Vertical Axis Wind Turbines (VAWT)

A vertical axis wind turbine (VAWT) has a shaft that spins vertical to the ground. Because of its orientation, the turbine never needs to be reoriented to face the wind. This generally means that it requires fewer moving parts and safety devices than the horizontal axis wind turbine (HAWT).

A Vertical Axis Wind Turbine (VAWT)

Windmills with a vertical shaft, though not often seen today, were, historically, the first windmills developed. The vertical design means that blades or sails pushed by the wind will turn the shaft to which they are connected, but these rotors must be designed to also be easily pushed back into the wind by other rotors. The simplest design to accomplish this is something like the design for an anemometer, which is basically a series of cups connected to arms spread from the central vertical axis. This design is said to be "drag based" because the cups are being pushed. The cups can never go faster than the wind speed.

Most modern designs make use of special rotor blades that rely on lift. One side of the blade has a longer, more curved, surface than the other side of the rotor. This creates less pressure on one side which results in "lift". This lift can actually cause the blades to move at speeds faster than the wind. But not too much faster. This is one of the disadvantages of the vertical axis wind turbines in producing electricity, seldom do the speeds of the shafts exceed 100 revolutions per minute. Electrical generators like speeds in the range of 1000 rpms. Gearing can be used to equalize these speeds, but the introduction of every gear brings with it the concomitant loss of efficiency.

The Darrieus lift-type vertical-axis turbines were designed in France in the 1920s. They look like egg-beaters. Because of their size and weight, they are generally mounted close to the ground where they get the most turbulent winds. They have another disadvantage: at certain wind speeds they become unstable.

Generally, vertical axis wind turbines are less efficient than HAWTs. Models produced for either commercial or residential use are rare.

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