Competitors in the men's and women's marathons at the 2000 Sydney Olympics had an exciting glimpse of the future. The pace vehicle that led them round the 42-kilometre circuit looked like a typical family wagon, but looks were deceptive. Under the bonnet was a stack of fuel cells, not an internal combustion engine. And as the car glided silently forward it emitted no smelly fumes or greenhouse gases — just a little water vapour.

The car was powered by hydrogen, the simplest and most abundant of all chemical elements. The fuel cells under the bonnet converted the hydrogen directly into electricity.

Many experts think hydrogen will replace petrol, diesel and natural gas as the main fuel for cars, buses and trucks over the next few decades. Already car manufacturers around the world have invested billions of dollars in research and development.

The advantages of hydrogen are enormous: no more smog-forming exhaust gases, no more carbon dioxide emissions that contribute to global warming, no more worries about diminishing oil supplies and rising prices.

But some tricky questions need to be answered before mass-produced hydrogen cars start appearing on the streets: 

* Where will the hydrogen come from? 

* How will motorists fill up?

* How will cars store the fuel?

The choice — combustion or fuel cells?

Two kinds of engines can use hydrogen as a fuel — those that have an internal combustion engine converted to use hydrogen and those that are made up of a stack of fuel cells.

Internal combustion engines

Internal combustion engines have powered cars since they first began to replace horse-drawn carriages more than 100 years ago. These engines can be converted to run on a variety of fuels, including hydrogen. When hydrogen burns, the only by-product is water — not the polluting cocktail given off by burning petrol and other fossil fuels.

BMW successfully demonstrated this technology in a fleet of 15 sedans used to ferry people to and from EXPO 2000, the world fair in Hanover, Germany. The fact that no major changes need to be made to the basic internal combustion engine design is a major attraction.

Fuel cell engines

Most car makers think that fuel cells powering an electric motor offer a better alternative. Electric cars are hardly a new idea, but the need to recharge heavy stacks of batteries after relatively short journeys has stopped them becoming popular. Now fuel cells have made electric cars practical.

Unlike batteries, which store electricity, fuel cells make electricity as they go. Recent developments in technology have greatly increased the amount of power that a stack of cells — small enough to fit under a car's bonnet — can provide. This has opened up the prospect of non-polluting electric cars with the levels of performance we expect from conventional vehicles.

Fuel cell technology sounds simple. The hydrogen fuel reacts with oxygen from the air to produce water and electricity — the reverse of the familiar electrolysis process that releases oxygen and hydrogen from water. In reality, however, it's much more complicated.

The big advantage of a fuel cell engine over an internal combustion engine running on hydrogen is its greater efficiency. The same amount of hydrogen will take a fuel cell car at least twice as far as one with a converted internal combustion engine.

Fill 'em up please

Hydrogen has many advantages as a fuel for vehicles, but a big disadvantage is that it is difficult to store. This is because at normal temperatures hydrogen is a gas. The hydrogen must be packed tightly into a car's tank, otherwise a filling stop will be needed every few kilometres.

The obvious solution is to strongly compress the hydrogen, or liquefy it. However, large amounts of energy are needed for this — an estimated 20-40 per cent of the energy content of the fuel. Also, tanks designed to hold hydrogen at extremely high pressures, or at temperatures approaching absolute zero, are heavy and expensive.

A futuristic filling station kept EXPO 2000's fleet of converted BMWs running. Drivers pulled up at the pump, pressed a button on their dashboard, and watched from inside the car as a laser-guided robotic arm connected the store of liquid hydrogen to their tank. Filling took about 3 minutes. It was wise to keep well out of the way — at minus 253℃, liquid hydrogen is unimaginably cold.

The special insulated tanks in the BMWs held 140 litres of hydrogen, enough to drive at least 300 kilometres. (That's a reasonable range, although a 95 litre tank of petrol would take the same cars twice as far.) The hydrogen-powered marathon car at the Sydney Olympics also ran on liquid hydrogen. Its much smaller tank (75 litres) gave it a range of about 400 kilometres, a sign of the greater efficiency of fuel cell cars.

High cost and the large amount of energy needed to liquefy the fuel are likely to be the main problems with refuelling with liquid hydrogen. Filling up with compressed hydrogen gas will probably prove more practical, even though it may reduce the distance between fills. Cars could store the hydrogen in high pressure tanks similar to those used for compressed natural gas. Or, if current research proves successful, some high-tech alternatives could be employed.

Scientists have found that various metals can absorb up to a thousand times their own volume of hydrogen gas. Specially treated carbon may also hold large amounts. These discoveries could shape the fuel tanks of the future.

But where will the hydrogen come from?

There's no risk that we'll ever run out of hydrogen, it's by far the most plentiful element in the universe. On Earth, however, it exists naturally only in chemical compounds, not as hydrogen gas. Water and the main components of coal, oil and natural gas are prime examples of these compounds.

Natural gas currently provides most of the hydrogen used in industry. The relatively simple technology employed — steam reforming — could also produce hydrogen gas for cars at central plants or filling stations. Alternatively fuel tanks could be filled with petrol or methanol, with the cars using on-board 'reformers' to generate hydrogen for their fuel cells. This serves as a transitional measure while research proceeds on the problems of storing hydrogen.

In steam reforming the hydrocarbon fuel reacts with water at high temperatures to produce hydrogen gas. A major drawback is that carbon dioxide and smog-causing gases such as nitrogen oxides are given off too, although emissions per kilometre of car travel would be less than from petrol-burning vehicles.

An alternative approach now under development, autoreforming, should increase the attractiveness of on-board hydrogen production. Use of a catalyst will allow the reforming to occur at much lower temperatures — too low for the production of nitrogen oxides.

Water is the only potentially pollution-free source of hydrogen. Researchers are looking at new ways of producing hydrogen — using algae, bacteria or photovoltaic cells to absorb sunlight and split water into hydrogen and oxygen. But the technology most likely to be adopted on a large scale is electrolysis, which uses an electric current to split water into oxygen and hydrogen.

Is it safe?

"Remember the Hindenburg" — that's a phrase often heard when hydrogen is discussed. This German passenger airship, kept aloft by hydrogen, crashed in flames as it came in to land at Lake-hurst, New Jersey, USA in May 1937. Thirty-five people died. Nowadays helium, which can't burn, is the gas of choice for lighter-than-air craft.

Hydrogen is highly flammable, but recent research has indicated that the airship's fabric, not hydrogen, was the culprit in the Hindenburg disaster. Properly handled, there's no reason to think hydrogen is any more dangerous as a fuel than petrol, the explosive liquid now carried safely in the tanks of untold millions of motor vehicles.

Looking forward

Recent technological advances, particularly in fuel cell design, have made hydrogen-powered cars a practical proposition, and car makers expect to start mass-producing them within the next decade or so. Their power and acceleration should match those of today's petrol-powered vehicles, but they may have to be refuelled more often.

The best ways to produce, distribute and store the hydrogen still have to be sorted out. In the short term fossil fuels may remain in demand as a hydrogen source. However, the idea that in the not too distant future most of us will be driving non-polluting cars fuelled by hydrogen from a clean, renewable source is no longer a flight of fantasy.

— www.sciam.com