On the Thermal Integration of Metal Hydrides with Phase Change Materials: Numerical Simulation Developments towards Advanced Designs

Hydrogen plays a crucial role towards the decarbonization of the transport sector, whilst most of the challenges for a widespread diffusion of hydrogen-based technologies are related to storage technologies. The use of Metal Hydrides (MH) has been widely recognized as a potential solution thanks to their advantages in terms of high degree of safety, high volumetric storage density, comparatively low operating pressure, the possibility of operation at room temperature and relatively low cost. Since the hydrogenation and dehydrogenation of MH are respectively highly exothermic and endothermic reactions, thermal management of the storage tank is one of the most critical issues to ensure safe and effective operations. The integration of Phase Change Materials (PCMs) in the MH tank design is a potential solution for the self-contained thermal management of MH-based hydrogen storage systems, aiming at substantial charge/discharge performance improvements and ease of integration with the other hydrogen system sub-components. Although several simulation-based studies have been recently proposed about the integration of MH and PCM storage systems, most of them typically include engineering-grade assumptions that oversimplifies the thermo-chemical and thermo-physical phenomena occurring within the MH and PCM domains. Typical examples include: the thermal equilibrium assumption within the heterogeneous (gas + metal alloy) MH bed; neglecting buoyancy-driven convection during the PCM melting phase; neglecting the variation of PCM thermophysical properties with temperature and between phases. The current work aims to propose an improvement in the numerical simulation framework for a better dissection of the physical phenomena occurring while integrating PCM and MH technologies and their effects towards transport-oriented advanced designs. More specifically, User Defined Functions (UDFs) have been implemented within the state-of-the-art ANSYS® Fluent commercial CFD package in order to model thermochemistry and heat transfer within the MH bed and to efficiently couple its operation with a PCM-based thermal buffer. In this initial development stage, the study has been oriented towards the analysis of a full set of parameters related to the thermal buffer configuration, including: buoyancy (i. e. natural convection) characteristics with respect to the optimal temperature difference design, PCM thermophysical properties, geometry of the containment volumes and heat transfer surfaces. Results show that including parameters such as buoyancy is crucial for a comprehensive performance evaluation of the MH/PCM storage system, especially during MH charging/PCM heating.