A vertical-axis wind turbine (VAWT) is a type of wind turbine in which the axis of rotation is perpendicular to the wind stream and the ground. VAWTs work somewhat like a classical water wheel in which water arrives at a right angle (perpendicular) to the rotational axis (shaft) of the water wheel. Vertical-axis wind turbines fall into two major categories: Darrieus turbines and Savonius turbines. Neither type is in wide use today.

 

The basic theoretical advantages of a vertical axis machine are:

 

· The generator, gearbox etc. may be placed on the ground, and a tower is not essential for the machine to operate.

· A yaw mechanism isn't needed to turn the rotor against the wind

 

The basic disadvantages are:

 

· Wind speeds are very low close to ground level, so although a tower isn't essential, the wind speeds will be very low on the lower part of the rotor.

 

· The overall efficiency of the vertical axis machines is not impressive.

 

· The machine is not self-starting, i.e. a Darrieus machine needs a "push" before it will start. This is only a minor inconvenience for a grid-connected turbine, however, since the generator may be used as a motor drawing current from the grid to to start the machine.

 

· The machine may need guy wires to hold it up, but guy wires are impractical in heavily farmed areas.

 

· Replacing the main bearing for the rotor necessitates removing the rotor on both a horizontal and a vertical axis machine. In the case of the latter, it means tearing the whole machine down.

 

See also horizontal-axis wind turbine.