Is there a general time scale for hydrogen storage with metal hydrides or activated carbon?

The charging and discharging times are some of the very important factors for the design of highly efficient hydrogen storage chambers with metal hydrides or activated carbon. However, experimental research or numerical simulations have to be performed to determine these factors, which could be time consuming and expensive. In this study, the possible existence of a general time scale for hydrogen storage chambers with large amount of sorption heat is discussed. By scale analysis of the energy equation and rapid heat generation and slow cooling assumption, it is found that there likely exists a general time scale for hydrogen charging and discharging. The time scale is determined as the thermal diffusion time scale of the porous storage bed. Its validity is demonstrated by comparing with the available metal hydrides and activated carbon data in previous literature. Furthermore, a minimum bed thickness scale for a specific charging time is also derived according to the developed general time scale. The proposed temporal and spatial scales may serve as a low cost and efficient tool for the design and optimization of hydrogen storage devices with metal hydrides or activated carbon.