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.
3.17.3 The Risø aerofoils
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 3.70 and 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.
| Aerofoil | Max t/c % | x/c at max t/c | y/c at TE | Re × 10−6 | α ο | cl max | Design α | Design cl | Max cl/cd |
|---|---|---|---|---|---|---|---|---|---|
| Risø‐A1‐15 | 15 | 0.325 |
0.0025
|