Abstract: In order to obtain the expression of the ideal horizontal axis wind turbine power coefficient with tip-speed ratio and airfoil lift-drag ratio changes, as the theoretical extreme value of an actual wind turbine power coefficient, the ideal blade chord expression is deduced by blade element-momentum (BEM) theory, and the ideal wind turbine power coefficient is obtained by integrating along the ideal blade span. Let the drag coefficient in the power coefficient expression equal to 0, the power coefficient limit formula associated with the tip-speed ratio is obtained; and further let the tip-speed ratio approach to infinity, Betz limit (16/27) is obtained. To find the actual wind turbine power coefficient limit, and to facilitate theoretical analysis, ideal wind turbine power coefficient for lift-drag ratio in the range of 10 to 1 000 and tip-speed ratio of 1 to 10 is calculated in detail, and the corresponding data form is given. The research shows that the ideal wind turbine power coefficient is only the function of tip-speed ratio and the lift-drag ratio, and the power coefficient approaches Betz limit only when both the tip-speed ratio and the lift-drag ratio is close to infinity. The results also show that the actual wind turbine power coefficient is impossible to exceed 0.538 if the lift-drag ratio is less than 100, and is difficult to exceed 0.500 if the effects of limited blade number and blade shape are considered.

Key words: Aerodynamic efficiency, Betz limit, Ideal horizontal axis wind turbine(IHAWT), Lift-drag ratio, Power coefficient limit, Tip-speed ratio, Wind energy utilization, CSP, EEG, Rehabilitation training, Upper-limb exoskeleton