As previously discussed, the volumetric energy density of gaseous hydrogen at room temperature is very small and falls short of the DOE targets. One strategy to increase its volumetric energy density is to compress H2 inside hollow glass microspheres or microcapsules in general.
Loading and unloading of hydrogen gas in and out is based on the fact that gas permeation through the solid shell is a thermally activated process, that is, gas permeation increases exponentially with temperature. Hollow glass microspheres are also called microcapsules, microcavities, microbubbles, or microballoons.
Practically, hydrogen loading in hollow microspheres can be performed at industrial scale in a batch process off vehicle in an autoclave at high temperatures (≈400°C) and pressures (>450 bar) to accelerate hydrogen permeation through the container’s shell. Then, the hollow glass microspheres are cooled to room temperature so that hydrogen gas remains trapped inside due to significant reduction in the H2 permeation with decreasing temperature.
The hollow glass microspheres, then, can be safely transported to distribution points at room temperature and atmospheric pressure. Hydrogen refueling of a vehicle would consist of sucking the spent microspheres out of the tank and pouring loaded ones in the tank in a manner very similar to current gasoline or diesel refueling. On-demand hydrogen release from the microspheres would be induced by an onboard electric heater or a heat lamp integrated in the tank. Alternatively, unloading could also be achieved by mechanically or thermally destroying the microspheres, thus releasing their H2 content.