I've just found this group and it's got some great discussions on it, I was particularly interested in Bob Hunt's wind turbine, but I believe there is another more efficient sail based VAWT, which I (and many others in the past) have invented.
This is the basic design of my VAWT, (top down view):
and you can read more information about it here:
http://sailturbine.envy.nu/
In the picture above, I didn't illustrate the sails themselves properly, they would obviously be curved, and would (I believe) also provide a lift force on the upwind side, as well as a drag force that would also push each blade round the upwind side. This design produces force all the way around the circle, unlike most other (if not all) sail based VAWTs (not that there are many other designs!).
This would overcome some of the unnecessary problems in Bob Hunt's design, as far as I can see.
I think there's going to be a bit of a shakeup in the wind turbine industry when somebody actually does some proper computer modelling of sail based VAWTs, as there's practically zero research into them, yet everybody and his dog just presume they are 'inefficient' without providing much evidence for this.
The problem with blade based HAWTs is that as you scale them up, the tip speed ratio remains the same, so the angular velocity will continue to decrease as the size goes up, so more and more wind is simply passing unused through the huge spaces in between the blades (watch an anemometer mounted at the top of a large turbine and notice how it changes it's speed as each blade passes it - the rest of the time it's receiving the full power of the wind. Then simply look at the huge swept area of the blades and you can clearly see how 90% of the wind is simply passing through unused. (Unless anybody can explain to the contrary). With sail based VAWTs this will never happen, even if the VAWT was a mile high - the sails still intercept far more wind than a HAWT of the same size.
Wind tunnel tests done on HAWTs using sub 10m models are next to useless when it comes to scaling up to the large scale HAWTs that exist today.
A similar VAWT is here:
http://energy.murdoch.edu.au/Solar2004/Proceedings/Wind/Cooper_Paper_Wind.pdf
but uses aerofoils instead of sails, but I think it would be much more powerful using sails. Such is the almost religious nature of modern wind technology that the writers of the paper probably hadn't even considered the possibility of using such archaic technology as sails!
