Environmental Engineering Reference
In-Depth Information
containers or storage materials, m s . Thus, the effective mass energy density
ρ M of a hydrogen storage system can be expressed as,
H
m
M
H
M
m
m
H
H
H
0 .
ρ
=
×
=
×
=
ρ
(5.4)
M
M
m
+
m
m
+
m
m
+
m
H
s
H
s
H
s
Since ρ 0
1
. MJ kg is a constant, the effective mass energy
density is proportional to the hydrogen's mass percentage
=
119 716
m
H
( % ,
m
+
m
H
s
in the storage system or materials, which means for a hydrogen storage
system or materials, the mass energy density is only a fraction of that in pure
hydrogen. Therefore, one can use the hydrogen mass percentage to describe
the gravimetric capacity of hydrogen storage systems. Similarly, the effective
volumetric energy density ρ V of a hydrogen storage system or material can
be expressed as,
H
V
m
M
m
V
H
0
H
ρ
=
×
=
ρ
,
(5.5)
V
M
where V is the total volume of the storage system or materials. Thus, one
can use the effective hydrogen density
m
V
H
(
)
g L
−1
,
to describe the volumetric capacity of hydrogen storage systems.
For onboard hydrogen applications, a vehicle needs to carry 5-13  kg
hydrogen in order to make a drive distance great than 300 mi. With the con-
finement of vehicle design, this imposes many requirements for hydrogen
storage systems. U.S. Department of Energy (DOE) has set several targets
for onboard hydrogen storage systems for light-duty vehicles [1]. Its ultimate
target is 7.5% gravimetric capacity and 70  g·L −1 volumetric capacity. The
target for year 2017 is 5.5% and 40 g·L −1 gravimetric and volumetric capaci-
ties. Figure 5.1 shows a summary of the different hydrogen storage systems
in use or under development in terms of their gravimetric and volumetric
capacities as compared with DOE's targets. Clearly, the physical means to
store hydrogen, for example, the compressed hydrogen and liquid hydrogen
systems, are very close to the DOE targets.
 
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