i was looking at this some time ago. i came across a company from chicago who built something like a dna molecule. you know? like a double helix. the idea was just as you suggest - make it work in light wind. i wanted to try and mount one on our front porch, but never got around to it. i'll root around to see if i can't find more for you about that company.
I am constantly responding to old posts. I click on the new post button and scroll down and wa la. I wish there was some way old inactive post would delete itself. I guess I should look at the date before responding but I am still curious about the 'vertical windmills', lol.
I have a perfect piece of land for one. We have an up flow breeze just about all the time. Sometimes it is nit a great speed but I don't think that matters. I just don't want something ugly standing out in my gardens. This post made me google it and I found a nice one that I wouldn't mind having. It looks like one of those whirly gigs...I will do some homework on this.