[myself6]

I don't think that is correct. A diesel sub can stay down a predetermined amount of time before refueling. Therefore the amount of space it has for fuel equals x amount of energy. A nuclear sub is about the same size as a diesel sub and I believe the fuel rods will keep it going for many years. If people didn't have to live on these things they would never have to stop. The amount of space for fuel on a nuclear sub equals infinite (relatively speaking) amount of energy to do work.

Obviously I was taking liberties with the language here but the concept is clear. 20 gallons of gas and a 20 gallon tank of compressed hydrogen do equal the same amount of energy.

[chaosagent]

Ah, but the hydrogen is compressible and the gasoline is not.

The amount of energy in 20 gallons of gasoline doesn't change.

The amount of energy in a 20 gallon tank of hydrogen depends how much pressure it's under.

It's best to talk about the mass (weight) of the two.

20 gallon of gasoline weighs about 125 pounds.

125 pounds of hydrogen (however much it's compressed) does not contain near the energy that 125 pounds of gasoline does.