Introduction to Modern Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 52

General Conigurations of Horizontal-Axis Wind Turbines
Figures 2-3 and 2-5 illustrate two general conigurations of modern HAWTs. The princi-
pal subsystems which make up the total wind energy conversion system are (1) the rotor, (2)
the power train, (3) the nacelle structure, (4) the tower, (5) the foundation, and (6) the ground
equipment station.
Figure 2-5. Principal subsystems of a HAWT. (a) Two-bladed teetered-hub upwind rotor
(b) Two-bladed rigid-hub downwind rotor. Many current HAWTs have three-bladed rigid-
hub upwind rotors
The Turbine Rotor Subsystem
HAWT rotors are often described as “propeller-type”, indicating correctly that many of
the aerodynamic and structural-dynamic principles that are basic to aircraft propeller technol-
ogy are also applicable to HAWT rotors. Of course, HAWT rotors decelerate rather than ac-
celerate the air, and their tip speeds (typically less than Mach 0.4) are much lower than those
of aircraft propellers. As shown in Figure 2-5, the main components of the HAWT rotor are
its blades fastened to a central hub . HAWT rotors usually contain either two or three blades.
One-bladed rotors with counterweights are technically feasible but rare. As the rotor turns,
its blades generate an imaginary surface whose projection on a vertical plane is called the
swept area . For purposes of calculating the swept area, blades are assumed to be undeformed
by applied loads.