It sounds almost too good to be true, but cars which run on hydrogen fuel cells emit only water vapor into the atmosphere and in contrast to oil, hydrogen is not a fossil fuel with only limited reserves. In fact it is the most common element on the planet.

Car manufacturers are seemingly uneasy about propagating this form of motive power. Research has been going on for decades and numerous breakthroughs have been announced. So far though, only a handful of prototypes and some limited series production vehicles have appeared.

Ford spokeswoman Monika Wagener in Cologne said that a volume fuel cell car is not practicable in the near future. This did not mean that the technology was a mere public relations gag.

"The fact that the large companies are investing in this technology in a big way shows that they mean business," Wagener said, adding that nearly all the carmakers are probing the fuel cell.

Two sides of the coin

A fuel cell car is basically an electric vehicle since the fuel cell stack is used to combine hydrogen with atmospheric oxygen. The chemical energy from the reaction is then converted into electric power which drives the vehicle.

Matthias Brock from Daimler in Stuttgart said he believes that the fuel cell is very promising when it comes to zero-emission driving in local areas. Mercedes has around 100 test vehicles underway -- the world's largest fleet of fuel cell vehicles. The company is currently working on a B-class with fuel cell power which could be produced in limited numbers from 2010.

"We estimate that between 2012 and 2015 the car could be ready for series production," Brock said.

Other manufacturers, such as Volkswagen in Wolfsburg, are more reserved about committing themselves to future fuel cell models.

"We expect fuel cell power to come on stream after 2020," said Hartmuth Hoffmann at Volkswagen in Wolfsburg.

Taming hydrogen does pose engineers with a complex set of problems, including how to produce it, how to transport the substance to customers and last but not least: "how can it best be stored on-board?"

According to Hoffman, "there are some basically very sound ideas out there but these are not yet suitable for volume production."

Filling up

Assuming fuel cell technology had matured sufficiently for everyday use, owners would be hard put to find somewhere to fill their vehicle tanks. Fuel outlets are a rarity. Both the German cities of Hamburg and Berlin are involved in a pilot project called "Clean Energy Partnership,, which is designed to probe the regular usage of hydrogen as a fuel yet, apart from this exemplary project, there are hardly any hydrogen fuelling facilities in the country.

The need for fuel outlets in close proximity is one of the reasons why field trials using fuel cell vehicles tend to take place in the United States where filling stations are more plentiful. Japanese maker Honda is gearing up to market 200 examples of the FCX Clarity model over the next three years and will offer them to commercial partners on a leasing basis.

General Motors (GM) intends to introduce onto US roads 100 fuel cell prototypes in a US-based project called "Driveway", GM technical spokesman Andrew Marshall confirmed. GM believes too that fuel cell cars are a long way from entering the showrooms.

"Having said that, the fuel cell offers the only truly zero-emission drive system around," Marshall said. "At the moment we cannot see an alternative."

Emission standards

Whether cars need to be completely emission-free is another issue. Andreas Ostermeier from the German federal environment ministry in Dessau said that the notion of zero-emissions is an irrelevance. Petrol and diesel engines no longer pose a threat to air quality.

Petrol engines which comply with the Euro-4 standard laid down by the European Union use the latest technology to reduce toxins to levels far below those of even a decade ago. The same will apply to diesels when Euro 6 becomes legally binding from 2014.

"Nowadays fuel consumption and a reduction in carbon dioxide emissions are at the top of the agenda," Ostermeier said.

Cost analysis

Hydrogen is complex to produce which puts the fuel at a further disadvantage as conventional fuels continue to get cleaner. It is also an expensive fuel source as one example shows. Hydrogen can be gained by using steam to reform natural gas.

"The trouble is that so much energy is used to drive the process that it is cheaper to pump the methane gas used straight into a gas-powered car," Ostermeier said.

The German expert does not share the car industry's optimism that one day it will be possible to produce sufficient hydrogen using renewable energy derived from wind, biodegradable refuse and hydro generation.

Ostermeier said it could take until 2030 before enough electricity from renewable sources becomes available. From an overall energy point of view it was probably better to rely until then on conventionally-powered cars, especially since the coming years are set to see the arrival of electric cars with far more efficient batteries and a wider range than those currently on the market.

Dirk Breuer from Toyota Germany in Cologne said he believes there is no single path to zero-emission vehicles.

"Many roads lead to this destination," he said.

In regions such as South America, bio-fuels could be the most viable alternative whereas in highly-populated urban regions in Western Europe the pure electric car offers the most potential for the future.