Wind Turbine Aerodynamics Part A: Basic Principles - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 340

Figure 5-40. General configuration of a Darrieus VAWT rotor blade.

Increasing Q in turn decreases the rotor aspect ratio. Thus, in broad terms one can say that
decreasing the height-to-diameter ratio of a VAWT rotor will increase K P , max . Carried to the
limit, a VAWT with a circular rotor will have a higher K P , max than one with straight blades of
the same length. However, a high value of K P , max is not particularly desirable, since it may
force the power train to be too large to be cost-effective.
Next, consider the effect of rotor shape on VAWT performance in low winds (higher tip-
speed ratios), when power is obtained from the straight sections of the blades. In low winds
equatorial sections have very low angles of attack, to the point where power produced by
lift is insufficient to overcome power lost by drag. As the angle Q is increased, the straight
segments of the rotor become longer (with a corresponding increase in aspect ratio) and the
low-wind power output of the rotor increases. When the chord varies over different sections
of the blade, the previous observations will hold as long as the blade chord remains fixed on
each section.

Torque Ripple

Each section of a VAWT blade sees a wide variation in angle of attack during each rotor
revolution. As a result, the torque produced by a blade, even in steady winds, is characterized
by a cyclic time history termed torque ripple . Figure 5-41 shows typical variations in torque
coefficient with blade azimuthal position, for a 17-m Darrieus operating at several tip-speed
ratios. The torque coefficient is defined by

Q b
0.5 r U 2 AR

(5.53)

C Q , b =

where C Q , b = blade torque coefficient

Q b = time-varying torque produced by one blade (N-m)

Over the front portion of the path, for about 40 percent of the revolution (y from about 30 to
160 deg), a blade produces positive torque. However, torque production on the remainder
of the circuit is low or negative, mostly because of the lower angles of attack experienced on
the rear path. At a tip-speed ratio of 11, the average torque is negative in this example. The