Finite element model for charge and discharge cycle of activated carbon hydrogen storage

One of the main challenges to introduce hydrogen on the energy market is to improve onboard hydrogen storage and develop more efficient distribution technologies to increase the amount of stored gas while lowering the storage pressure. The physisorption of hydrogen on activated carbons (AC) is being investigated as a possible route for hydrogen storage. The objective of this work is to study the performance of adsorption-based hydrogen storage units from a "systems" point of view. A realistic two-dimensional axisymmetric geometric model which couples mass, momentum and energy balances is established based on the thermodynamic conservation laws using finite element method as implemented in COMSOL Multiphysics (TM). We consider the charging and discharging of the storage unit at a rated pressure of 9 MPa, and at an initial temperature of 302 K. The results are compared with experimental data obtained at the Hydrogen Research Institute of the University of Quebec at Trois-Rivieres. The storage tank is cooled by ice water. Research results show that both the simulated variations of pressure and temperatures during charge and discharge processes are in good agreement with the experimental data. The temperatures in the central region of tank are higher than those at the entrance and near the wall at the end of charge time while they are lower than those at the entrance and near the wall at the end of discharge time. The velocities are largest at the entrance, and decrease gradually along the axis of the tank. Owing to thermal effects, the larger flow rates result in less amount of adsorption in the condition of the same charging pressure. Hence measures of increasing heat transfer should be adopted, such as increasing the thermal conductivity of the storage bed. From the point of view of storage capacity, it is therefore possible to realize rapid hydrogenation, which is conducive to the use of such systems for on-board hydrogen storage based on activated carbon adsorption. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.