you can't at scale.
using current tech you could probably electrolyse enough water annually to cover about 1% of the private vehicles in London, I think.

And it uses more energy to electrolyse water than you get by burning hydrogen and it always will do, that's not a problem that can be "solved" because it's a fundamental thermodynamic principle. So, it's not free anything. It's a higher cost both financially and in energy terms, it's just a load of smoke and mirrors to make it appear not to be.
( the mighty badger Aphrodite, on a bar stool, by your side, Thu 18 Oct 2012, 14:05, 1 reply, 13 years ago)

That is rendered nearly moot if the energy conversion has been done already.
Those fossil fuels required energy input to get to their current state, it just took many millions of years.

If we sweep up solar deuterium from the surface of an asteroid, the sun has already done the hard bit for us.

In theory of course.
(Bicycle Repairman "you're also a bit of a wanker", Thu 18 Oct 2012, 14:22, Reply)

that might well be fine for a fusion reactor
it's not gonna power a hydrogen car though, which is what we're on about here.

Although, I'm prepared to take at least a small punt that the energy required to get a harvesting vehicle clear of the earth's gravitational pull might be more than you could ever recover from the deuterium it could collect.
( the mighty badger Aphrodite, on a bar stool, by your side, Thu 18 Oct 2012, 14:25, Reply)

Well, we already send vehicles with electrolyticly separated hydrogen and oxygen to the moon
Which is a far better use of it than simply cracking it to power millions of cars, wasting gigawatts of energy in the process merely as a sop to the big oil companies who are investing in hydrogen storage technology.
(Bicycle Repairman "you're also a bit of a wanker", Thu 18 Oct 2012, 14:39, Reply)