The ideal ultra-green hydrogen cycle for autos would be something like this I think:
A desalinization/demineralizing plant on the coast pumps distilled quality fresh water to a solar water splitting plant in a location with lots of annual sunshine. The minerals extracted from the sea water are then sent back into the sea, preferably in an area with good sea currents so there is good mixing and the minerals do not concentrate.
The solar powered water splitting plants then produce the hydrogen for distribution, and either release the oxygen into the atmosphere, or sell it for medical or industrial uses. The existing energy companies (who may own and run the water splitting plants) provide the infrastructure to store and distribute the hydrogen, since they know how to build and operate such an infrastructure from their hydro-carbon experience.
Autos, buses, trucks, and trains use something like the high efficiency Ballard fuel cells to generate electric power for use in mechanical locomotion. Oxygen taken from the atmosphere combines with the hydrogen in the fuel cell producing distilled steam-water as the only chemical by-product. The steam-water is then cool-condensed and collected in a storage tank within the vehicle for periodic dumping at designated dumping stations.
Why is this scenario ultra-green?
Water supply: Inexhaustable
Solar energy: Inexhaustable
Ultimate energy source for the complete hydrogen cycle: The Sun
Net effect on the oceans: zero
This is because the fresh water produced in the fuel cells eventually will end up back in the earth's water cycle and back into the oceans. There will be no mineral buildup in the oceans since the fresh water will eventually return there.
Net effect on cities: zero plus
The steam-water from the fuel cells will have to be collected and dumped since a large city with large urban traffic would become a sauna without this. The water collection and dumping would also provide an alternative source of fresh water which might be handy in desert communities.
Net effect on the oxygen content of the atmosphere: zero
Oxygen released (or sold) from the water splitting plants when eventually released into the atmosphere, would balance the oxygen used in the fuel cells.
Net thermal effect on the earth's environment: zero
The solar heat that would have been released by sun warming of the ground (but is used instead in the water splitting), will eventually be released in the fuel cells, steam-water and electrical locomotion, since all the energy released will eventually end up as heat.
Net chemical effect on the atmosphere: zero
No chemical pollutants would be released into the atmosphere from this process.
The issues with airplanes have to do with the weight density of hydrogen and the strength of the pressure vessels required to store it.
I understand that while hydrogen has about twice the BTU's available per pound as either gasoline or Jet fuel, it's also much lighter in liquid form and thus requires more volume to store for a given weight. Also hydrogen has to be stored at a high pressure to remain liquid, especially at altitude. This requires thick walled fuel tanks which would add weight to an aircraft. Obviously a fuel cell would never work in anything other than a very light aircraft, so a hydrogen fueled turbine would have to used.
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Safe flying to you...
[This message has been edited by Flight Safety (edited 12 January 2001).]