Environmental Engineering Reference
In-Depth Information
The US Department of Energy (DOE) has established a series of targets which
should be met by hydrogen storage tanks, including those related to overall system
performance [
110
]. Both volumetric and gravimetric storage system performances
are considered basically crucial; the relative target for the year 2010 is fixed in
1.5 kWh/l and 2 kWh/kg as regarding system volumetric and gravimetric capacity,
respectively. These targets imply for the hydrogen storing material volumetric
density values of at least 45 g/l and gravimetric density not lower than 6 wt%. The
following sections contain an overview on hydrogen storage in molecular form
(compressed gas and liquid) and some details about the main technological
improvements related to the storage by absorption or adsorption processes.
2.3.1 Hydrogen Storage in High Compressed Gas Form
The compression in cylinders with a maximum pressure of 200 bar is a reliable
well-established technology, today widely used for hydrogen and natural gas
storage. Higher pressures permit the volumetric capability to be improved but not
negligible power consumption has to be taken into account when high compression
ratios are reached.
The ideal compression (in isothermal conditions) work can be calculated
according to the following equation:
p
p
0
L
¼
DG
¼
RTZ ln
ð
2
:
28
Þ
where DG is the Gibbs free energy variation (corresponding to the ideal useful
work L), R is the gas constant, T the Kelvin temperature, Z the compressibility
factor and p/p
0
the ratio between the final and initial pressure value. Z factor is
approximately 1 in the range 1-100 bar, when the hydrogen behaviour can be
approximately assumed as ideal, while at high pressures the density of H
2
affects
its compressibility and Z significantly increases in dependence of critical param-
eters of temperature and pressure.
It can be derived that for p/p
0
= 700 an energy loss due to ideal compression
results about 2.2 kWh/kg, but in a real compression stage the energy losses are
significantly higher because of the irreversibilities that also determine non-iso-
thermal operation. Pressure increments could affect in a not negligible way the
impact of the hydrogen compression step on the well-to-wheel efficiency evalu-
ation for hydrogen fuel cell vehicles [
111
], the energy consumption at 700 bar is
about 10% of the H
2
LHV [
112
]. Additionally, the heat produced during the
compression stage gradually reduces the gas density, limiting the positive impact
of compression on energy density of the tank.
Dynamic compressors, commercially used for natural gas compression, could be
used in case of moderately high compression (p/p
0
up to 200-300), while alter-
native volumetric devices are necessary for higher pressure ratios. With respect to
