Is huge 'power tower' just pie in the sky?

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The Independent Online

Science Editor

A "power tower", seven kilometres high, which could produce pollution- free energy from the North Sea is being proposed by researchers in the Netherlands.

Its proponents claim that the tower could generate nearly twice as much electricity as Drax, Britain's largest power station, by harnessing the same natural system that causes water to evaporate from the oceans and fall as rain

The four-mile-high structure would be nearly 30 times taller than Britain's highest building, Canary Wharf Tower (244 metres) and only 1,848 metres lower than the summit of Everest. But civil engineers doubt that it could be built, and renewable energy experts said there were better ways of generating non-polluting energy.

At the top of the "MegaPower Tower", ammonia or hydrogen gas would condense in the cold of the upper atmosphere and then fall through a turbine to the bottom, where heat flowing from the warm sea would evaporate it and start the cycle over again.

According to New Scientist magazine, a year-long feasibility study carried out by Novem, the Netherlands energy and environmental agency, on behalf of the government, concluded that such a vast structure should not be impossible to build.

However, since the tower would be more than 10 times taller than the highest man-made structure - the 646-metre Warsaw radio mast in Poland - this claim has been greeted with scepticism. The mast fell down in 1991, just 17 years after it had been erected. Even if the tower's construction were feasible, energy experts doubt that it would be much use.

One of Britain's foremost advocates of renewable energies, Dr John Twidell, of De Montfort University in Leicester, pointed out that conventional gas turbines generate electricity efficiently - converting at least 50 per cent of the heat energy into electricity - because of the high temperatures of combustion. But the proposed tower would produce a temperature difference of only about 20C to 30C and so "in engineering terms, the thermodynamic efficiency would be very low" he said. "The tower would be very inefficient."

Dr Twidell continued: "It is a fallacy to believe we are short of energy - there is an abundance - the challenge is to harness that in a cost effective way. It is clearly cheaper to have a wind turbine that goes up 50 metres than a tower that is seven kilometres high."

Adrian Fox, of the engineering consultants Arup, told New Scientist: "It's an exciting idea, but when you start conceiving of structures that are so huge, you can't really make reliable comments without doing a detailed study. If feasible, it would require significant breakthroughs in technology."