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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.

Diameter	Type	Aerofoil thickness	Primary	Tip	Root
3–10 m	Variable speed Variable pitch	Thick	‐‐‐	S822	S823
10–20 m	Variable speed Variable pitch	Thin	S802	S802 S803	S804
10–20 m	Stall regulated	Thin	S805 S805A	S806 S806A	S807 S808
10–20 m	Stall regulated	Thick	S819	S820	S821
20–30 m	Stall regulated	Thick	S809 S812	S810 S813	S811 S814, S815
20–40 m	Variable speed	—	S825	S826	S814
	Variable pitch				S815
30–50 m	Stall regulated	Thick	S816	S817	S818
40–50 m	Stall regulated	Thick	S827	S828	S818
40–50 m	Variable speed Variable Pitch	Thick	S830	S831 S832	S818

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.