Parasitic Heating In Cryo-Adsorbent Tanks for Vehicular Hydrogen Storage

In cryo-adsorbent storage, hydrogen is adsorbed at cryogenic temperature (nominally 77 K). The temperature difference between the cold pressure vessel and ambient, which is between -30 and 50°C per the 2010 DOE specification for operability, results in parasitic heat transfer across the hydrogen lines, structural supports, and thermal insulation. As heat is transferred into the pressure vessel, the adsorbent temperature increases and hydrogen is desorbed to the tank void volume. Thus, parasitic heating of a closed tank increases the gas-phase pressure until it reaches the allowable limit of the tank, whereupon hydrogen is vented via a combustor or directly to the atmosphere. The ultimate DOE technical target for loss of useable hydrogen, including permeation and leakage, is 0.05 g/h per kilogram useable H2 stored. Parasitic heating also affects the metric known as dormancy, or the amount of time before the onset of venting in an idle vehicle. This metric is not developed within the DOE technical targets but should be maximized wherever possible to encourage customer acceptance of cryo-adsorbent technology. In this work, the parasitic heat transfer is calculated for a baseline tank geometry configured in two ways: a tank with structural supports fashioned from G-10 composite, like that developed by other investigators for cryo-compressed hydrogen storage, as well as a tank employing tensile Kevlar fiber isolation. The thermal analysis accounts for diurnal cycling of the tank temperature during hot and cold conditions, and includes the effects of ortho-para conversion in the cryogenically stored hydrogen. The Kevlar-suspended tank described herein is shown to be a significant improvement over the G-10 supported cryo-adsorbent tank. Thus, a detailed mechanical vibration analysis of the Kevlar-suspended tank is developed and used to validate the design by providing insight pertaining to maximum strain during increased loading as well as tolerances for creep and mechanical joining.