Environmental Engineering Reference
In-Depth Information
methane the hydrogen compression introduces some specific difficulties, related to
the different volumetric energy densities and to the choice of dedicated fixings
[
107
]. On the other hand, at higher pressures the gravimetric density of the tank
progressively decreases because of the increasing thickness of cylinder walls.
Austenitic stainless steel or aluminium alloys are widely used for 200 bar
cylinders but recent advances in lightweight composite cylinders have permitted
hydrogen storage systems reliable up to 700 bar to be developed. Composite
materials are constituted by high-strength fibres, mainly carbon-based that are
wrapped around the cylinders in layers. Several types of advanced high-pressure
cylinders could be designed to improve volumetric and gravimetric density in
dependence of the weight ratio between metal and composite: from a whole metal
to a plastic liner or a fully wound composite overwrap, passing by different per-
centages of metal liner and composite overwrap.
In order to match the safety issues, future pressure vessels include three layers:
an inner polymer liner, overwrapped with a high-strength and high-elasticity
carbon fiber composite, and an outer layer of an aramid-material capable of
withstanding mechanical and corrosion damage [
107
].
Recent approaches aimed at improving the unsatisfactory gravimetric and
volumetric capacities are based on compressed cryogenic technique [
110
].
In particular by cooling a tank to nitrogen liquefaction temperature (77 K) the
volumetric capacity results three times higher with respect to conventional high
pressure tank.
2.3.2 Hydrogen Storage in Liquid Cryogenic Form
The liquid cryogenic technology might represent a valid alternative to high-
pressure gas storage approach [
113
]. It can be used to increase significantly
(see Table
2.2
) the unsatisfactory volumetric density values of hydrogen stored as
gaseous compound. At a very high pressure (700 bar), the gravimetric density of
hydrogen in gaseous form does not reach 40 kg/m
3
, while as liquid it reaches about
70 kg/m
3
. However, some technical aspects need to be considered for an overall
analysis of this technology.
The Linde cycle is a simple cryogenic process based on Joule-Thompson
effect. It is composed of different steps: the gas is first compressed, then
preliminarily cooled in a heat exchanger using liquid nitrogen, finally it passes
through a lamination throttle valve to exploit the benefits of Joule-Thomson
expansion. Some liquid is produced, and the vapour is separated from the liquid
phase and returns back to the compressor through the heat exchanger. A simplified
scheme of the overall process is reported in Fig.
2.9
.
The process is rather expensive because of high electricity costs for compres-
sion and the low Joule-Thomson inversion temperature of hydrogen (203 K), that
involves high energy consumption necessary to maintain the hydrogen continu-
ously cooled (about 30% of its LHV) [
114
].
