Google X will soon be testing a 84-foot version of their Makani airborne wind turbine. It flies at 1,500 feet altitude, where wind speeds are much faster, and is tethered to the ground. Wind at altitude is also considerably more variable than on the ground, especially where the test will be, and Google says they want a few crashes so they can determine just how far the devices can be pushed. Each one can generate a not insubstantial 600kW.
The energy kite simulates the tip of a wind turbine blade, which is the part of a turbine that makes most of the energy. The kite is launched from the ground station by the rotors, which act like propellers on a helicopter. Once in the air, the kite generates power by flying in large circles where the wind is strong and consistent. Air moving across rotors mounted on the kite forces them to rotate, driving a generator to produce electricity, which travels down the tether to the grid.