Winds 300 to 600 metres above the ground are, on average 5 to 8 times as strong as those at the surface.

Altaeros, a four-year-old company in Massachusetts, has developed its Buoyant Airborne Turbine (BAT) to take advantage of these stronger, more consistent winds to deliver power to remote sites.

The BAT is a 18 metre diameter helium aerostat with a three-blade-rotor spinning inside. It rises 300 metres off the ground and sends back power through high-strength tethers.

Altaeros, which has investment from a group controlled by the Indian industrialist Ratan Tata, recently announced a $1.3 million demonstration project in Alaska that will supply power to about a dozen homes and is working on deals for remote sites in Australia and Canada.