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Carmakers tout green motors in Geneva

But where's the juice ultimately coming from?

The only other option for here-and-now technology is hydrogen, with the first hydrogen cars now appearing in limited numbers. Hydrogen's big attraction is that an engine burning it emits only harmless water vapour.

BMW's Hydrogen 7 demonstration vehicle debuted last November, and the company is leasing out a few hundred for consumer trials in a sort of motoring beta test. The Hydrogen Seven is essentially no more than an existing BMW 760 limo with a barrel-sized insulated cryogenic tank in the boot. The engine is a normal big-car job, which runs happily enough on hydrogen rather than petrol although – in the current state of the technology – it loses a little power when doing so. It can also use the car's ordinary petrol tank, which is just as well as there are only seven hydrogen filling stations in Europe.

There are some snags in view for aspiring Hydrogen Seven drivers. For one, the cryogenic fuel will all boil off after nine days whether it is used or not. This also means that the car cannot be left parked in an enclosed space – a garage, for example – without risking a Hindenburg-style explosion. For another, the car will only go a bit over a hundred miles on a tank of hydrogen.

Most big firms don't see this kind of vehicle as the way ahead, preferring to use hydrogen to generate electricity in a fuel cell and then drive the car's wheels with hybrid-style motor-generator units.

Honda's FCX, due to beta-test from 2008, will probably be the first example in normal drivers' hands. It's already used by a few organisations as a Honda marketing tool. The FCX, however, involves very serious performance compromises.

Unlike the Hydrogen Seven, which handles pretty much like a normal Beemer in hydrogen mode, the FCX tops out at 80 horsepower. This has enabled it to act as the official pace car for the Los Angeles Marathon, but not for any motorsport events.

At the moment, hydrogen is at least as big an environmental nightmare as fossil fuel, as its manufacture requires not only power but natural gas. The Hydrogen Seven is estimated to have as much environmental impact under current production methods as a heavy diesel truck. However, the motor firms are looking forward to some future day when hydrogen would be cracked from water using cleanly-generated electricity.

When pressed, car execs usually speculate that this electricity might be from solar, wind or wave plants. Even radical greens, however, don't really think that these methods could yield enough power for current domestic/industrial use, let alone colossal amounts more to produce clean fuel for road transport (sample quote from Greenpeace (pdf): "Renewable energy, combined with the smart use of energy, can deliver half of the world's energy needs by 2050"). This problem probably applies to ethanol production as well.

Few are saying so openly, but it appears that any truly zero-carbon car of the future would be ultimately dependent on nuclear power. ®

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