"When the wind blows into a conventional three-bladed, single-rotor wind turbine less than 40 per cent of its energy is converted into electricity. The rest escapes, much of it in the air wake that’s created behind the blades. That wake spins in the opposite direction (i.e. counter-clockwise) to those blades. If a second rotor with another set of blades is right behind the first rotor, and if it is designed to also spin counter-clockwise, it can capture energy from that wake. The end result is a turbine system that harnesses much more energy from the initial flow of wind." - LANjackal from Bookmarklet