Enhancing Metal Hydride – Phase Change Material Hydrogen Storage Systems Efficiency with Expanded Graphite

Abstract

Hydrogen storage systems and, specifically, metal hydride-based systems, hold a significant potential when it comes to finding safe, affordable, and efficient energy storage solutions [1-3]. A challenge often associated with most metal hydride compounds is building an efficient Heat Management System to prevent the hydride temperature to diverge from equilibrium and, thus, slow down the storage process [4]. We analyze a hybrid Metal Hydride – Phase Change Material (MH-PCM) configuration, where the PCM surrounds the MH powder and works as Thermal Storage Unit (TSU). During desorption of hydrogen (endothermic), the PCM provides heat to the MH by using the same energy that it had previously stored during the absorption stage (exothermic) [5]. However, PCMs suffer from low thermal conductivities, thus several Thermal Augmentation Systems (TAS) might be employed to try and solve this issue. Among them, we focus our attention on the addition of Expanded Natural Graphite (ENG) into the PCM. ENG has a high thermal conductivity and can be easily mixed within the PCM to form a composite. The reduction in gravimetric and volumetric density is a negative side effect of using ENG. In this work, we numerically assess the impact of ENG by comparing the absorption and desorption processes of a baseline MH-PCM design and other layouts with increasing amounts of ENG. The results show that the overall cycle time is reduced by 20.9% when increasing the ENG volume fraction from 0% to 25%. The gravimetric density drops by 12.3%, thus suggesting the increase in the storage system weight and size. The average inlet and outlet power increase from 2.7 kW to 3.6 kW and from 1.2 kW to 1.45 kW, respectively.