A Power Plant That Flies to 4,000 Metres Just Flew Over a City

A kite-like generating system flew in Yueyang, Hunan, this week, reaching altitudes of up to 4,000 metres — the first time a system of this class has been flown in a city rather than at a test range.

The machine, called S4000, is 67 metres long, 40 wide and 23 high, and is held aloft by a helium-filled platform. Lightweight generating modules ride up with it and the power comes back down a tethered cable at 10 to 35 kilovolts. Its design life is 20 years.

The appeal of high-altitude wind is that it is stronger and steadier than what reaches a tower. Wind speeds rise with height, and at a few thousand metres the flow is less disturbed by terrain. The resource is large and almost entirely unexploited.

Two architectures are being tried. In ground-based designs the heavy generator stays on the ground and a kite or blimp pulls a cable to drive it. In airborne designs — the one used here — the generator goes up with the platform and the electricity comes down the tether.

The city flight was not mainly about generating power. Dun Tianrui, the system's chief architect, said the test was about grid connection: whether running the generator disturbs power quality and the platform's own stability. It was also about whether the machine can fly safely near people, which is a different question from whether it can fly.

That difference matters because the customers are where the people are. Cities are the core of electricity demand, so a generating system that has to stay in the desert solves less of the problem than one that can operate at the edge of a city.

S4000 completed a full-altitude run at a test base in the northwest in August, under plateau conditions with strong ultraviolet light and wide temperature swings. Cai Rong of the Chinese Academy of Sciences said the city flight showed the system could be delivered and deployed quickly in real conditions — the groundwork for airworthiness certification.