Kiteplanes power up

Flying a kite in the park has inspired an innovative concept to generate electricity from powerful winds miles above the earth.

The ‘Laddermill’ concept, envisaged by a team at Delft Technical University in the Netherlands, is a chain of wings or kites attached to a looped cable stretching up to 10km high. The ‘kiteplanes’ exert an upwards force on one side of the loop and a downwards force on the other, causing the cable to rotate and power a generator in the base station. The kiteplanes’ angle and shape can be altered to generate the opposing forces needed.

Delft University team leader Prof Wubbo Ockels, an ex-astronaut and head of ESA’s education office, said the wind energy at 30,000ft is 20 times more powerful than at sea level. ‘Above a certain altitude there is a massive amount of wind power,’ he said.

‘Kites that can tap into that wind can generate a great deal of energy.’ One Laddermill could generate 100MW, the researchers claimed, compared to only a few MW for conventional wind turbines.

At the recent European Wind Energy Conference, the The Laddermill team announced plans to test a variety of kiteplane designs next year using inflatable and lightweight materials. A working model will be built in the next four years.

The apparent dangers of a structure extending kilometres above the earth could be overcome, the researchers said.

‘If the wind dropped the Laddermill would drift gently to the ground’, said Ockels. ‘We want to be safe. Flexible or inflatable kites wouldn’t be hazardous; the worst that would happen would be the kite becoming dirty when it landed,’ he said. ‘We would only operate the system with a good forecast and the wings would be adapted to weather patterns.’ The ground station could deploy and retrieve the Laddermill in hours, he added, and they would only be built in uninhabited areas.

The cable connecting the wings would be made of materials such as Dyneema, said Ockels. Cable breaks would be unlikely, because the cable could be inspected as the system rotated through the base station, and sections removed for maintenance. Each kiteplane would have a GPS sensor to inform air traffic control of its position.

Stability of the wings is important in the unpredictable, turbulent winds near the ground, but the kites higher up the system would support the lower cable, said Ockels.

The most critical aspect is controlling rotation about the cable axis. The team plans to control this by changing the kites’ shape, or by fitting small propellers, which could be controlled remotely or by ‘smart kites’ that reposition themselves.