Although hydrogen has one of the highest energy densities on a mass basis, it has one of the lowest energy densities on a volumetric basis. Consequently, storing large amounts of hydrogen is not so direct as putting it in an unpressurized tank. For example, a ten gallon tank of hydrogen at ambient pressure and temperature holds only as much energy as 2 teaspoons of gasoline.

Hydrogen can be compressed, liquefied, or used as a feedstock to boost its energy density to levels useful for storing significant quantities. Commercially available fuel cell electric vehicles use hydrogen compressed to 700 times atmospheric pressure (700 bar or about 10,000 psi). It takes about 30 gallons of hydrogen at 700 bar (5 kg) to drive a car about 300 miles.

The amount of energy used to compress hydrogen to these high pressures can be from 5-17% of the energy stored in the hydrogen itself depending on the pressure. Liquefying hydrogen can take 30% of the energy in the hydrogen.

Converting hydrogen to methane makes it completely interchangeable with natural gas for transportation and storage. Hydrogen and carbon dioxide combine to form methane in a well-developed “methanation” process. There is a 20% energy loss (as potentially useful waste heat) in that process.

Hydrogen can be used as a feedstock for other carrier chemicals such as ammonia, formic acid, methyl-cyclohexane, et al. These each have their own energy losses, generally in excess of the ones described above.

Volumetric energy density of compressed and liquid hydrogen compared with other common fuels. (Hydrogen Storage Fact Sheet, US DOE Fuel Cell Technologies Office, March 2017)