On 20/04/2019 17:58, NY wrote:
"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.

Propane in a closed vessel is at 6.4 barg at 16°C. It is actually a
liquid in the tank, with a layer of gas over it. As you take gas out,
the pressure drops and the liquid boils, restoring the pressure, until
there is no liquid left.

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

The pressure is determined by the vapour pressure of the gas. The bulk
of it is liquid in the tank. Higher pressures would add little mass, as
the bulk is the incompressible liquid.

SteveW