Practical modeling of metal hydride hydrogen storage systems

A new approach is introduced to model the discharge behavior of a metal hydride hydrogen storage bed. The reversible reaction kinetics and the empirical van't Hoff relationship used in a typical reactor model are replaced by a solid-phase diffusion equation and a semiempirical equilibrium P-C-T relationship. Two new semiempirical P-C-T models are also introduced based on modified virial and composite Langmuir expressions. By varying the heat- and mass-transfer coefficients, the model was calibrated to experimental pressure and temperature histories obtained from a commercially viable metal hydride bed containing Lm(1.06)Ni(4.96)Al(0.04). Overall, the results of this study showed that a fairly simple numerical model can do a reasonable job in predicting the discharge behavior of a fairly complicated metal hydride hydrogen storage bed over a wide range of hydrogen flow-rate demands. The extreme theoretical limits of isothermal equilibrium (analytical model), adiabatic equilibrium, nonadiabatic equilibrium, isothermal nonequilibrium, and adiabatic nonequilibrium conditions were also studied and compared to the actual behavior under nonadiabatic nonequilibrium conditions. These limiting cases revealed that the metal hydride hydrogen storage vessel was definitely heat-transfer-limited and only minimally mass-transfer-limited over a wide range of hydrogen discharge flow rates.