Environmental Engineering Reference
In-Depth Information
almost completely (up to 90%) the efficiency of the Carnot cycle, which represents
the reference cycle for the cooling process. However, the practical feasibility of an
integrated plant based on this method is far to be experimentally verified.
2.3.3 Hydrogen Storage in Solid Materials
Hydrogen storage devices based on high pressure gas and cryogenic liquid tech-
nologies do not reach the storage performance of conventional transportation
liquid fuels. On the other hand, they suffer also of important safety issues related
to the utilization of severe operative conditions necessary to improve, even if only
partially, the very low gravimetric and volumetric energy densities typical of
hydrogen molecules. An interesting alternative is the storage of hydrogen based on
the principle of forming hydrogen containing carriers from which the fuel can be
extracted when necessary. Then the idea is to develop materials able to soak up
and release hydrogen, thank to absorption and/or adsorption processes [
116
].
Metal hydrides are 'hydrogen atom'-carriers usable as storage system [
107
,
110
,
116
,
117
]. The method is based on specific properties of some metals that
readily absorb gaseous hydrogen, thanks to their capability to accept hydrogen
atoms, derived by its molecule splitting, inside their interstitial sites in reasonable
operative conditions.
The absorption process is a reaction of hydrogen gas with a metal, and results
moderately exothermic. Then during the charging phase, which occurs typically at
30-60 bar, some heat needs to be removed, while during discharging phase slight
changes in operative conditions (temperature and pressure) permit the quantitative
amounts of gaseous hydrogen molecules previously charged to be released,
according to:
M
þ
x
=
2H
2
MH
x
ð
2
:
29
Þ
where M indicates the metal used for hydrogen storage.
A schematic hydrogen absorption mechanism consists in the following three
steps:
1. mass transport of gas molecules to the solid surface
2. physical adsorption with successive molecule dissociation
3. transition of hydrogen atoms to the metal bulk with formation of new solid
phases.
During the third step, at a specified temperature value, the metals bind hydrogen
(previously dissociated as atoms in the adsorption step) producing an intermediate
solid solution [
107
,
118
]. Increasing the pressure the interaction between hydrogen
atoms and metal framework sites becomes stronger, nucleation and growth steps
start bringing to the formation of a hydride solution. The concentration limits
relative to the formation of the two above solutions (intermediate and hydride)
