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Vertical Axis Wind Turbines
Wind turbines, as the name indicates, are designed to convert the kinetic energy of wind into mechanical and consequently electrical energy. The main characteristic of a vertical axis wind turbine (VAWT) is the direction of the main rotor shaft that runs vertically, and this has certain advantages compared to the horizontal axis wind turbines.
VAWT are suitable for low wind conditions and don't need to face the wind's direction in order to operate efficiently. The gearbox in these turbines is placed at the bottom of the structure so that the tower doesn't have to endure the extra weight moment arm. However this type of windmills is susceptible to pulsating torque during turbine rotation.
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Types of Vertical Axis Wind Turbines
There are several types of vertical axis windmills. The most popular include the Savonius type and the Darrius type, along with many variations. A number of new VAWT types have also been introduced.
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Darrieus Wind Turbines
One type of VAWT is the Darrieus wind turbine that uses the lift forces of the wind to rotate the aerofoils of the machine. The tip speed ratio (TSR) indicates the rotating velocity of the turbines to the velocity of the wind. In this case, the TSR has a higher value than 1, meaning that the velocity rotation here is greater than the velocity of wind and generates less torque. This makes Darrieus turbines excellent electricity generators. The turbine blades have to be reinforced in order to sustain the centrifugal forces generated during rotation, but the generator itself accepts a lower amount of force than the Savorius type. A drawback to the Darrieus wind turbines is the fact that they cannot start rotation on their own. A small motor, or another Savonious turbine, maybe needed to initiate rotation.
Variants of the Darrieus wind turbine are the giromill, the helical turbine, and the cycloturbine:
- The Giromill: This type of wind turbine uses vertical blades and horizontal supports to keep them attached to the main vertical axis. It is the cheapest Darrieus design and can endure strong winds, however it lacks the efficiency of other turbine designs.
- The Helical Turbine: The Helical Blade Turbine is a variation of the Giromill, where the three blades have the shape of a DNA structure or a helical twist of approximately 60o. This special shape is designed in order to reduce the pulsating torque during rotation.
- The Cycloturbine: Another variant of the Darrieus is the Cycloturbine, where the tilt of the blades can be mechanically oriented in order to take advantage of both drag and lift forces and achieve higher turbine acceleration and self-starting.
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Savonius Wind Turbine
This is a drag-type vertical axis wind turbine, meaning that it operates using drag forces, the same way an anemometer does. In this case, the cups of a Savonius Turbine cannot rotate faster than the speed of the wind and the TSR is always 1 or less.
In the Savonius Turbines, 15% of the mechanical energy is converted into electrical energy, and this is due to the limited number of rotations that result in low voltages. The low conversion percentage makes them suitable for applications other than generating electricity- water pumping and grain grinding are among them.
Because of the high torque during rotation, the motor parts must be sturdy and serviced regularly. This type of turbine is quiet, easy to manufacture (even hand-made), and can endure strong winds without being damaged.
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Other Types of Vertical Axis Wind Turbines
Less known designs of VAWT involve the following:
- The Rotational-Sail Windmill: In this type of windmill, the three sails can contract and expand depending on the speed of the wind. A magnetic unit controls the whole process, so that the windmill can operate with either low or strong wind.
- The Neo-Aerodynamic Turbine: The airfoil base of this turbine has the ability to harness torque through the kinetic energy of the air flow, lacking this way, any turbulence problems. It is suitable for low wind areas, has low cost, and doesn't require a tower. Another benefit of this design is that it is operational with other fluids as well, such as water (rivers, creeks).
- The Windstar Turbine: The straight aerodynamically shaped aluminum blades of this type are attached at each end to a central rotating shaft. The characteristic feature here is the operation in linear arrays with each rotor’s blades passing within two feet of its neighbor. This configuration enables the center rotors to gain an increase in output and efficiency through the “couple vortex" effect.
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The Best VAWT
Based on the above, the choice of the right VAWT depends on the intended purpose and conditions of use. Most of the types described are suitable for domestic use and can be hand-made.