Environmental Engineering Reference
In-Depth Information
Figure 9.6 Possible single-pass low-pressure hydrogen liquefaction cycle, proposed [116] by Gas
Equipment Engineering Corp. In this diagram, the resistor-like symbols are heat exchangers, and the
cooling occurs as the He
Ne gas mixture, initially pressurized, is allowed to expand.
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transported on a larger scale by ship and railroad tank car. On a large scale, hydrogen
transport as a liquid is economic for some applications.
On the smaller scale, to replace the gas tank of an automobile, liquid storagemay be
a less viable option, since the fuel would disappear over relatively short storage times.
At present, the most common hydrogen storage is in a high-pressure tank.
9.7
Surface Adsorption for Storing Hydrogen in High Density
Alternative possibilities for dense hydrogen storage are by reaction of hydrogen to
form chemicals such as metal hydrides and by temperature-dependent adsorption of
hydrogen onto lightweight high-area substrates, which may include decorated
graphitic surfaces. Hydride storage tanks have been tested in connection with autos
and trucks. A
figure of merit of such a storage medium is the weight density of
hydrogen that can be achieved. Apractical benchmark of 6.5% by weight of hydrogen
has been chosen by the U.S. Department of Energy.
A large surface area on which hydrogen adsorbs with a small binding energy is
suggested in the cover image (lower left) of this topic. The concept shown is a
nanoporous structure made purely of carbon, using graphene planes and carbon
nanotube pillars. Such a structure is not manufacturable at present, but illustrates
properties that are desired. The weak adsorption energy allows hydrogen to be
desorbed by a small increase in the temperature, and the surface density is clearly
high in such an imagined structure.
A simpler storage model based on graphene layers is shown in Figure 9.7. The
modeling associated with Figure 9.7 suggests that graphene layers optimally spaced
at 0.8 nm may, with some uncertainty, accommodate hydrogen at the 6.5 wt % level
