Page 94 - Fluid Power Engineering

72 Chapter Five

There are three primary methods of regulating power in a turbine.
Pitch-regulated turbines control the output of power by

changing the pitch of the blade in order to maintain the out-
put power at a constant value at wind speeds higher than v rs .
The blade is turned along its longitudinal axis to increase the
pitch and decrease the angle of attack (see Fig. 5-1).
Active stall-regulated turbines are similar to pitch-regulated

in that the blade is turned along the longitudinal axis, but
instead of turning against the wind, the blades are turned
to decrease the pitch and increase the angle of attack (see
Fig. 5-1). The power curve looks similar to pitch-regulated
turbines.
Passive stall-regulated turbines do not control pitch; the pitch

remains constant and as the wind speed increases, the angle
of attack increases leading to higher drag. Stall-regulated tur-
bines are characterized by a hump in power production and
sharp fall in power output after the hump (see Fig. 5-6).

Vertical Axis Wind Turbine (VAWT)
In the early days of wind turbine commercialization (1970s and 1980s),
large VAWT were popular. As technology evolved, HAWT became
popular and VAWT were phased out. In recent years, there are no
large VAWTs in production, to the best of the author’s knowledge.
Several small wind turbines are commercially available in the VAWT
category. There are three types of VAWT turbines:

Savonius VAWT. Savonius is a drag-based machine. An ex-

ample of this was presented at the end of Chapter 4. With
cup-type blades, the maximum power coefficient is below 0.2.
These turbines have rotors that are typically made of simple
materials and are, therefore, inexpensive. However, the rela-
tive inefficiencies do not warrant their use.
Darrieus VAWT. The blades of these turbines are aerodynamic

and the forces generated are based on aerodynamic lift. From
an efficiency standpoint, the theoretical efficiency of Darrieus
3
VAWT is 0.554 compared to 0.593 for HAWT. The blades are
typically in the shape of an eggbeater. The advantages are
that the generator and other components are on the ground
for easy maintenance. The disadvantages are that these tur-
bines cannot be installed high off the ground, so the wind
speeds experienced by the rotor are much lower. Since the
blades are rotating about the vertical axis, the angle of attack