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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	3.00	−4.0	1.50	6.0	1.13	168
Risø‐A1‐18	18	0.336	0.0025	3.00	−3.6	1.53	6.0	1.15	167
Risø‐A1‐21	21	0.298	0.005	3.00	−3.3	1.45	7.0	1.15	161
Risø‐A1‐24	24	0.302	0.01	2.75	−3.4	1.48	7.0	1.19	157
Risø‐A1‐27	27	0.303	0.01	2.75	−3.2	1.44	7.0	1.15	N/A
Risø‐A1‐30	30	0.300	0.01	2.50	−2.7	1.35	7.0	1.05	N/A
Risø‐A1‐33	30	0.304	0.01	2.50	−1.6	1.20	7.0	0.93	N/A

Figure 3.71 The Risø‐P series of aerofoil profiles.

The Risø‐P family of just four aerofoils, shown in Figure 3.71 and 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 3.72 and Table 3.6.

Table 3.5 The principal characteristics of the Risø‐P 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‐P‐15	15	0.328	0.0025	3.00	−3.5	1.49	6.0	1.12	173
Risø‐P‐18	18	0.328	0.0025	3.00	−3.7	1.50	6.0	1.15	170
Risø‐P‐21	21	0.323	0.005	3.00	−3.5	1.48	6.0	1.14	159
Risø‐P‐24	24	0.320	0.01	2.75	−3.7	1.48	6.0	1.17	156

Figure 3.72 The Risø‐B series of aerofoil profiles.

Table 3.6 The principal characteristics of the Risø‐B 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‐B1‐15	15	0.278	0.006	6.00	−4.1	1.92	6.0	1.21	157
Risø‐B1‐18	18	0.279	0.004	6.00	−4.0	1.87	6.0	1.19	166
Risø‐B1‐21	21	0.278	0.005	6.00	−3.6	1.83	6.0	1.16	139
Risø‐B1‐24	24	0.270	0.007	6.00	−3.1	1.76	6.0	1.15	120
Risø‐B1‐30	30	0.270	0.01	6.00	−2.1	1.61	5.0	0.90	N/A
Risø‐B1‐36	36	0.270	0.012	6.00	−1.3	1.15	5.0	0.90	N/A

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_l is high so that a blade will be most efficient at low solidity.