This subject has been on my mind for the past 2 days.
All wind turbines seem to be blades mounted vertically on a horizontal
axle/axis.
These can only be 100% efficient if the wind blows directly onto the face
of the blades.

Can never be 100% efficient. or else you would be left with still air behind
the blades, thus blocking the incoming. Usually significantly less the 50%
efficeincy to allow the air after the blades to have some energy left to
carry it away.