Numerical simulation of heat and mass transfer in metal hydride hydrogen storage tanks for fuel cell vehicles

This paper presents a two-dimensional mathematical model to optimized heat and mass transfer in metal hydride storage tanks (hereinafter MHSTs) for fuel cell vehicles, equipped with finned spiral tube heat exchangers. This model which considers complex heat and mass transfer was numerically solved and validated by comparison with experimental data and a good agreement is obtained.     

Design of a large-scale metal hydride based hydrogen storage reactor: Simulation and heat transfer optimization

An optimized design for a 210 kg alloy, TiMn alloy based hydrogen storage system for stationary application is presented. A majority of the studies on metal hydride hydrogen systems reported in literature are based on system scale less than 10 kg, leaving questions on the design and performance of large-scale systems unanswered. On the basis of sensitivity to various design and operating parameters such as thermal conductivity, porosity, heat transfer coefficient etc., a comprehensive design methodology is suggested. Following a series of performance analyses, a multi-tubular shell and tube type storage system is selected for the present application which completes the absorption process in 900 s and the desorption process in 2000 s at a system gravimetric capacity of 0.7% which is a vast improvement over similar studies. The study also indicates that after fifty percent reaction completion, heat transfer ceases to be the major controlling factor in the reaction. This could help prevent over-designing systems on the basis of heat transfer, and ensure optimum system weight.     

Hydrogen storage in metal hydride tanks equipped with metal foam heat exchanger

This paper presents a two-dimensional mathematical model to evaluate transient heat and mass transfer in a metal hydride tank (hereinafter MHT) with metal foam heat exchanger. The model is validated by comparison with experimental data. A good agreement is obtained.     

Optimization of hydrogen storage in metal-hydride tanks

The storage time of hydrogen in metal-hydride tanks (MHTs for short) is strongly influenced by the rate at which heat can be removed from the reaction bed. In the present work a two-dimensional mathematical model is developed and validated against experimental results. This model is used, first, to evaluate the impact of the tank wall thermal mass on the hydriding process. Walls in steel and in brass are tested and the obtained results show that there is no significant effect on hydrogen storage time. Then, the established model is used to study the dynamic behaviour inside various designs of MHTs: i) a cylindrical tank, ii) a cylindrical tank with external fins, iii) a cylindrical tank with a concentric tube filled with flowing cooling fluid and iv) a cylindrical tank with a concentric tube equipped with fins. Optimization results indicate that almost 80% improvement of the storage time can be achieved over the case where the tank is not optimized.     

Simulation of transient heat and mass transfer during hydrogen sorption in cylindrical metal hydride beds

A CFD analysis of heat and mass transfer in cylindrical metal hydride beds is carried out using the commercial code Fluent 6.2. The effect of bulk diffusion is considered for mass transfer in the solid phase. Temporal and spatial variations of temperature and concentration in hydride bed are plotted. Emphasis is given to monitor the motion of hydrogen within the bed and to the influence of the L/D$L/D$ ratio and porosity. It is observed that a concentration variation in the bed is the driving force for hydrogen flow in hydride beds. The gas movement is observed to be from saturated cooler peripheral region towards the unsaturated hotter core region of the bed.     

Experimental analysis of a metal hydride hydrogen storage system with hexagonal honeycomb-based heat transfer enhancements-part B

This study is a continuation of the computational analysis of the reactor equipped with hexagonal honeycomb based heat transfer enhancements, performed in Part A of the study. In the present study, the performance of the metal alloy and the reactor is investigated experimentally. The gravimetric capacity and reaction kinetics of the alloy La_0.9Ce_0.1Ni₅ are determined. The performance of the reactor under different external environments is noted. The influence of operating conditions such as supply pressure, heat transfer fluid, heat transfer fluid temperature on the reactor performance is investigated. Evaporative cooling as a heat removal technique for metal hydride based hydrogen storage reactors is tested for the first time and compared to conventional heat removal methods. It is found to improve the heat transfer from the alloy bed significantly.     

Simulation of heat transfer in a metal hydride reactor with aluminium foam

A 1-D model has been developed to evaluate various designs of metal hydride reactors with planar, cylindrical or spherical geometry. It simulates a cycling loop (absorption–desorption) focusing attention on the heat transfer inside the hydride bed, which is considered the rate-limiting factor. We have validated this model with experimental data collected on two reactors, respectively, containing 1 and 25 g of _LaNi5${\mathrm{LaNi}}_5$, the second being equipped with aluminium foam. The simulation program reproduces accurately our experimental results. The impact of the foam cell size has been studied. According to our model, the use of aluminium foam allows the reactor diameter to be increased by 7.5 times, without losing its performance.     

Effect of design parameters on enhancement of hydrogen charging in metal hydride reactors

The effects of heat transfer mechanisms on the charging process in metal hydride reactors are studied under various charging pressures. Three different cylindrical reactors with the same base dimensions are designed and manufactured. The first one is a closed cylinder cooled with natural convection, the fins are manufactured around the second reactor and the third reactor is cooled with water circulating around the reactor. The temperatures of the reactor at several locations are measured during charging with a range of pressure of 1–10 bar. The third reactor shows the lowest temperature increase with the fastest charging time under all charging pressures investigated. The effective heat transfer coefficients of the reactors are also calculated according to the experimental results and they are found to be 5.5 ± 1 W m⁻² K⁻¹, 35 ± 2 W m⁻² K⁻¹ and 113 ± 1 W m⁻² K⁻¹, respectively. The experimental results showed that the charging of hydride reactors is mainly heat transfer dependent and the reactor with better cooling exhibits the fastest charging characteristics.     

Optimized heat transfer fin design for a metal-hydride hydrogen storage container

This paper presents a heat transfer fin optimization for a LaNi₅ hydrogen storage container. In this simplified approach, a one-dimension fin model is proposed in order to avoid geometrical restrictions and constrains associated to a particular technological solution. Therefore, the presented model can be utilized as a general framework for the development of containers with inner fins.     

A new algorithm for solving transient heat and mass transfer in metal–hydrogen reactor

A new algorithm based on the lattice Boltzmann method (LBM) is proposed as a potential solver for two-dimensional and dynamic heat and mass transfer in metal–hydrogen reactor. To check the validity of this algorithm, computational results have been compared with the experimental data and a good agreement is obtained. The advantages of the proposed numerical approach include, among others, simple implementation on a computer, accurate CPU time, and capability of stable simulation.     

Experimental study of a metal hydride vessel based on a finned spiral heat exchanger

The reaction time of hydrogen in metal hydride vessels (MHVs for short) is strongly influenced by the heat transfer from/to the hydride bed. In the present work an experimental study of the geometric and the operating parameters of a finned spiral heat exchanger has been carried out to identify their influence on the performance of the charging process of the MHV. The experimental results show that the charge time of the reactor is considerably reduced, when finned spiral heat exchanger is used. In addition, the effect of different parameters (flow mass and temperature of the cooling fluid, applied pressure, and hydrogen tank volume) has been discussed and obtained results show that a good choice of these parameters is important.     

Three-dimensional modeling of hydrogen sorption in metal hydride hydrogen storage beds

This paper conducts a three-dimensional (3D) modeling study to investigate the hydrogen absorption process and associated mass and heat transport in a metal hydride (LaNi₅) hydrogen storage tank. The 3D model is further implemented numerically for validation purpose and the detailed investigation on absorption process. Results indicate that at the very initial absorption stage the bed temperature evolves almost uniformly, while it varies greatly spatially at the latter stage. At the initial seconds, most hydrogen is absorbed in the region near the cooling wall due to the better heat removal. The absorption in the core is slow at the beginning, but becomes important at the very end stage. It also shows that the initial hydrogen flow in the bed is several-fold larger than the latter stage and the flow may provide extra cooling to the hydriding process. By analyzing the Peclet number, we find that the heat convection by the hydrogen flow may play an important role in local heat transfer. This work provides an important platform beneficial to the fundamental understanding of multi-physics coupling phenomena during hydrogen absorption and the development of on-board hydrogen storage technology.     

Performance simulation of metal hydride hydrogen storage device with embedded filters and heat exchanger tubes

A practical metal hydride based hydrogen storage device would consist of many filters to distribute hydrogen gas and heat exchanger tubes to cool or heat the hydride bed depending on whether hydrogen is being absorbed or desorbed. This paper presents the simulation of such a device with LaNi₅ as the hydriding alloy. A study of the geometric and operating parameters has been carried out to identify their influence in the hydriding performance of the storage device.     

Charging dynamics of metal hydride hydrogen storage bed for small wind hybrid systems

Small hybrid wind systems are capable of storing and supplying power for residential applications. In this paper, the excess wind energy is converted into hydrogen by electrolysis and is stored in a metal hydride. Metal hydride beds are known for their high volumetric capacity compared to the compressed hydrogen storage, and offers hydrogen storage at a reasonable operating temperature and pressure. A system simulation model is developed in Matlab/Simulink platform for the dynamics of the metal hydride hydrogen storage system, which is charged by the wind energy. The thermal loads of the metal hydride storage system is met by passing water at ambient temperature for cooling the bed while hydrogen is being absorbed. The effect of the transient turbulent wind velocity profile on the storage system is analyzed. The thermal management of the storage system plays an important role in the overall design, and hence it is discussed in detail.     

Enhancement of heat and mass transfer characteristics of metal hydride reactor for hydrogen storage using various nanofluids

The execution of metal hydride reactor (MHR) for storage of hydrogen is greatly affected by thermal effects occurred throughout the sorption of hydrogen. In this paper, based on different governing equations, a numerical model of MHR filled by MmNi_4.6Al_0.4 is formed using ANSYS Fluent for hydrogen absorption process. The validation of model is done by relating its simulation outcomes with published experimental results and found a good agreement with a deviation of less than 5%; hence present model accuracy is considered to be more than 95%. For extraction or supply of heat, water or oil is extensively used in MHR during the absorption or the desorption process so as to improve the competency of the system. Since nanofluid (mixture of base fluid and nanoparticles) has a higher heat transfer characteristics, in this paper the nanofluid is used in place of the conventional heat transfer fluid in MHR. Further the numerical model of MHR is modified with nanofluid as heat extraction fluid and results are presented. The Al₂O₃/H₂O, CuO/H₂O and MgO/H₂O nanofluids are selected and simulations are carried out. The results are obtained for different parameters like nanoparticle material, hydrogen concentration, supply pressure and cooling fluid temperature. It is seen that 5 vol% CuO/H₂O nanofluid is thermally superior to Al₂O₃/H₂O and MgO/H₂O nanofluid. The heat transfer rate improves by the increment in the supply pressure of hydrogen as well as decrement in temperature of nanofluid. The CuO/H₂O nanofluid increases the heat transfer rate of MHR up to 10% and the hydrogen absorption time is improved by 9.5%. Thus it is advantageous to use the nanofluid as a heat transfer cooling fluid for the MHR to store hydrogen.     

Numerical modelling and heat transfer optimization of large-scale multi-tubular metal hydride reactors

Heat management during the absorption/desorption process is a key aspect in improving the performance of large-scale hydrogen storage systems. In this article, the absorption and desorption performance of a multi-tubular hydride reactor is numerically investigated and optimized for 60 kg mass of LaNi₅ alloy. The 90% absorption with 7, 14, and 19 tubes is achieved in 985, 404, and 317 s with an overall reactor weight of 78.46, 88, and 88.2 kg, respectively. The 14-tube reactor performance is investigated by introducing the longitudinal fins inside the tubes. The reactor performance is enhanced by allocating fins into different pairs of half and full fins constrained by overall fin volume. A thermal resistance network model is presented to investigate the effect of fin distribution and coolant velocity on equivalent resistance of the metal hydride reactor. Storage performance obtained from numerical model validates the thermal resistance analysis from heat transfer viewpoint. With six full fins, 90% hydrogen absorption is achieved in 76 s. However, tubes with 6, 8, and 12 pairs of half and full fins require 74, 58, and 54 s, respectively. The 14-tube reactor with 8 pairs of half and full fins is used for quantifying the augmentation in the absorption performance in response to operating conditions (supply pressure and heat transfer fluid temperature). A design methodology is outlined for the development of a large-scale multi-tubular hydride reactor based on a heat transfer optimization strategy.     

Predictive calculation of the effective thermal conductivity in a metal hydride packed bed

The effective thermal conductivity of a metal hydride packed bed was calculated by considering the influence of expansion during hydrogen absorption and contraction during hydrogen desorption. The porosity was calculated using an experimental formula developed by direct observation, which was used in combination with other referenced methods. However, none of the methods could express the reported experimental value response to pressure change using only the experimental porosity formula. The area contact model was modified so that the porosity and the contact area changed with expansion and contraction. The contact area change was calculated by assuming a simple geometrical deformation caused the difference between the particle expansion and the packed bed expansion. The calculation results of the improved area contact model with the deformed factor and the shape factor were in good agreement with the reported experimental data. This calculation method of the effective thermal conductivity with the influence of expansion and contraction is expected to be useful for designing of heat transfer enhancement of a hydrogen storage tank.     

Experimental formula for estimating porosity in a metal hydride packed bed

An experimental formula for estimating porosity in a metal hydride packed bed is presented. The formula was developed by direct observation of the volume changes of a metal hydride packed bed under free expansion in a vessel. The experimental results showed that the cycles of expansion and contraction were repeated at large porosities above 60% after a rapid state change caused by early particle breakup. The formula for porosity was expressed as a function of the reacted fraction and as a function of the cycle number. The function formula of the reacted fraction can be used to compute different values of porosity for expansion by absorption and for contraction by desorption. The coefficients assuming 100% hydrogen storage based on the experiments with LaNi₅ were an expansion ratio of 16.7% and a contraction ratio of 8.4%, on average. This experimental porosity formula is useful for effective thermal conductivity calculations and for numerical simulations of metal hydride packed bed behavior.     

Numerical analysis of heat and mass transfer during absorption of hydrogen in metal hydride based hydrogen storage tanks

In this paper, hydriding in a cylindrical metal hydride hydrogen storage tank containing HWT5800 (Ti_0.98Zr_0.02V_0.43Fe_0.09Cr_0.05Mn_1.5) is numerically studied with a two-dimensional mathematical model. The heat and mass transfer of this model is computed by finite difference method. The effects of supply pressure, cooling fluid temperature, overall heat transfer coefficient and height to the radius ratio of the tank (H/R) on the hydriding in the hydrogen storage tank are studied. It is found that hydride formation initially takes place uniformly all over the bed and hydriding processes take place at a slower rate at the core region. Supply pressure, cooling fluid temperature and overall heat transfer coefficient play significant roles during the absorption of hydrogen. At the H/R = 2 both maximum bed temperature and the average bed temperature are the highest, and the hydride bed takes the longest time to saturate.