The wind-turbine-on-a-car article on the front page has a few people confused about Betz' law and wind turbine aerodynamics. I like Betz' law, I think it's an interesting bit of logical physics that massively increases understanding of wind turbines. So I'm using this space to promote the comment I made.
And to invite you all to read the quite good wiki page on wind turbine aerodynamics.
If you want more, read the hugely interesting crosswind kite power paper from Lloyd, written in 1980. This single paper spawned a huge research effort. About 40 teams are developing crosswind kite power systems, all based on this single paper. And better yet, it's understandable if you've read the wind turbine aero wiki page. Also, this paper proposes a modified C5 Galaxy to be used as a power generating kite.