Introduction to Modern Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 80

above-rated operation is indicated on the Mod-5B curve by the shaded region in which pow-
er coeficients are purposely reduced at lower tip-speed ratios. Rotor power coeficients have
little signiicance in the above-rated regime. The power limit at above-rated wind speeds is
determined by an economic trade-off study in which the value of the power lost is balanced
by the reduced cost and maintenance expense of a smaller-capacity power train.
Figure 2-18 shows sample HAWT and VAWT power output density curves (power out-
put per unit of swept area vs. wind speed) calculated from rotor power coeficients, speciied
air density, schedules of rotor speed vs. wind speed (for a variable-speed turbine), mechani-
cal and electrical losses in the power trains, and power lost in transmission to the speciied
system output point. The calculation procedure is illustrated in Table 2-2.

Figure 2-18. Sample HAWT and VAWT power density curves. The HAWT power is
limited by blade pitch control, while the VAWT power is limited by aerodynamic stall.

Before calculating gross annual energy output, it is necessary to determine the design
frequency distribution of free-stream winds at the mid-elevation of the area swept by the ro-
tor. Weibull factors for this elevation are obtained by interpolation within Table 2-1. For the
example wind turbines, these factors are
Mod-5B HAWT:

z m = 61.0 m
C m = 10.04 m / s
k m = 2.70

(2-21a)