Effect of Number of Blades

The variation in power output with TSR from a wind turbine is affected by the number of blades. If there are no nonideal losses to consider, the resulting behavior is typified by Fig. 13.12. Notice that increasing the number of blades (or increasing solidity) does not affect the maximum efficiency of the turbine, but it does affect the tip speed ratio (or wind speed) where maximum efficiency is obtained. Increasing the number of blades or solidity also reduces the range of tip speed ratios over which high power coefficients can be obtained. Too many blades or too much solidity gives a steep power curve that peaks at low tip speed ratios. This is not very useful in practice, besides the fact that the blades will also be approaching stall at these conditions (high wind speeds). The addition of viscous drag (see Section 13.8.2) changes the situation somewhat and the efficiency will decrease with increasing numbers of blades (increasing solidity). There is no effect on efficiency if the solidity is kept constant and only viscous drag is assumed. However, consideration of tip losses (see Section 13.8.3) will tend to favor turbines with larger numbers of blades (for a given overall solidity) because this will minimize tip-loss effects (i. e., induced losses will be minimized and the turbine will be closer to an ideal actuator disk).

Figure 13.13 shows how the power output from a wind turbine depends on the viscous drag of the airfoils that comprise the blades of the turbine. Clearly maximum efficiency favors airfoils with lower drag coefficients, all other factors being held constant, but this is rather obvious. At the Reynolds numbers encountered on wind turbines most airfoils tend to have drag coefficients in the range 0.01 < < 0.02 unless surface finish

is degraded by erosion, dead insects, frost, or ice. Some airfoil sections are more sensitive to these effects than others. The absence of specific information on the airfoil section characteristics from measurements would suggest the use of Cd0 = 0.01 as an initial assumption for preliminary design studies.