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Published online 13 August 2008 |
Nature
454,
810-811
(2008)
| doi:10.1038/454810a
Updated online: 20 August 2008
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Four wheels good?
With the world's love of cars showing little sign of abating, manufacturers are under increasing pressure to make vehicles less polluting and oil dependent. Duncan Graham-Rowe explores some of the technologies that could keep us on the road.
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The world will just have to get over its love of cars. It will never be possible to make a car "energy efficient" as long as the vehicle weights several times more than the passengers who occupy it. Passenger cars are an extravagant use of energy that will not have a place in our energy-deprived future. On your bikes everyone! Michael Lardelli Adelaide, Australia
Although hydrogen's normal boiling point is indeed a little below -250 Celsius, it could be stored as a slightly less dense liquid at temperatures up to its critical 'T', up to, if I recall correctly, 32.976 K, minus 240.174 K. Its storage as 20-K liquid on board several independently built prototype cars dating back to the mid-1970s is a fact, so Graham-Rowe's discussion of such tanks as if they were entirely hypothetical is annoying. Also, no on-board mechanism cools them, rather, the cold liquid hydrogen cools them at refuelling time and they then allow the outside world to warm up their fuel charge at a rate that is reasonably low. So for instance the BMW car's hydrogen tank increases in pressure for a few days if the car is not used, and then the relief valve starts letting hydrogen go to a fuel cell that safely oxidizes it. Very seldom would any hydrogen be wasted in this way in a car that was being used with normal frequency.
Michael, I agree with you. Unfortunately, by the same criteria, we might have to rule out most, if not all, forms of public transport. Underutilized trains and buses are just as environment-unfriendly as passenger cars.
"At the cathode hydrogen is oxidized forming water?" No, molecular hydrogen, H2 was oxidized at the anode when its electrons were stripped and it fell apart to 2H(+) + 2e.
Too many people have been dazzled with the energy density of hydrogen in weight/energy while ignoring the weight and bulk of the container A fuel cell using sacrificial aluminum anodes doesn't need cryogenics, doesn't need high pressure and doesn't leak greenhouse gasses. One would have to off-load Al2O3 for resmelting when one stopped to take on a new aluminum anode.