Self-contained thermal management of metal hydride hydrogen storage using phase change materials for solar-hydrogen systems
Creators: name: Nguyen, Huy
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401703
subject: Energy generation, conversion and storage (excl. chemical and electrical)

description: Heat is an inherent part of hydrogen charging and discharging processes in metal hydride (MH) hydrogen storage systems. In order to maintain charge and discharge rates of hydrogen at desired levels and maximise the utilisation of MH storage capacity, the MH must be thermally managed such that the heat released from exothermic reaction is effectively removed from the MH bed during charging process and required heat is transferred to the MH during discharging. In a solar-hydrogen system in which MH hydrogen storage is used for absorbing hydrogen and releasing it at stable rates matching the supply by electrolyser (EL) and demand by fuel cell (FC), respectively. Conventional solutions for MH thermal management or addressing the low discharge rate of MH, including oversizing the hydrogen storage capacity or using external heat sources (i.e., cooling/heating systems), suggest significant increase in capital costs, complexity, and parasitic energy. Heat recovery from FC to boost MH hydrogen discharging is an alternative solution, but this approach diminishes the benefit of utilising FC heat for other onsite needs such as hot water supply, space heating. 

Considering the significant amount of heat released/demanded by MH hydrogen storage system, the idea of utilising a phase change material (PCM) for storing the heat released during hydrogen charging mode and utilising it later for supporting the MH during hydrogen discharging is promising. This approach helps avoid the need for external heating/cooling arrangements for thermal management of MH required during both charging and discharging. The use of PCM for thermal management of MH hydrogen storage (MH-PCM) has been very limitedly mentioned in the literature; however, this has never been investigated, experimentally or in an applied context. Moreover, the studies in which the use of PCM is mentioned for this thermal management purpose are all looking at charging mode, where the PCM is used for cooling the MH canisters. 

Addressing these gaps, in this study, we proposed and investigated a PCM-based arrangement for self-contained thermal management of MH hydrogen storage during both charging and discharging. This is a useful approach for systems in which the MH undergoes frequent charging and discharging modes. Hence, we conducted this study in practical context of a standalone solar-hydrogen system that experiences such a scenario. Moreover, after conducting a thorough theoretical investigation, this study was supported by a comprehensive experimental investigation. The study was designed to understand the feasibility and performance of this solution by showing how it helps with matching the hydrogen absorption and desorption rates of the MH with those supplied by the electrolyser and demanded by the fuel cell (i.e., in solar-hydrogen system), respectively. This study also highlighted the key limitation of organic PCMs with low thermal conductivity. Hence, the use of metal foam (MF) as a heat transfer enhancement technique was investigated as a practical approach to overcome this challenge. Moreover, the impacts of operating parameters on thermal behaviours, charging/discharging performances, and energy efficiencies of the system in the presence a PCM thermal management solution were investigated. Overall, this study aims to contribute to understanding and quantifying heat transfer behaviour of MH-PCM arrangement during hydrogen charging and discharging. 

A mathematical modelling approach, exergy and entropy analyses, and an experimental study have been employed to achieve the objectives of this study. HOMER simulation tool was first used to optimally size a standalone solar-hydrogen system for a case study of a small energy-efficient passive dwelling located in southeast Australia. A mathematical model, which is based on simplified heat and mass transfer of MH and PCM, was then developed in MATLAB to analytically investigate thermal behaviours and the effects of operating parameters on charging/discharging performance of the MH-PCM hydrogen storage system used in the case study. Exergy and entropy analyses using the formulations of the first and second laws of thermodynamics were conducted to quantify and qualify the energy performances of the solar-hydrogen system which used MH-PCM unit for hydrogen storage.  

The simulation results confirmed that using PCM for self-contained thermal management of MH offered a technically viable solution to avoid the parasitic energy required for cooling and heating of the MH. The PCM can store the heat released from MH during hydrogen charging and transferring it back to the MH during discharging purposes. Thermodynamics analyses showed that the energy and exergy efficiencies of such storage arrangement are 100% and 98.8% respectively. This underlines the advantages of this thermal management solution compared to conventional systems (e.g., external heating/cooling). Due to oversizing the solar PV system to guarantee the continuity of power supply throughout the year, the energy and exergy efficiencies of the solar-hydrogen system are approximate 3.5% and 3.8%, respectively. 

This research was further supported by an experimental study by looking at a MH-PCM hydrogen storage unit in which hydrogen charging and discharging at the rates supplied by the EL and demanded by FC were simulated. The experiments involved charging and discharging processes while both pure PCM and that enhanced with MF (i.e., copper metal foam) were used. These experiments created an insight into the performance of the PCM and MH hydrogen storage system in the PCM-MH arrangement used in this study. It also helped with validating some outcomes of the theoretical models created as a part of this study. The experimental work shed light on technical challenges that this novel self-contained arrangement faces and then showed how the use of MF can address them.  

Low thermal conductivity (i.e., 0.2 W/mK) of pure PCM was identified as the key challenge of this solution that limit its capacity to serve its purpose. Improving the ETC of PCM was then considered for practical implementation of this novel self-contained thermal management arrangement. The analytical model suggested that the ETC of PCM used in this thermal management application should be at least greater than 1.5 W/mK to address this challenge. This can be obtained by solutions such as inserting high thermal conductivity solid matrices such as metal fins, wire matrixes and metal foams.