Environmental Engineering Reference
In-Depth Information
Fig. 2.9 H
2
liquefier block
diagram (Linde cycle)
H
2
Compressor
N
2
out
1. Heat
exchanger
1
1
2. Flash
evaporator
LN
2
2
1
Joule - Thomson
valve
2
LH
2
Furthermore, the boil-off effects must be considered because of the liquefaction
temperatures (21 K). A very low external thermal exchange fluctuation could
produce appreciable vaporization. The font of these variations could be originated
by the exothermic transformation of electronic configuration of hydrogen molecule
from ortho to para [
107
], by the thermal irreversibility of fluid compression, or by
the different heat exchange mechanisms with outside. This problem is emphasized
for tanks of small size (100 l, typical value for cars), because heat leaks are
proportional to the surface to volume ratio, and the high initial density could
significantly decrease during the time.
The materials used in the different size vessels are based on stainless steel or
aluminium alloys, but also on polymeric materials, such as Teflon. The sphere
form is ideal for the minimization of vapour release, due to its minimum surface to
volume ratio with respect to all other possible geometries, and for the limitation of
stress and uniformity of strain distribution. On the other hand, the manufacturing
costs for spheres are too high, in compared with cylindrical shape which is the
most economical solution.
Recently, a new liquefaction approach based on the magnetocaloric effect has
been proposed [
115
]. The method is based on the characteristics of some magnetic
materials that heat up when placed in a magnetic field and cool down when
removed from it. The magnetic refrigeration can develop its potentialities in terms
of overall efficiency for liquefaction process, as it is theoretically able to exploit
