vertical-axis wind turbine (VAWT)
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:
generator, gearbox etc. may be placed on the ground, and a tower is not
essential for the machine
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
See also horizontal-axis
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