When inventor Saul Griffith arrived at MIT in 1999, he encountered a problem no other Ph.D. student faced. Where to kitesurf? “Without other options, I became the first person to kitesurf on the Charles River,” says Griffith, 35. “And, yes, it was just as terrible as you’d imagine.” Now he’s tackling a slightly bigger issue: the global energy crisis. The folks at Google have kicked in $15 million to his clean-energy company, Makani Power, and the MacArthur Foundation tapped him in 2007 for a prestigious $500,000 “genius” fellowship. His hobby, it seems, has paid off. Griffith and his colleagues at Makani are now developing huge, wing-shaped kites to harness the energy of high-altitude air—which is much stronger and more consistent than the ground-flow breezes that traditional windmill-based turbines rely on. The concept is simple: The yo-yo effect of the kite being pulled in and out by the wind churns a turbine and creates energy. But for every advance, Griffith says, he’s realized there’s a drawback. “Now whenever the wind blows, I have to go test our technology—and not go kiting.”

THE BRUSH (Altitude: 60'): American Charles F. Brush was the first to design an automatic wind turbine, in 1887. The giant cedar contraption powered his 17-room mansion for 20 years.

THE WINDMILL (Altitude: 262'):Indiana’s Fowler Ridge I, one of the largest wind farms in the U.S., operates 222 turbines, each of which produces enough energy to power some 365 households.

THE KITE (Altitude: 4,100'): In 2007 Dutch scientists debuted a kite capable of powering five homes. But soon they plan to test this “Laddermill,” which they say could power 50 houses.

THE MAKANI (Altitude: 31,680'):Makani has kept its design under wraps, but Griffith says the kites could fly six miles high and produce enough energy for 100,000 homes.—Andrea Minarcek