FRANKFURT (Reuters) - Astronomical costs for delivering hydrogen-powered cars and the network to run them mean clean vehicles are years away from competing against today's polluting rivals, even if oil prices stay at record highs.

Proponents of the technology still insist that 5 million to 10 million cars running on hydrogen-fed fuel cells and emitting only water vapour could be on the road within 15 years, with the number ballooning to 350 million by 2050.

But the dream will only come true if the companies driving the change can make fuel cells better and cheaper, while securing the billions of dollars needed to finance comprehensive hydrogen infrastructure at a time of stretched public coffers.