Solid-state Hydrogen storage: influence of storage capacity in physisorption

Hydrogen storage in solid-state is a promising alternative to conventional technologies to overcome some of their limitations, particularly in terms of the amount of hydrogen stored per unit volume. For instance, liquid hydrogen (stored at 1 bar and 20 K) is characterized by a 70.8 kg/m3 volumetric density, but a great amount of energy is required for liquefaction (about 12.5 kWh/kg); whereas, hydrogen stored as compressed gas (350 - 700 bar) at ambient temperature reaches a density ranging from 24.5 to 41.4 kg/m3, however the compression work is significant, and the high storage pressure determines safety issues. Regarding solid-state hydrogen storage, chemisorption is a solution, where hydrogen is stored at moderate pressure and temperature, reaching a volumetric density of 80-110 kg/m3, but between hydrogen and metal hydrides chemical bonds born, which require a significant amount of energy to be break them; moreover, the time required for absorption and desorption is very low. In the context of solid-state storage, carbon materials can be an interesting solution due to their low cost, and their ability to store hydrogen reversibly within a porous structure through physisorption mechanisms, under supercritical conditions. This study numerically investigates the effect of the variation of the tank capacity on the charging, dormancy, and discharging processes. Two similar axi-symmetric geometries are considered by doubling the tank volume from V1 = 2.5 L to V2 = 5 L. The physisorption mechanism has been modeled according to the modified Dubinin-Astakhov isotherm model. Initially, the CFD model has been validated on V1 tank against experimental results available in the literature. Then, simulations have been carried out on V2 tank to estimate the effect of a storage capacity increase. The simulations on the V2 tank have been set keeping the same boundary conditions in terms of velocity, density, and temperature at the inlet by doubling the overall amount of hydrogen stored with respect to V1 tank. The results show corresponding trends in temperature and pressure profiles with only slight differences in their behaviors; in particular, in the final stage of the charging process.