The technology isn't new-a patent for a hydrogen internal combustion engine was issued in 1928, and a 20hp fuel-cell tractor was demonstrated in the late 1950s.

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It's powered by a hydrogen internal combustion engine that turns a 44-kilowatt electric generator that runs eight propellers and whatever communications equipment is loaded on board.

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There is, however, an alternative: burn the hydrogen in a conventional internal-combustion engine.

ECONOMIST: Hydrogen-powered cars