On the selection of metal foam volume fraction for hydriding time minimization of metal hydride reactors

Here we examine how the hydriding time of a metal hydride reactor (MHR) varies with the volume fraction, φmf, of a metal foam installed in the reactor. Technically, an experimentally validated mathematical model accounting for the hydrogen absorption kinetics of LaNi5 is used to compute the heat and mass transport in a cylindrical MHR. We then demonstrate that, with a fixed amount of metal hydride powder sealed in the reactor, saving a relatively small fraction (say, 1%) of the MHR internal volume to accommodate a metal foam usually suffices to substantially facilitate heat removal from the reactor, thereby greatly shortening the MHR hydriding time. However, for a metal foam of fixed apparent size, increasing φmf would reduce the metal hydride content, and hence the maximum hydrogen storage capacity, of the MHR. Consequently, if a prescribed amount of hydrogen is to be stored in the MHR, the hydriding time would decrease with increasing φmf at first (due to heat conduction augmentation), reach a minimum at an “optimal” φmf value, and then increase drastically due to metal hydride underpacking.