"Tim Streater" wrote in message
.. .
Hydrogen has very high energy density -
https://www.energy.gov/eere/fuelcells/hydrogen-storage says about 120
MJ/kg, compared with about 45 MJ/kg for petrol or diesel ...

... 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.

As a very rough estimate, propane cylinders have a nominal weight of 47 kg
of gas. They are about 1.2 m high and have a diameter of about 30 cm, so the
volume is about 1.2 * pi * 0.15^2 = 0.085 m^3 (85 litres). Propane's density
is 490 kg/m^3 so at atmospheric pressure you'd get 493*0.085 = 42 kg.
Interesting - so propane cylinders are at only slightly above atmospheric
pressure (1 bar). I'd expected it (at a very rough guess) to be something
like 5 bar to get a reasonable flow rate to *fight* against atmospheric
pressure of air as the gas comes out of the burner.

Maybe LPG in cars is dispensed and stored in the car's tank at a higher
pressure than propane for central heating.