A dynamic model is used to characterize cryogenic H-2 storage in an insulated pressure vessel that can flexibly hold liquid H-2 and compressed H-2 at 350 bar. A double-flow refueling device is needed to ensure that the tank can be consistently refueled to its theoretical capacity regardless of the initial conditions. Liquid H-2 charged into the tank is stored as supercritical fluid if the initial tank temperature is >120 K and as a subcooled liquid if it is <100 K. An in-tank heater is needed to maintain the tank pressure above the minimum delivery pressure. Even if H, is stored as a supercritical fluid. liquid H-2 will form as H-2 is withdrawn and will further transform to a two-phase mixture and ultimately to a superheated gas. The recoverable fraction of the total stored inventory depends on the minimum H-2 delivery pressure and the power rating of the heater. The dormancy of cryogenic H-2 is a function of the maximum allowable pressure and the pressure of stored H-2; the evaporative losses cannot deplete H-2 from the tank beyond 64% of the theoretical storage capacity. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.