Heat and Mass Transport in Metal Hydride Based Hydrogen Storage Systems

Hydrogen storage technologies based on solid-phase materials involve highly coupled transport processes including heat transfer, mass transfer, and chemical kinetics. A full understanding of these processes and their relative impact on system performance is required to enable the design and optimization of efficient systems. This paper examines the coupled transport processes of titanium doped sodium alanates (NaAlH4, Na3AlH6) enhanced with excess aluminum and expanded natural graphite. Through validated modeling and simulation, we have illuminated transport bottlenecks that arise due to mass transfer limitations in scaled-up systems. Individual heat transport, mass transport, and chemical kinetic processes were isolated and experimentally characterized to generate a robust set of model parameters for all relevant operational states. The individual transport models were then coupled to simulate absorption processes associated with rapid refueling of scaled-up systems. Using experimental data for the absorption performance of a 1.6 kg sodium alanate system, comparisons were made to computed results to identify dominant transport mechanisms. The results indicated that channeling around the compacted porous solid can contribute significantly to the overall transport of hydrogen into and out of the system. The application of these transport models is generally applicable to a variety of condensed-phase hydrogen sorption materials and facilitates the design of optimally performing systems.