Robert Clark wrote:

G. R. L. Cowan wrote:
...
Hydrogen is one-ninth the mass of the water it is in ...
but if you want to bring water to somewhere it isn't,
9 kg of it in a 0.5-kg tank beats
1 kg of liquid hydrogen in a 15-to-40-kg tank,
even if oxygen is free at the destination.

(Very big liquid hydrogen tanks able to contain
tens or hundreds of tonnes of it can have more favorable
containment-to-payload mass ratios. 15.3 is the lowest I've
heard of at car scale, however.)



The hydrogen won't be in liquid form otherwise I would have no problem
getting the water to liquify. Perhaps mildly pressurized, 4 bar.


4 bar?! That makes the choice (a) carry 9 kg of ready-made water
in a 0.5-kg tank or (b) carry 1 kg of hydrogen in, like, a 1.8-m-dia
spherical tank, maybe 160 kg if walled with 2-mm steel.

But I suppose if you never let the pressure get down near 1 bar,
it can have a tension wall, and not be rigid. Not quite so heavy.


--- G. R. L. Cowan, former hydrogen fan
Boron: internal combustion, nuclear cachet:
http://www.eagle.ca/~gcowan/Paper_for_11th_CHC.html