"Tim Streater" wrote in message
.. .
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.

It's all that carbon it contains. That's the rub.

Good point. That's why butane is more energy efficient than propane which is
more energy efficient than methane, in terms of the heat energy when burned
for a given amount (mass) of gas. That's why you only get bottles of butane
and propane (*), not methane. It would be more efficient in terms of the
volume of gas that needed to be supplied through the gas mains, to use
butane or propane - but nature has given us methane instead ;-)

(*) Although propane is a lower energy density, it does have the big
advantage that its boiling point is lower so it is less likely to turn to
liquid in cold weather - and you don't want liquid butane squirting out of
the gas jets if it hasn't all boiled off to gas by the time it reaches the
jet.