Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 76

wind at high wind speeds, even if the rotor is not turning. An advantage of the Savonius wind tur-
bine is the ease of construction.
5.3 ORIENTATION OF THE ROTOR AXIS
Wind turbines are further classified by the orientation of the axis of the rotor with respect to the
ground: horizontal-axis wind turbine (HAWT) and vertical-axis wind turbine (VAWT; Figures 1.7
and 5.6). The rotors on HAWTs need to be kept perpendicular to the flow of the wind to capture the
maximum energy. This rotation of the unit or rotor about the tower axis, yaw , is accomplished by
a tail on upwind units (small wind turbines, up to 10 kW, although there have been tails on some
50 kW units), by coning on downwind units (Figure 5.7), or by a motor (electric or wind [fan tail
rotor]) to drive the unit around the yaw axis. Coning is where the blades are at an angle from the
plane of rotation.
VAWTs have the advantage of accepting the wind from any direction. However, the Darrieus
wind turbine is not reliably self-starting, as the blades have to be moving faster than the wind to
generate power. So the induction motor/generator or another motor is used for start-up to get the
blades moving fast enough that they generate positive power. The giromill ( Figure 5.8 ) may have
articulated blades, which can change angle on the rotational cycle, so it can be self-starting. Another
advantage of VAWTs is the speed increaser and generator can be at ground level. There are two
disadvantages: the rotor is closer to the ground, and there is cyclic variation of power on every
revolution of the rotor.
FIGURE 5.6 Horizontal-axis wind turbine, 10 m diameter, 25 kW, and vertical-axis wind turbine, Darrieus,
17 m diameter, 100 kW, USDA-ARS, Bushland, Texas.