Describing hydrogen as "dense" is a bit fraught. It is very dense on a joules per kilogram basis, but it is sparse on a kilograms per liter or joules per liter

Describing hydrogen as "dense" is a bit fraught. It is very dense on a joules per kilogram basis, but it is sparse on a kilograms per liter or joules per liter basis. And the design constraint in cars is the volume of the tank, not the weight of hydrogen it carries. From wikipedia, compressed hydrogen (690 bar at 15°C) has 3x the specific energy (MJ / kg) but 1/8 the energy density (MJ / L) versus gasoline. http://en.wikipedia.org/wiki/Energy_density http://en.wikipedia.org/wiki/Energy_density