DUAL OR TWIN ROTOR CONCEPT

[MR FIXIT]

Why not use this on a savonious mill?  And why not build a two turbine system instead of just talking about it .  See how it works for yourself.  Talk is cheap.

[chadking]

You know it's been studied at Harvard that two counter-rotating savonius mills placed very close together, with the power side of the rotors close to one another will cause the two rotars to "phase lock", and this combination will create more power than two individual rotors, due the Magnus Effect.

The layout would be something like this:

In practical use this isn't really feasible, for the fact that most VAWT's take wind from multiple directions, and not just one prevailing wind direction.  So a permanent layout of two VAWTs wouldn't always be placed in the optimum direction.

Though it's not something that seems easily usable in most cases, it's another 'effect' of counter rotation that most people don't take into consideration.  

[MR FIXIT]

How about one savonious above the other on the same axis.  What ever you get out a system like this would be a plus.  Even if you get 30 to 40% more or less because one turbine is turning faster than the other , it is still a plus.

[chadking]

Another rotor on top on an existing vertical axis rotor will increase output.  It does so because it is directly increasing the swept area of the system.  Your efficiency isn't going to increase any, but yes, the output will be higher.