The drawback is that hydrogen must first be produced, requiring a primary energy source, and this is where scientists see major obstacles.

When environmentally friendly wind electricity is used to generate hydrogen, only one quarter of the energy generated by the wind turbine is eventually used to move a car.

The rest is lost during transport and energy conversion, said Ulf Bossel of the European Fuel Cell Forum, which held its annual fuel cell conference in Lucerne, Switzerland this week.

"With hydrogen energy you only have 25 percent efficiency to turn wind power to (car) wheel power," he said. It's much more efficient to transport that electricity directly into a car battery, via the grid, and use 90 percent of its power," he told Reuters in a telephone interview.

Hydrogen is being discussed at the conference because it is one of the fuels for cells that can generate electricity and heat in an electrochemical conversion.

US President Bush said in April that hydrogen was the fuel of the future. In meetings with US House and Senate leaders in May, the big three US motor companies pushed for a scenario involving ethanol and hydrogen fuel cell cars.

Even oil and gas giant Royal Dutch Shell has started a project in Rotterdam to fuel city buses with hydrogen. Shell Chief Executive Jeroen van der Veer said recently such projects are important to raise awareness of alternative energy.

Bossel said most renewable energy will be harvested as electricity through wind and solar power and should be used directly.

BIODIESEL BETTER

But he accepted that today's economy is based on fuels and that cars will need some form of liquid fuel for long journeys, rather than have to recharge batteries every few hundred kilometres.

Even when this liquid energy is harvested from biomass, it makes sense to turn it into a biodiesel rather than hydrogen, said Wim van Swaaij, professor of thermo-chemical conversion from Twente University in the Netherlands.

Biofuels are easy to handle, like today's fuels. Hydrogen, in its pure form, needs to be stored under high pressure which also consumes energy. Biofuels themselves contain hydrogen but in a much more stable form.

"Through steam reforming technology we can turn 40 to 50 percent of the original fuel content in the biomass into biofuel. The percentage is even higher for hydrogen, 50 to 60 percent, but we will also have to store that hydrogen, and biofuels are the easiest and most efficient way to store hydrogen," Van Swaiij said.

The carbon particles in the biofuel will not make a net contribution to heating up the earth through the greenhouse effect if the fuel is harvested from biomass, because the plants consume carbon dioxide as they grow, Van Swaaij added.

Bossel also said that producing hydrogen, either through electrolysis using nuclear or renewable electricity, or refined from biomass or fossil fuels, requires massive amounts of water. One kilogramme of hydrogen requires nine litres of water.