Environmental Engineering Reference
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Figure 5-26. Power curve of the 38-m diameter Mod-0 HAWT with fixed blade pitch.
a relatively high tip speed of 111 m/s. Full-blade pitch was used to hold power constant at 2
MW in high winds, and pitch was held constant in below-rated winds.
Figure 5-27 shows measured and predicted power output for the Mod-1 HAWT as a
function of free-stream wind speed. Strip theory power predictions are shown based on both
smooth
and
half-rough airfoil
lift and drag data. Half-rough data are obtained by averaging
reference smooth and
rough
airfoil data [Abbot
et al.
1945]. (Rough airfoil performance is
unrealistically low for wind turbine blades.) Smooth airfoil data leads to predictions which
closely match measured power at levels greater than one-half of rated power, where wind
turbines produce the majority of their energy output (see Table 5-1).
Mod-2 HAWT Performance
The 2.5-MW Mod-2 turbine design had a two-bladed teetering rotor that used airfoils
in the NACA 23000 series. It had a rotor solidity of 0.036 and tip speed of 84 m/s, which
places these parameters midway between those of the Mod-0 and the Mod-1 rotors. The
outboard 30percent of each blade had variable pitch for starting, stopping, and controlling
power output. In contrast to the wind turbines examined previously, the Mod-2 control sys-
tem employed pitch change during operation in below-rated winds. The test data presented
here are for the rotor without vortex generators, and in this configuration the pitch schedule
below rated power was as follows (tips are fully-feathered at a pitch angle of -90 deg): For
power outputs between 0 and 1.0 MW, the tip pitch varied linearly with power, from -5 deg
to -2 deg; from 1.0- to 2.5-MW the turbine was operated at a fixed blade pitch of -2 deg.
Figure 5-28 is a plot of test data reported by Boeing [1982] compared with predicted
power output obtained using modified strip theory [Wilson and Walker 1984]. A correction
for the 0.3-m gaps between the fixed- and variable-pitch portions of the rotor (developed
earlier in this chapter) is included in the predicted power. At the “knee” of the power curve,
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