In the tables above, the ‘design cl’ is the value of the lift coefficient that corresponds to the maximum lift to drag ratio and the ‘design α’ the corresponding angle of attack. An optimised variable‐speed turbine should be designed so that the blade sections operate at this angle of attack. It is a design feature of the Risø aerofoils that the design cl is high so that a blade will be most efficient at low solidity.
3.17.4 The Delft aerofoils
The Delft University of Technology in the Netherlands has also developed a number of aerofoils for wind turbine rotors (Timmer and van Rooij 2003). As with the NREL and Risø aerofoils, the principal feature driving the designs was surface roughness insensitivity, but more emphasis was placed upon seeking designs for thick aerofoils to gain a structural advantage. The Delft University series of aerofoil profiles are illustrated in Figure 3.73 and listed in Table 3.7.
The design tool for the Delft aerofoils was the RFOIL code, a modification made at Delft of the XFOIL code to include the effects of stall delay.
The two thickest of these aerofoils have not been tested in a wind tunnel, and the characteristics have been determined by calculation.
3.17.5 General principles for outboard and inboard blade sections
The aerofoil sections of the outboard half of the blade are responsible for extracting the major part of the wind energy. These sections should therefore be efficient with a high lift/drag ratio, hence reasonably thin, consistent with adequate structural strength. Thickness ratios around 18% are usual with relatively high CLmax so that the operating CL where the best CL/CD ratio occurs is significantly below CLmax. This allows efficient operation while keeping sufficiently clear of the stall to avoid its adverse effects when wind gusts momentarily push up the angle of attack too quickly for pitch regulation to respond sufficiently.
Figure 3.73 The Delft University series of aerofoil profiles.
Table 3.7 The principal characteristics of the Delft University 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 |
|---|---|---|---|---|---|---|---|---|---|
| DU 96‐W‐180 | 18 | 0.3 | 0.0018 | 3.00 | −2.7 | 1.26 | 6.59 | 1.07 | 145 |
| DU 00‐W‐212 | 21.2 | 0.3 | 0.0023 | 3.00 | −2.7 | 1.29 | 6.5 | 1.06 | 132 |
| DU 91‐W2–250 | 25 | 0.3 | 0.0054 | 3.00 | −3.2 | 1.37 | 6.68 | 1.24 | 137 |
| DU 97‐W‐300 | 30 | 0.3 | 0.0048 | 3.00 | −2.2 | 1.56 | 9.3 | 1.39 | 98 |
| DU 00‐W‐350 | 35 | 0.3 | 0.01 | 3.00 | −2.0 | 1.39 | 7.0 | 1.13 | 81 |
| DU 00‐W‐401 | 40.1 | 0.3 | 0.01 | 3.00 | −3.0 | 1.04 | 5.0 | 0.82 | 54 |