Michael Graham - Wind Energy Handbook

Table 3.5 The principal characteristics of the Risø‐P series.

Figure 3.72 The Risø‐B series of aerofoil profiles.

Table 3.6 The principal characteristics of the Risø‐B series.

In the tables above, the ‘design c l’ 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 c lis high so that a blade will be most efficient at low solidity.

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 3.73and 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.

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 C Lmaxso that the operating C Lwhere the best C L/C Dratio occurs is significantly below C Lmax. 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.

Figure 3.74 Flat‐back aerofoil derived from DU‐97‐W‐300.

The inboard sections of a wind turbine blade are much more strongly dictated by structural bending strength requirements. Hence increasingly thick sections are used as the radius reduces to the root. Blade sections at the root end may be up to 40% thick. Inboard of the root end of the aerodynamic sections of the blade, the blade often merges continuously into a circular or other bluff section joining the blade to the hub. To accommodate these very thick sections and at the same time retain a high C Lmaxfor power purposes, blades are often fitted with vortex generators (VGs) (see later) near the location of early separation, and so‐called ‘flat‐back’ sections with blunt trailing edges have been designed. An example is the aerofoil shown in Figure 3.74, which is derived by thickening the rear half of the more conventional DU‐97‐W‐300. A computational analysis of the aerodynamics and aeroacoustics of this aerofoil has been given by Lynch and Smith (2009).

There are a number of small devices that can be added (and sometimes are) to existing wind turbine blades post‐design without compromising the blade structural performance. These devices have often been derived from aircraft usage and are usually incorporated either to improve performance that has turned out to be below the designed level or, more often, to provide additional performance beyond the intended where circumstances dictate. An example of the latter is the desirability of increasing the design lift coefficient for a turbine blade that is to be operated in significantly reduced air density due to the altitude of the wind turbine site.