> isn't very dense, about 7% of that of water That's not a good metric. Hydrogen has about 2-3x energy density per mass vs carbon fuels. H₂ is about 120MJ/kg,

> isn't very dense, about 7% of that of water That's not a good metric. Hydrogen has about 2-3x energy density per mass vs carbon fuels. H₂ is about 120MJ/kg, while carbon fuels are about 35-55 MJ/kg, gasoline/diesel is 45 MJ/kg. Per volume, gasoline - 34 MJ/l, liquid methane 22 MJ/l, liquid hydrogen 8.5 MJ/l. Much smaller, but not laughable 7% (14x).