Starting your own wind farm but lack the backyard space? Try vertical-axis windmills. When grouped properly they're more efficient than the more common propeller-style, horizontal-axis type, says Robert Whittlesey of the California Institute of Technology. In the right arrangement, vertical-axis turbines benefit from drafting, much like fish swimming in a school, making it possible to pack vertical turbines into just 1 percent of the space that would be required by an equivalent number of horizontal-axis turbines. And the faster they rotate, the more visible their cylindrical shape appears, making them safer for birds than traditional windmills.