China has flown an airborne wind energy system to 4,000 metres (13,123 feet) above sea level and generated electricity from the sky, in what state broadcaster CCTV described as a world first.According to a CCTV report on Saturday, the prototype S4000 craft completed a full work cycle – ascent, station-keeping, power generation and recovery – at a test site in northwestern China, which the broadcaster called a breakthrough for high-altitude wind power and a step towards engineering...