Design of Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 110

used for the global post-stall method for the prediction of post-stall 3-D airfoil characteristics to be
used with the 2-D airfoil data.
PROPID [24] is a program for personal computers for the rotor design and analysis of horizontal-
axis wind turbines, and the executable program is available online [25]. The strength of the method
is its inverse design capability. PROPID is based on the PROPSH blade element/momentum code,
and it includes a 3-D post-stall airfoil performance synthesization method for better prediction of
peak power at high wind speeds.
Most wind turbine blades use the same airfoil for the entire blade; however, twist and chord length
change from the root to the tip of the blade. The surface of the blade should have a smooth transi-
tion along the length. The Alternative Energy Institute also fabricated test blades for the Carter 25,
which used new airfoils designed specifically for wind turbines by NREL [26]. The criteria for
the design of thin airfoils were high lift/drag for the inboard blade portion, restrain maximum lift
coefficient of the outer part of the blade to limit peak power, and provide insensitivity to surface
roughness. Because three different airfoils were used, a computer program was developed to calcu-
late blade fairness (no waves) along the blade. The program used cubic spline under tension [27, 28]
and is available from the Alternative Energy Institute. The basic input to the program consists of
specified airfoils, blade radius, root cut-out, and wind distribution. Additional input can be specific:
spanwise airfoil stations, specified twist, and taper distributions. Different tension parameters result
in a different continuous spanwise airfoil distribution. Optimization is achieved by iteration through
computer codes to determine the surface based on annual energy output and predicted blade load
history for a specified wind distribution. Computer design of blades is of little value if the blade
cannot be practically constructed. Therefore, various input constraints are allowed on twist, taper,
and sharpness of edges and corners.
The blade fairness program determined the airfoils at ten sections from the three input airfoils
(Figure 6.11). The templates were cut out on a numerical control milling machine and assembled

Root airfoil

S807

Primary airfoil

S805

Tip airfoil

S806

S806A

S805A

S807

S808

FIGURE 6.11 Thin airfoil series for wind turbine blade, and their input placement for a 5 m blade.