Wind Turbines Theory and Practice

Chapter 4 extends the aerodynamic discussions of Chapter 3 to show how the rotor net loads (power, thrust, and torque) are developed. The dimensionless power coefficient (Cp) curve is introduced, and the relationship between rotor tip speed ratio and optimum solidity is explained. The variation of thrust loading with wind speed on an ideal pitch-controlled rotor is explained from simple theory, and illustrated with measurements from a full-scale turbine. Equations governing the chord and twist distributions for an optimised blade are given and discussed in the context of some historic blade types, with illustrations. Rotor aerodynamic control is explained with reference to fixed-pitch stall regulation and variable blade pitch (both positive and negative). The influence of blade number is examined, with discussion of the advantages and disadvantages of one-, two-, and three-bladed wind turbines. The method by which annual energy capture is derived from the power curve and wind speed distribution is explained, with example. The chapter concludes with a brief overview of alternative aerodynamic control devices including tip vanes and ailerons, and downwind rotors (with examples).