Heat And Mass Transfer Studies On Li-Mg-N-H Based Hydrogen Storage Tank

Hydrogen is considered as a promising alternative energy carrier for the future stationary and mobile applications due to its high calorific value and environmental friendliness. However, safe and efficient hydrogen storage is still the most critical technical barrier for practical applications of hydrogen energy, especially for automotive applications. Li-Mg-N-H is believed to be one of the most promising hydrogen storage materials for the onboard applications because of its moderate operating temperatures and a relatively high reversible hydrogen storage capacity of 5.6 wt%. Nowadays, the researches on the Li-Mg-N-H material mainly focused on its hydrogen sorption properties and reaction mechanism, while the study on the Li-Mg-N-H based hydrogen storage tank is still not sufficient until to now. The researches on the Li-Mg-N-H based hydrogen storage tank need to solve a series of problems. (1) The batch preparation process and conditions must be determined for the Li-Mg-N-H material with excellent hydrogen storage properties. (2) We should develop solutions for improving the apparent density and effective thermal conductivity of the Li-Mg-N-H material, and understand the effects of these methods on the basic physical properties and hydrogen storage properties of the Li-Mg-N-H material (3) We should fully understand the influences of the volume change of the compacts, a’nd the air and water on the hydrogen storage properties of the Li-Mg-N-H material, which can be used to guild the safe use of the Li-Mg-N-H material. (4) We should have a comprehensive understanding about the hydrogen absorption/desorption process and the heat and mass transfer properties of the Li-Mg-N-H based hydrogen storage tank, and develop a numerical model to guild the optimization of the tank.The work of this paper mainly focused on the batch preparation and the investigation on the hydrogen storage properties of the Mg(NH2)2-2LiH-0.07KOH material, the hydrogen absorption and desorption properties, the numerical simulation and optimization of the hydrogen storage tank. The main works of this paper are as follows. Firstly, the Mg(NH2)2-2LiH-0.07KOH material with excellent hydrogen storage properties was prepared by using mechanical ball-milling method. Secondly, the effects of the compaction pressure and the ENG content on the basic physical properties, the hydrogen storage properties and the safety performance of the Li-Mg-N-H material were discussed in detail. Thirdly, a cylindrical Li-Mg-N-H hydrogen storage tank is designed and fabricated, and the hydrogen absorption/desorption properties, and the heat and mass transfer properties of the tank were also investigated. Finally, based on the comprehensive understanding of the heat transfer, hydrogen transport, and the thermodynamic and kinetic characteristics of hydrogen desorption for Li-Mg-N-H based hydrogen storage tank, a numerical model was developed and used to guild the optimization of the tank.The batch preparation of the material:The Mg(NH2)2-2LiH-0.07KOH material with excellent hydrogen storage properties was prepared by using mechanical ball-milling method, and using the Mg powder, LiNH2 and KOH as the main starting materials. Moreover, the mass of the material prepared in a batch reached to hundreds of grams. The as-prepared material starts to absorb hydrogen at 75℃, and its absorption rate reaches to the maximum at 145℃. And the material starts to desorb hydrogen at 100℃, and its desorption rate reaches to the maximum at 215℃. The reversible hydrogen storage capacity of the as-prepared material is 4.64±0.04 wt%.The study on the hydrogen storage properties of the as-prepared material: Compaction and ENG addition have a great influence on the hydrogen storage properties of the Mg(NH2)2-2LiH-0.07KOH material. The compaction pressure only affects the hydrogen desorption properties of the material from the 1st to 5th cycle. As the hydrogen absorption/desorption process proceed, the volume of the compact expands, and the hydrogen desorption kinetics is accelerated. After 5 sorption cycles, the compacts prepared at different compaction pressures exhibit the same hydrogen desorption kinetics. However, the effect of the ENG content on the hydrogen desorption properties exists all the time. As the ENG content increases, the effective thermal conductivity of the material is improved obviously, and the hydrogen desorption kinetics is accelerated significantly.The study on the safety performance of the as-prepared material:The safety performance of the as-prepared material can be enhanced obviously by powder compaction. The powder sample reacts violently with water, and its hydrogen desorption capacity is reduced to almost zero after exposed to the air. The compact reacts with water in a quiet and calm manner. When the compact is exposed to the air, only the surface material of the compact reacts with O2 and H2O. The hydrogen desorption capacity of the compact reduces from 4.68 wt% to 3.32 wt%, and the desorption kinetics declines correspondingly.The study on the hydrogen absorption and desorption properties of the hydrogen storage tank:During hydrogen absorption, the temperature of the hydride bed initial quickly elevates to a high temperature, and then decreases and returns back to the set-temperature. Moreover, the applied hydrogen pressure strongly affects the hydrogen absorption properties of the tank. With increasing the applied hydrogen pressure, the temperature rise of the hydride bed increases, and the hydrogen absorption capacity of the tank also increases. The hydrogen desorption process of the tank exhibits 3 stages, and the hydrogen desorption of the Mg(NH2)2-2LiH-0.07KOH mainly carries out at stage II accompanied with a significant temperature drop. In addition, the hydrogen flow rate has a great influence on the hydrogen desorption properties of the tank. As the hydrogen flow rate increases, the endothermic dehydrogenation of the hydride degrades the bed temperature to a lower value, and the amount of hydrogen released also decreases at stage Ⅱ. Increasing the ENG content obviously improves the effective thermal conductivity of the hydride bed, which results in the rise of the bed temperature and the increase of the hydrogen desorption time at the constant flow rate. Increasing the compaction pressure significantly decreases the hydrogen permeability of the hydride bed, but the entire desorption process of the tank changes a little.The development of the numerical model and the optimization of the hydrogen storage tank:The optimization of the hydrogen storage tank consists of four aspects, such as the length-to-diameter ratio of the cylindrical hydrogen storage tank, the effective thermal conductivity of the hydride bed, the overall heat transfer coefficient between the hydrogen storage tank and the outside, and the hydrogen permeability of the hydride bed. A. As the length-to-diameter ratio of the cylindrical hydrogen storage tank increases, the temperature drop of the hydride bed decreases, and the reaction fraction at the constant flow rate increases during hydrogen desorption of the tank. B. Increasing the effective thermal conductivity of the hydride bed and the overall heat transfer coefficient between* the hydrogen storage tank and the outside can increase the reaction fraction at the constant flow rate during hydrogen desorption of the tank. C. The hydrogen permeability of the hydride bed also affects the hydrogen desorption properties of the tank. Increasing the hydrogen permeability of the hydride bed can lead the reaction fraction at the constant flow rate to be increased obviously.