Michael Graham - Wind Energy Handbook

Because of data availability, a popular range of aerofoil sections for wind turbine blades was, but less so now, the NACA six‐digit series, an example of which is discussed in Section 3.9. Although more tolerant to leading edge roughness, the NACA six‐digit series is no better overall than the NACA four‐digit series described in Appendix A3. The main reason for the popularity of the NACA aerofoils is because high quality experimental data is available from tests that were carried out in the 1930s in the pressurised wind tunnel built by NACA (superseded by NASA in 1959). The NACA technical reports are available free on the NASA website, and much of the force data is given in Theory of Wing Sections by Abbott and von Doenhoff (1959).

The development of special‐purpose aerofoils for HAWTs began in 1984 jointly between the National Renewable Energy Laboratory (NREL), formerly the Solar Energy Research Institute (SERI), and Airfoils, Incorporated (Tangler and Somers 1995). Since that time, nine aerofoil families (see Table 3.3) have been designed for various size rotors. The principal requirement, depending to some extent on Reynolds number and hence rotor size, is that they have a maximum lift coefficient that is maintained in the presence of leading edge surface roughness.

The primary design tool was based on the work of Eppler (1990, 1993), who developed a method of determining the nature of the 2‐D viscous flow around an aerofoil of any profile. The Eppler method includes flow separation in the initial stages of stall and has proved to be very successful.

In addition, several different aerofoil families have been designed for stall‐regulated, variable‐pitch, and variable‐rpm wind turbines.

For stall‐regulated rotors, improved post‐stall power control is achieved through the design of aerofoils for the outer sections of a blade that limit the maximum lift coefficient. The same aerofoils have a relatively high thickness to chord ratio to accommodate overspeed control devices.

For variable‐pitch and variable‐speed rotors, outer section aerofoils have a high maximum lift coefficient, allowing low blade solidity.

Generally, aerofoil cross‐sections with a high thickness to chord ratio give structural designs of high stiffness and strength without causing a large weight penalty, and aerofoils of low thickness result in less drag.

Table 3.3 Summary of the NREL aerofoils and their applications.

Annual energy capture improvements that are claimed for the NREL airfoil families are of the order of 23–35% for stall‐regulated turbines, 8–20% for variable‐pitch turbines, and 8–10% for variable‐rpm turbines. The improvement for stall‐regulated turbines has been verified in field tests.

The aerofoil shape coordinates for some of the NREL aerofoils are available on the website of the National Wind Technology Center (NWTC) at Golden, Colorado. Measured aerofoil data for some aerofoils is also available. A licence must be purchased for information about those aerofoils that are restricted.

Some of the NREL large blade aerofoil profiles are illustrated in Figure 3.69.

The Risø National Laboratory in Denmark have also developed families of aerofoil designs for wind turbines with similar objectives to the NREL series (Fugslang and Bak 2004). Although the aerodynamic design techniques of the two laboratories were different, there is, perhaps not surprisingly, a significant similarity about the actual designs.

The design tools for the Risø aerofoils were the X‐FOIL code developed by Drela (1989), a development of the work of Eppler (1990, 1993), and the Ellipsys‐2D CFD code developed at the Technical University of Denmark by Sørensen (1995).

Three families of aerofoils have been developed at Risø – Risø‐A, Risø‐P, and Risø‐B. The Risø‐A family was designed in the 1990s and was intended for stall‐controlled turbines; however, sensitivity to surface roughness was found to be higher than expected in field tests. The Risø‐A family of aerofoil profiles is illustrated in Figure 3.70and listed in Table 3.4.

Figure 3.69 NREL aerofoil profiles for large blades.

Figure 3.70 The Risø‐A series of aerofoil profiles.

Table 3.4 The principal characteristics of the Risø‐A series.

Figure 3.71 The Risø‐P series of aerofoil profiles.

The Risø‐P family of just four aerofoils, shown in Figure 3.71and Table 3.5, was designed to replace the corresponding profiles in the Risø‐A series for use on variable‐pitch and variable‐speed rotors.

The Risø‐B family was designed as six separate aerofoils with an extended range of thickness to chord ratio from 15% to 36%. The aerofoils, generally, have high maximum lift coefficients for use on multi‐megawatt size rotors with low solidity, flexible blades having variable‐speed pitch control. This family of aerofoil profiles is shown in Figure 3.72and Table 3.6.