Metal hydride-based storage system design. System modeling was performed using the Metal Hydride Storage Design Tool (MHSDT) to estimate the hydrogen storage system mass and volume for a specified amount of usable hydrogen. A schematic of the modeled system is shown in Figure S-5. MHSDT uses the physical properties of the metal hydride storage material to estimate the size of the entire storage system, including the metal hydride, heat exchanger, combustor, cooling tubes, and other balance of plant components. Heat transfer properties are used primarily to determine the number of cooling tubes required during refueling, which is an exothermic process. (The MHSDT does not specifically account for either thermal expansion or lattice expansion occurring upon rehydrogenation, so as a general approach we assume that the bed contains 30% open pore volume.) The MHSDT estimates the necessary vessel wall thickness for a given tank design (specifying either diameter, length, or volume). The thickness of the cylindrical section of the tank's wall is primarily determined using the traditional thin-walled pressure vessel hoop stress formula. The end cap geometry is assumed to be perfectly hemispherical with wall thicknesses equal to the cylindrical section. The accuracy of the MHSDT has been verified using finite element analysis (FEA), showing that the wall thicknesses predicted by the estimation tool results in an acceptable stress state. Whereas previous versions were limited to canonical tank wall materials such as 316SS and 6061-T6, this tool has recently been expanded with additional high-performance alloys that we use in this investigation.