To have the maximum efficiency in low wind speed, there exists an optimum blade angle. This angle is between the wind direction and blade or it is an angle between the blade and direction of blade rotation?
I’m afraid I don’t understand what you are asking, but I’ll throw some information at you in hopes that it helps.
Blades are generally twisted and the blade pitch angle is somewhat arbitrary because the twist distribution is arbitrary. Some people like to define the twist distribution as 0 at the 70% span. I (and others) prefer to define it such that the twist at the tip is 0. The resulting angle between the chord at a given span and the plane of rotation is the local twist plus the blade pitch.
The blade pitch to choose in Region 2 (cut-in to rated) is the one that gives you the maximum Cp. To determine what that optimal angle is, I run a performance tool such as our WT_Perf to compute Cp as a function of tip-speed ratio and blade pitch and use the pitch that gives me the largest Cp.
The optimal twist distribution and pitch setting depend on the airfoil choice and chord distribution, so there is no one angle that works for all turbines. Running a performance optimizer such as our HARP_Opt will find the optimal airfoil, chord, and twist distribution for a given set of airfoils, rotor radius, and rated power.