Also, the mass-specific lower heating value of hydrogen
is almost three times as high as that of gasoline or diesel.
However, hydrogen is gaseous at ambient conditions with
a density that is several orders of magnitude lower than
that of liquid fuels. Increasing the energy storage density is
a topic of continuing research, but outside of the scope of
this paper. Currently, used storage systems are either
pressurized (commonly used storage pressures are 20 and
35 MPa, with 70 MPa already demonstrated) or liquefied
(requiring cooling to 253 C). Other possibilities to store
hydrogen are metal hydrides or liquid organic hydrogen
carriers [10].