the ambient wind speed.
The power from the Savonius turbine design is based on the difference in air pressure across the blades as one set of blades retreat from the wind and the other set of blades advance into the wind. This is in turn related to the difference in the drag coefficients associated with the convex side of the blade and the concave side of the blades. Generally, compared to other forms of wind turbine designs, the Savonius rotors has fairly low efficiencies.
Lift-driven vertical axis wind turbine rotors however, are more commonly used for electrical power generation. These VAWT rotor blades are called lift-driven turbine blades because when the wind is blowing across the turbine, the rotor blades will experience a lifting force. Typically lift-driven rotors have long curved blades and are often called a Darrieus rotor design after its inventor.
There are several advantages associated with the Darrieus Wind Turbine. First of all, the Darrieus turbine is omnidirectional and does not require any special yaw mechanisms to continuously orient itself toward the wind direction. Secondly, its vertical drive shaft simplifies the installation of the gearbox and the electrical generator on the ground, making the structure and maintenance much simpler. On the negative side, the vertical axis machine has not been widely used because its output power cannot be easily controlled in high winds simply by changing the blade pitch.
Although there are hundreds of commercial available vertical axis wind turbine designs readily available for the home owner to choose from, in practice the vertical axis wind turbine has not been used nearly as widely as have their horizontal axis wind turbine cousins. The reasons have to do with the nature of the aerodynamics of the vertical rotor, the structural loads on each blade vary greatly during each rotation resulting in the blades becoming twisted and bent as they rotated in the wind contributing to a high fatigue failure and damage.
In addition, the vertical axis wind turbine is not suited to being installed on top of a tall tower as many VAWTs require large bearings or supports at the top of the design to permit rotation of the shaft. This means that the vertical axis wind turbine generators tend be located close to the ground or on top of buildings in a region of relatively low wind speeds.
Although VAWT designs can capture ground-level winds, just like any turbine installed on a short tower, this makes them sensitive to turbulence and ground drag. Both ground drag and turbulence in lower-level winds diminish the power available to any turbine mounted close to the ground resulting in less efficiency than that of a horizontal axis turbine of equivalent rated power, but on a taller tower.
Vertical Axis Wind Turbine Generators tend to be less reliable and less efficient than their equivalent “Horizontal Axis Wind Turbine” cousins. As VAWTs are mounted nearer to the ground, both ground drag and turbulence in lower-level winds diminish the power available so there is very little extractable energy in the wind in such locations and the lower the wind speed, the less electricity a turbine will produce.
The majority of vertical axis wind turbine generators currently installed generate electricity. Whether or not these turbines are grid connected they need to produce an electricity supply which is of constant frequency or else many common appliances will not function properly. So just because a vertical axis wind turbine can be mounted at ground level or on the roof of a building does not mean it will produce enough electricity to be worthwhile.
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