Wouter, a long long time ago ( over 30 years ) I was working with a guy who was experimenting with horizontal turbines rather than vertical. Basically he had 4 wing sections on long horizontal poles, the wings were set up so they started to tilt when over a certain speed making them less efficient and thus governing the speed. His big problem was to get them spinning in low winds to then create lift which powered them up. His reasoning was that a small wing section on a long pole generates a lot of force rather than the inefficiencies of a propellor, much easier and simpler to build as well. Anyway why has this type of turbine not developed rather than the props we know of today