Wind Turbine Airfoils and Rotor Wakes - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 383

rotors, and that it is caused by the surface roughness precipitating blade stall at a relatively
low lift coefficient. Consequently, peak power occurs at a lower tip-speed ratio than that
for which the stall control was designed. Large-scale rotors are less affected because of
their higher elevation (above most insects and dust particles) and because debris thicknesses
are smaller fractions of the leading-edge radius and the airfoil thickness.
Airfoil Selection
In the present state of the art there is no unambiguous, rational procedure to determine
the ideal airfoil for a given wind turbine rotor, or even for a given radial station on a blade.
This is the result of both the very wide range of angles of attack over which a blade
operates and the widely different geometrical combinations of airfoil section, chord, and
twist possible in the blade design. A further factor relates to the differing spanwise
aerodynamic requirements of the general rotor. For these reasons the current approach in
airfoil selection is to use a rotor performance computer code [ e.g. , Wilson et al. 1976,
Tangler 1987, McCarty 1993, Buhl et al . 1998] that takes into account (as adequately as pos-
sible) all the forces on the rotor that vary with tip-speed ratio and radial position. The aerody-
namicist can then vary the rotor geometry until an acceptable performance results. Inherent
in this process is the assumption that the analytical models in the code properly account for
the effects of these geometric changes on performance.
Standard Aircraft Airfoils
Many different standard airfoils developed for aircraft have been used on wind turbines
with no special advantages from any particular choice when only the impact of the basic
airfoil properties on power output is considered. Five of these are shown in Figure 6-6.
The NACA 230XX series and the NACA 44XX series airfoils (where the XX stands for the
thickness-to-chord ratio, in percent) have been used on many modern HAWT units, with
thickness-to-chord ratios varying from about 28 percent at the root to about 12 percent at the
tip. In some respects, these standard airfoils have unsatisfactory characteristics. For example,
airfoils in the NACA 230XX series have maximum lift coefficients that are very sensitive
to surface fouling, and their performance deteriorates with increased thickness more rapidly
than that of other airfoils.
NACA 63-2XX series airfoils have demonstrated the best overall performance
characteristics of the NACA families, and they provide reasonable resistance to roughness
losses. Airfoils in the LS(1)-04XX series were designed to tolerate surface fouling, but
HAWTs with these airfoils ( e.g. , the ESI 80 and Carter 300 ) have experienced large power
losses induced by roughness. This airfoil series also has a very high nose-down pitching
moment which can result in excessive elastic blade twist in thin blades and undesirable
performance changes.
For most VAWTs, a symmetrical airfoil such as the four-digit series NACA 00XX is
normally used, with thickness ratios varying from 12 percent to 15 percent.
Current Designs of Airfoils for Wind Turbines
Special airfoils designed for wind turbine applications have been developed since the
mid-1980s. Classes of these for HAWTs are described by Tangler and Somers [1985, 1995]
and Tangler [1987], and are shown in Appendix D. Some airfoils specially tailored for
VAWTs are described by Klimas [1984]. Typical design goals for new HAWT airfoils are