"Tim Streater" wrote in message
.. .
... but a much poorer one when measured as MJ/litre. Which is the
important measure when filling a tank of a given size. Diesel is five
times better than liquid hydrogen by that measure.

True, but a gas can be compressed whereas a liquid can't, so a 50-litre
tank can contain 50 litres of petrol/diesel (very roughly 40 kg) but it
can contain far more than 50 litres of hydrogen because that can be
compressed to several atmospheres of pressure (limited by strength of
tank!) so you may be able to fit a greater weight of hydrogen in than of
diesel. I'm not sure what the typical tank pressure of hydrogen and LPG
is.

I'm not talking about compressed gaseous hydrogen. I'm talking about
liquid hydrogen, which, according to the gravimetric vs. volumetric
density chart in the link you gave above, shows liquid hydrogen
(9MJ/litre) a smidge better than hydrogen at 700 bar (5MJ/litre).
Compares poorly with diesel at 38MJ/litre. Good luck compressing liquid
hydrogen, or compressing gaseous hydrogen to a greater volumetric
energy density than liquid hydrogen.

Ah, right. I hadn't realised that liquid hydrogen (the ultimate compression
of gaseous hydrogen - until it liquifies) is still only 9 MJ/litre. Agreed -
liquid petrol and diesel are much greater energy density per litre than
this.

I wonder how thick the walls of a tank need be to contain a gas at 700 bar.
And I bet they need to be a lot thicker with a vaguely rectangular tank
(probably a very elongated egg shape) than a perfectly spherical one which
is harder to find room for in a car.

Is combustion of hydrogen in an internal combustion engine more or less
efficient than using the H2 in a fuel cell to generate power for electric
motors?