Originally Posted by KevinBarr
No one challenges how much it costs in terms of energy to refine oil into petrol. I wonder if it is more than to extract hydrogen from H2O...?
In a watt-hour for watt-hour comparison, yes it is.
Go have a look at the laws of thermodynamics and their applicability to potential energy and chemical energy. H2O (water) is a very stable substance and to extract H2 from it (unstable) requires a large amount of energy. Theoretically, the energy you get from recombining hydrogen and oxygen to make water (very high) is exactly the same as the energy it costs to extract hydrogen and oxygen from water in the first place.
Crude oil is, relatively, a high chemical energy compound (low stability -- it will "burn", or recombine chemically into more stable / low energy compounds by itself given starter energy) and extracting petrol from it is just a distillation process, it doesn't require an electrolysis reaction. The energy return you get from burning petrol is higher, because you're not just taking it back to crude oil, you're burning it to form carbon dioxide, water, and other gases (including nasty oxides of nitrogen but that's beside the point).
So crude oil is already a source of energy, whereas water is not.
The advantage of using hydrogen, despite the energy return being lower, are many, however. You get a cleaner burn (hydrogen burns to form water only, no carbon dioxide or other nasty gases). Hydrogen is a high energy compound and transporting it is one very efficient way to transport energy over long distances. And, of course, that the source compound (seawater) won't ever run out.
So, for example, a high energy (nuclear powered, for example) power station that is not ideal to have near centres of population could be used to extract hydrogen from seawater, and then the hydrogen generated could be transported and used in households or vehicles, whatever. So you could build a big stinking nuclear power station on a deserted coastline somewhere (north western Australia comes to mind, although parts of northern Canada may be just as suitable), use it to extract large amounts of hydrogen from the sea, and pipe or transport that to hydrogen fuelled power stations nearer cities which would then produce energy without any local pollution.
It's debatable whether the cost/energy trade-off from such a solution would be better or worse than, say, the process of growing biofuels and using these to generate electricity. Probably biofuels would be the winner because the technology required is more readily available, less objectionable to the public (doesn't matter where you build your nuclear power station, someone's going to picket it), and the biofuels are more stable in transit. Essentially the biofuel process captures solar energy (sunlight used to make the plants grow) and converts this into a form we can use in an existing oil or diesel fuel power station.
However as the total fossil fuel reservoir available to the planet declines to nothing over the next century or two, both solutions are probably going to need to be examined if we're going to have enough energy to fuel our current and future needs.