Effect of the Tank Volume and Aspect Ratios on the Storage Capacity in Solid-State Hydrogen Storage: A Numerical Assessment

Storing hydrogen in solid state is gaining attention enabling for overcoming simultaneously two specific limits of conventional hydrogen storage technologies: storage capacity and safety. Currently, hydrogen is stored as compressed gas (usually, at either 350 or at 700 bar), reaching densities ranging from 24.5 to 41.4 kg/m3, but requiring a significant compression work and determining several safety issues; otherwise, it can be stored in liquid state under cryogenic conditions (70.8 kg/m3 at 1 bar and 20 K), but requiring a great amount of energy for liquefaction (about 12.5 kWh/kg) and costs to keep it liquid. In solid state context, physisorption (also said adsorption) mechanism has been recognized as an attractive technique for its capability of storing hydrogen in a porous structure, with weaker bonds, differently from chemisorption mechanism where hydrogen molecules chemically bonds to the absorbing material forming, for instance, metal hydrides. During adsorption, heat is released and thermal management can become an issue when the tank volume is increased. This work aims to numerically investigate the effect of increasing the tank volume on the solid-state hydrogen storage capacity. Moreover, the effect of the tank aspect-ratio is considered. The investigation has been carried out through Computational Fluid Dynamic analyses. The modified Dubinin-Astakhov isotherm model has been taken into account to describe the isotherm sorption during charge, dormancy, and discharge processes. Initially, the model has been validated on a 2.5 dm3 tank, against experimental results available in the literature; then, the geometry has been scaled by a factor of 10, and several aspect ratio considered. The numerical study was conducted considering three different aspect ratios (i.e., 5, 8.5, and 15), whilst keeping the internal volume constant. In this work, it is shown that larger aspect ratios can lead to improved adsorbed mass and decreased maximum temperatures in the system.