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0.30 1005 9.5 20.7 2975 0.35 965 7.6 19.5 3400 0.40 925 6.1 18.7 3825 0.45 885 4.9 18.1 4250 0.50 845 3.9 17.6 4675 0.55 805 3.1 17.1 5100 0.60 765 2.4 16.6 5525 0.65 725 1.9 16.1 5950 0.70 685 1.5 15.6 6375 0.75 645 1.2 15.1 6800 0.80 605 0.9 14.6 6375 0.85 565 0.6 14.1 7225 0.90 525 0.4 13.6 8075 0.95 485 0.2 13.1 8500 1.00 445 0.0 12.6

Figure 3.44 The aerodynamic characteristics of the NACA632XX aerofoil series. (XX corresponding to the percentage thickness ratio of each section indicated.)

Figure 3.45 Angle of attack distribution for a range of tip speed ratios.

The importance of the outboard section of the blade is clearly demonstrated in Figure 3.47. The dramatic effect of stall is shown in the difference in torque distribution between the tip speed ratio of 4 and the tip speed ratio of 2. Note, also, the flat distribution of torque at the high tip speed ratio of 12; this is caused by the effect of drag, which reduces torque as the square of the local speed ratio and with the low angle of attack at λ = 12 drag causes a significant loss of power.

Figure 3.46 Distribution of the flow induction factors for a range of tip speed ratios (lines and symbols as for Figure 3.45).

Figure 3.47 Distribution of blade loads for a range of tip speed ratios (lines and symbols as for Figure 3.45).

Although the blade thrust coefficient increases with tip speed ratio as shown in Figure 3.48, it must be remembered that the actual thrust force increases with wind speed, as is demonstrated in Figure 3.49.

Figure 3.48 Variation of thrust coefficient with tip speed ratio.

Figure 3.49 Variation of the actual force with wind speed.

3.12 The performance curves

      3.12.1 Introduction

      The performance of a wind turbine can be characterised by the manner in which the three main indicators, power, torque, and thrust, vary with wind speed. The power determines the amount of energy captured by the rotor, and the torque developed determines the size of the gearbox and must be matched by whatever generator is being driven by the rotor. The rotor thrust has great influence on the structural design of the tower. It is usually convenient to express the performance by means of non‐dimensional, characteristic performance curves from which the actual performance can be determined regardless of how the turbine is operated, e.g. at constant rotational speed or some regime of variable rotor speed. Assuming that the aerodynamic performance of the rotor blades does not deteriorate, the non‐dimensional aerodynamic performance of the rotor will depend upon the tip speed ratio and, if appropriate, the pitch setting of the blades. It is usual, therefore, to display the power, torque, and thrust coefficients as functions of tip speed ratio.

      3.12.2 The C_P – λ performance curve

The theory described earlier in this chapter gives the wind turbine designer a means of examining how the power developed by a turbine is governed by the various design parameters.

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