Drag turbines like this Savonius design can only spin at a theoretical maximum equal to the wind speed. Turbines using aerodynamic lift spin faster and perform better.
There is a fundamental difference that separates the Savonius-type wind turbine from all the others in the efficiency table. The Savonius extracts power from the wind by leveraging the force of aerodynamic drag, while all the others leverage the more powerful aerodynamic lift force. While all surfaces exposed to airflow generate both lift and drag, lift offers far greater potential. A simple example of the difference between these forces is found in sailing. A sailboat going downwind can only go as fast as the wind pushes or “drags” it. However, a sailboat cutting across the wind can achieve speeds of more than twice the actual wind because of the lift force generated by the smooth flow over the sails. Any wind turbine that relies on blades being pushed or dragged downwind will be less efficient than one with blades that utilize lift.