Jeremy Bentham, head of Shell Hydrogen in the Netherlands, said that at present, about 50 million tons of hydrogen was consumed worldwide per annum.

A key advantage of hydrogen energy could be diversification from existing energy sources like oil and coal as well as a decrease in urban air emissions, he added.

The long-term benefits of hydrogen would be seen in improved energy supply and more environmentally friendly energy, Head of Hydrogen for Norway's Norsk Hydro Ivar Hexeberg said.

Hydrogen offered diversification as it could be produced from a number of sources such as natural gas, coal, biomass, renewable sources and nuclear generation, International Energy Agency executive director for France Claude Mandil said.

Hydrogen could also reduce emissions of carbon dioxide, if produced from renewable, nuclear, gas and coal sources, he added.

Hydrogen was also a carbon free energy source - it could provide a way of alleviating the build up of carbon dioxide, which would be the principle challenge for many generations to come, Mandil said.

A couple of years ago experts in the hydrogen energy sphere said that by 2005 there would be mass availability of hydrogen vehicles, but at present only hundreds of hydrogen prototype vehicles existed, Bentham said.

"It is very early days in the use of hydrogen energy," said Hexeberg.

"It could take decades before hydrogen became a major energy carrier," he added.

However, if a major technology breakthrough or events, like wars or natural catastrophes, triggered greater focus on hydrogen energy, the development of hydrogen technology could come a lot quicker, he added.

The key challenge related to hydrogen energy was in reducing the cost as well as increasing the storage capability and durability of hydrogen as an energy source, he added.

The most critical need was to drive down the cost of fuel cells, which could possibly be done by mass production of hydrogen fuel cells.

At present, hydrogen energy was not at all cost competitive relative to electricity and other existing energy sources, speakers said.

There was a need for public private partnerships between the energy industry and governments to address the challenges related to hydrogen, Hexeberg said