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Introduction

Conclusions

Cryogenics

At a temperature of about -260ºC, hydrogen becomes a condensed liquid with a density of 70.8kg/m3. The energy density increases correspondingly to 8.4MJ/L. An even greater increase in energy density can be obtained by creating “slush” hydrogen where the solid and liquid forms are in equilibrium.

There are two major drawbacks to using cryogenic hydrogen. One is the energy necessary to bring the hydrogen to such a cold temperature to begin with. The other is insulation. Any outside heat reaching the tank will cause almost immediate boil-off of the hydrogen. Therefore extremely good insulation is necessary to prevent dangerous boil-off, which can cause explosions if there is no method for alleviating the pressure.

Some insulation materials that are used in cryogenic hydrogen storage are polyurethane and polyvinylchloride foams, glass fibers, aluminum foil, glass paper laminate, and silica powder. The best forms of insulation are also evacuated, meaning that there is no air present in the insulation layer to transmit heat by conduction. The reflectivity of the glass and aluminum foil also prevents heat absorption. These insulations can have thermal conductivities as low as 10-5 W/mK.

However, even with the best insulation, there will always be heat absorption into the tank and boil-off of hydrogen. Therefore, a system using cryogenic hydrogen must have a constant low-level usage of fuel, otherwise material is lost or dangerous pressure build-ups occur. This makes cryogenic storage impractical for applications where the tank must last more than a day or two without losses.