Augumentation of effective thermal conductivity of metal hydride beds and its effect on the performance of energy conversion and storage devices

Abstract

Metal hydride beds have drawn attention of researchers owing to their capability of reversible absorption and desorption of hydrogen accompanied by exothermic and endothermic reactions respectively. Applications of hydriding materials for solid state hydrogen storage, hydrogen compression, thermal energy storage and sorption heating and cooling systems have been demonstrated successfully. However, the performance of these devices significantly depends upon heat and mass transfer characteristics of the Metal Hydride (MH) beds. One of the important parameters regulating heat and mass transfer in the hydriding bed is its Effective Thermal Conductivity (ETC), which is dependent on several operating parameters, such as, thermal conductivities of solid and gas, gas pressure, bed temperature, void fraction, hydrogen concentration, etc. In general, the value of ETC for these beds is low and needs to be enhanced.Mathematical simulation of ETC helps in understanding its dependency on different operating, geometric and material parameters. Several mathematical models are available for the estimation of effective thermal conductivity (ETC) of non-reactive packed beds. Keeping in view the salient differences between metal hydride beds in which chemisorption of hydrogen takes place and conventional non-reactive packed beds, modified models are proposed here to predict the ETC. Variation in properties, such as, solid thermal conductivity and porosity during hydrogen absorption and desorption processes are incorporated. These extended models have been applied to simulate the ETC of MmNi4.5Al0.5hydride bed and are compared with the experimental results. Applicability of the extended models for estimation of the ETC at different operating conditions such as pressure, temperature and hydrogen concentration are discussed. Reaction between metal alloy and hydrogen, being a complex phenomenon, causes considerable changes in bed porosity, solid thermal conductivity, etc. For accurate measurement of ETC of MH beds, the experimental set up and ETC cell need to be designed precisely. Experimental measurement of ETC of MH beds using one dimensional steady state, radial heat transfer, absolute method has been attempted in the present work.It is well known fact that the value of ETC of MH powder beds lie in the range of 0.1 to 1.5 W m-1 K-1. Many researchers have attempted augmentation of ETC of MH beds applying different techniques. Among the different techniques reported in literature, compaction and pelletization has resulted in the best level of ETC augmentation. The present work attempts to study the effect of combination of two augmentation techniques, such as, inclusion of high thermal conductivity metallic structure and pelletization. La0.8Ce0.2Ni5 and Mg + 50 wt% LaNi4.6Al0.4 are considered for metal hydride based hydrogen compression (MHHC) and metal hydride based energy storage (MHTES) applications respectively. Two types of pellets are fabricated. First type is made by mixing 6 wt% graphite flakes with metal alloy/composite powder, while the second type has a three dimensional structure made with copper wire mesh, embedded to it in addition to the constituents of first type. The ETC of the three types of beds namely, loose metal powder bed (LMP), bed of pellets of metal alloy/composite powder mixed with graphite flakes (PMPGF) and bed of pellets of metal alloy/composite powder mixed with graphite flakes and embedded with copper wire mesh structure (PMPGFCu) are measured and compared at different hydrogen pressures and concentrations and averagebed temperatures. The hydrogen absorption and desorption rates of three types of beds with both the materials are tested at a wide range of pressure and temperature conditions and their performances are compared with respect to hydrogen compression and energy storage applications.. This thesis presents important data related to simulation, experimental measurement and augmentation of ETC of MH beds. Additionally, the effect of ETC augmentation on sorption characteristics of MH beds and consequently on the performance of MHHC and MHTES is also presented. The results obtained will add value to the knowledge base of ETC augmentation and its effect on sorption characteristics of MH beds.