The Finnish engineer S J Savonius designed the first savonius windmill in 1922. It is a Vertical Axis Wind Turbine (VAWT) unlike most windmills today, who are of the Horizontal Axis Wind Turbine (HAWT) type. They are less efficient in turning wind power into energy. Compared to HAWT's that produce at around 35 % efficiency, the efficiency of the VAWT is a decent amount lower, in the 25 percent range, but they are cheap to build, and can be placed in dense populated areas more easily, because of their smaller visual and audio footprint. When seen from a distance, the savonius mostly looks like a rectangle, with slightly moving sides. You can have an electrical generator mounted at the bottom of the windmill. No need to hoist the generator up into the air 10 meter.
S-shaped savonius type are least efficient. Cut barrel type, where the 2 halves are shifted, are some degree more efficient. The most efficient is a shape like the cut barrel, but with 2 straight middles (that allow air to pass). Some are spiral shaped, to even out the load at all angles.
A vertical savonius functions the same from all horizontal wind angles. It is also possible to build horizontal savonius mills, but then you will need some turning-in-the-wind-device, or BIG tail of some sort. Most savonius mills are vertical, since they are structural easy to build.
It rotates because the air pressure is lower on one side, the hollow side. The cut barrel option tries to deflect some of the wind into the cup of the other blade, where it is deflected, making that blade turn a little harder to. The straightening of the lines in the middle makes the path shorter, and tries maximize that effect. By making the straight pieces larger, you capture a larger portion of the wind in a productive way. All air that directly flows into the cup can go out one way, to create a rotating motion, or the other way, towards the other cup, where it will create motion. This is all theory, but the efficiency of straight cut barrel type savonius is claimed to be getting into the 35 percent region. More study on this matter is required. So, why this wiki?
