Established Methods Based on Compression and Cryogenics - Hydrogen Generation, Storage, and Utilization - page 75

At 1 atm and 298.15 K (25°C), the volume occupied by 1 mole of H 2 is
V M = 24.46 L, and the volumetric energy density of H 2 is

∆ H
V M

ρ V = = 9 89 . MJ m ⋅

− 3

. (5.3)

Similar definition can be used for other fuels, such as methane, propane,
and gasoline. In fact, hydrogen has the highest mass energy density among
all the chemical fuels, but almost the lowest volumetric energy density beside
wood. Taken gasoline for example, its mass energy density is 45.7 MJ·kg −1
and volumetric energy density is 34,600 MJ·m −3 . Although gasoline has
smaller mass energy density, it has the highest volumetric energy density that
makes it really useful. For practical application, for example, it takes 10 gal
of gasoline for a light-duty vehicle to drive around 300 mi. If hydrogen is
used, one needs to burn a tank of 3495 gal of hydrogen to drive similar
distance. It is practically impossible to use such a big volume of H 2 to drive
a commercial vehicle. Therefore, one critical issue in using hydrogen is to
find new methods to improve the volumetric energy density of hydrogen
while keeping the mass energy density high, that is, to compress the large
volumes of the hydrogen gas.
One of the easiest ways to increase the volumetric energy density is to
compress the hydrogen or to liquefy hydrogen at low temperature. Liquid
hydrogen has a mass density of 70.8 kg·m −3 (at −253°C). This gives a volu-
metric energy density of 8.495 × 10 3 MJ·m −3 , which is about 860 times
higher than that of hydrogen gas at ambient conditions. This storage method
is based on changing the physical state of hydrogen, and usually requires
extra accessories, such as robust containers, valves, regulators, piping,
mounting brackets, insulation, added cooling capacity, and thermal manage-
ment components.
An alternative way to store hydrogen is to use hydrogen storage materials.
For some solid materials, the interaction of hydrogen atoms with the atoms
in the materials may be much stronger than the hydrogen-hydrogen interac-
tion. Therefore, hydrogen atoms could bind more closely together inside the
solid structures, generating much higher hydrogen density under ambient
conditions, or hydrogen could chemically react with the solid material to
form hydrides with relatively weak hydrogen bonds or metastable states.
Those materials could significantly improve the volumetric energy density
of hydrogen.
For hydrogen storage system, the mass of the energy storage system not
only includes the mass of hydrogen m H , but also should account for mass of