Hybrid H2/Ti dust explosion hazards during the production of metal hydride TiH2 in a closed vessel

The explosibility of hybrid H₂/Ti dust in the production of metal hydride TiH₂ was simulated and studied using a 20-L spherical vessel. The influential factors for the explosion performance of hybrid H₂/Ti dust, including particle size distribution and polydispersity, humidity, temperature, hydrogen content, inert gas and degree of reaction, on hybrid explosion were investigated. Results showed that both the mean particle sizes and particle size polydispersity had significant effects on the dust severity of hybrid H₂/Ti dust. The explosion severity of hybrid H₂/Ti dust was enhanced at a higher temperature in a certain range, and it presented a trend of increasing at the early stage and then decreased both for the increasing humidity and hydrogen pressure. Explosion inhibition effects of typical inert gases for hybrid H₂/Ti dust increased in the following order: argon < helium < nitrogen. The values of (dP/dt)_ex and V_f decreased along with the reaction process, while the value of P_ex kept stable, which showed that the hydrogen state had no obvious impact on P_ex but significantly affected the explosion risk of hybrid H₂/Ti dust, and special attention should be paid to the initial stages of the production process of TiH₂.     

Experimental investigations and a simple balance model of a metal hydride reactor

The metal hydride reactor filled with 5 kg of the AB₅-type (LaFe_0.5Mn_0.3Ni_4.8) alloy was investigated with respect to the hydrogen discharge rates classified using C-rate value, which is discharge of the maximum hydrogen capacity 750 st L within 1 h. The reactor cannot be fully discharged with a constant flow rate, for each temperature of hot water and flow rate there exists a moment of crisis at which the hydrogen flow drops under the constant value. The nominal capacity of the reactor reaches 80% of maximum capacity if sufficient heat transfer is provided. The simple balance model of a metal hydride reactor is developed based on the assumption of uniform temperature and pressure inside a metal hydride bed. The model permits to predict behavior of the metal hydride reactor in different operation regimes, quantitative agreement is obtained for low C-rates (less than 4) and sub-critical modes.     

A hydrogen purification and storage system using metal hydride

This work is to develop a new hydrogen purification and storage system for daily start and stop (DSS) operations. The new system enables us to minimize emissions of carbon dioxide by using compact and highly efficient fuel cells. The new system first removes carbon monoxide, which is poisonous to metal hydride, from reformed gas by using a special carbon monoxide adsorbent. After removing carbon monoxide, the reformed gas is introduced to a metal hydride bed to purify and store hydrogen. Some 100 NL/h Laboratory scale apparatus was operated in daily start and stop operations for 100 cycles for a total of 150 h with quite good efficiency. The new process has achieved an 83% hydrogen recovery ratio in one-month DSS operations.     

Active disturbance rejection control of metal hydride hydrogen storage

Metal hydride (MH) hydrogen storage is used in both mobile and stationary applications. MH tanks can connect directly to high-pressure electrolyzers for on-demand charging, saving compression costs. To prevent high hydrogen pressure during charging, hydrogen generation needs to be controlled with consideration for unknown disturbances and time-varying dynamics. This work presents a robust control system to determine the appropriate mass flow rate of hydrogen, which the water electrolyzer should produce, to maintain the gaseous hydrogen pressure in the tank for the hydriding reaction. A control-oriented model is developed for MH hydrogen storage for control system design purposes. A proportional-integral (PI) and an active disturbance rejection control (ADRC) feedback controllers are investigated, and their performance is compared. Simulation results show that both the PI and ADRC controllers can reject both noises from the output measurements and unknown disturbances associated with the heat exchanger. ADRC excels in eliminating disturbances produced by the input mass flow rate, maintaining the pressure of the tank at the charging pressure with little oscillations. Additionally, the parameters estimated by the ADRC's extended state observer was used to predict the state-of-charge (SOC) of the MH.     

Cyclic behaviors of a novel design of a metal hydride reactor encircled by cascaded phase change materials

The integration of a phase change material (PCM) with a metal hydride (MH) reactor has received considerable attention recently. In such a system, the exothermic and endothermic processes of the MH reactor can be utilized effectively by enhancing the thermal exchange between the MH reactor and the PCM bed. In this study, a novel design that integrates the MH reactor with cascaded PCM beds is proposed. Magnesium nickel (Mg₂Ni) alloy is used as the hydride reactor. Two different types of PCMs with different melting temperatures and enthalpies are arranged in series. A parametric study is carried out to identify the optimum distribution of the different PCMs. The results indicate that the proposed cascaded MH-PCM sandwich design improves the heat transfer rate which consequently shortens the time duration of the hydrogenation and dehydrogenation processes by 26% and 51%, respectively, compared to an MH-PCM sandwich design that includes only a single PCM.     

The effect of copper coated metal hydride at different ratios on the reaction kinetics

In this study, the process parameters that affect the improvement of hydrogen storage material properties were investigated. In order to accelerate the hydrogen charge/discharge processes and to obtain the required hydrogen at the desired flow rates in a short time, the thermal conductivity of the storage materials has been improved, and density analyses have been made. The ideal grinding time has been determined for the LaNi₅ material. Within the scope of the experimental studies, the thermal conductivity coefficients of LaNi₅ coated with copper and LaNi₅ ground for 5 h and coated with copper were increased by 500–750%, and the copper plating ratios were optimized. The materials obtained were characterized by XRD, SEM, and their density was measured with the Helium Pyknometer device and their thermal conductivity coefficients with the Hot Disk Thermal Constants Analyzer. In addition, the hydrogen storage of materials with increased thermal conductivity was investigated experimentally in the metal hydride reactor at the determined pressure. In the study, it was seen that the storage material coated with copper increases the heat transfer, reduces the hydrogen charging time in the metal hydride reactor, and increases the stable discharge time.     

Ammonia suppression during decomposition of sodium amide by the addition of metal hydride

The decomposition of NaNH₂ has been reported, mainly decomposing into NaH, N₂ and H₂. Ammonia is also produced in addition to N₂ and H₂. To the best of our knowledge, very few scattered reports on the effect of alkali hydrides on NaNH₂ exist in literature. Thus, we choose NaNH₂–MH (M = Li, Na, K, Mg, Ca) system to be investigated in detail. Since NaNH₂–NaH is the simplest combination due to same cation, it was tested for the establishment of reaction mechanism using transmission electron microscopy (TEM). It is observed that the entire process follows NH₃ mediated reaction similar to LiNH₂–LiH system. Sodium amide first decompose into Na metal and NH₃, then generated NH₃ reacted with added NaH to form NaNH₂ and release H₂. This process continues until the consumption of NaH, thus suppresses NH₃ evolution to a great extent. The investigation has been extended further to the other metal hydrides and it is found that the addition of other metal hydride i.e. LiH, KH, MgH₂, and CaH₂ have also effectively suppressed the NH₃ evolution. The detailed reaction mechanism has been elucidated for all the amide hydride systems. It is observed that the decomposition takes place through an intermediate step of double-cation amide formation.     

Dynamic model and experimental results of a thermally driven metal hydride cooling system

This paper presents a dynamic model and experimental results of a metal hydride cooling system based on a coupled pair of reactors: a hot reaction bed (alloy LmNi_4.91Sn_0.15) and a cold reaction bed (Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5). The driving power is waste heat removed at high temperature (130 °C). Metal hydride reactors can have interesting applications in thermal storage systems, refrigeration and heat pumps. The experimental setup is described, as well as the governing equations of the model. Correlations are used for the relationship between the equilibrium pressure, hydrogen concentration and temperature. An innovative approach is used for the modelling of hysteresis. The simulation results are compared and validated with experimental measurements during dynamic refrigeration cycles.     

Computational study on metal hydride based three-stage hydrogen compressor

A mathematical model for predicting the performances of a three-stage metal hydride based hydrogen compressor (MHHC) is presented. The performance of the MHHC is predicted by solving the unsteady heat and mass transfer characteristics of the coupled metal hydride beds of cylindrical configuration. The governing equations for energy, momentum and mass conservations, and reaction kinetic equations are solved simultaneously using the finite volume method. Metal hydrides chosen for a three-stage MHHC are LaNi₅, MmNi_4.6Al_0.4 and Ti_0.99Zr_0.01V_0.43Fe_0.99Cr_0.05Mn_1.5. Numerical results obtained for a single-stage MHHC using MmNi_4.6Al_0.4 are in good agreement with the experimental data reported in the literature. Using three-stage compression, a maximum pressure ratio of 28 is achieved for the supply conditions of 20 °C absorption temperature and 2.5 bar supply pressure. A maximum delivery pressure of 100 bar is obtained for the operating conditions of 20 °C absorption temperature and 120 °C desorption temperature.     

Thermal modeling and performance analysis of industrial-scale metal hydride based hydrogen storage container

In this paper, a performance analysis of a metal hydride based hydrogen storage container with embedded cooling tubes during absorption of hydrogen is presented. A 2-D mathematical model in cylindrical coordinates is developed using the commercial software COMSOL Multiphysics 4.2. Numerical results obtained are found in good agreement with experimental data available in the literature. Different container geometries, depending upon the number and arrangement of cooling tubes inside the hydride bed, are considered to obtain an optimum geometry. For this optimum geometry, the effects of various operating parameters viz. supply pressure, cooling fluid temperature and overall heat transfer coefficient on the hydriding characteristics of MmNi_4.6Al_0.4 are presented. Industrial-scale hydrogen storage container with the capacity of about 150 kg of alloy mass is also modeled. In summary, this paper demonstrates the modeling and the selection of optimum geometry of a metal hydride based hydrogen storage container (MHHSC) based on minimum absorption time and easy manufacturing aspects.     

Accurate simplified dynamic model of a metal hydride tank

As proton exchange membrane fuel cell technology advances, the need for hydrogen storage intensifies. Metal hydride alloys offer one potential solution. However, for metal hydride tanks to become a viable hydrogen storage option, the dynamic performance of practical tank geometries and configurations must be understood and incorporated into fuel cell system analyses. A dynamic, axially-symmetric, multi-nodal metal hydride tank model has been created in Matlab–Simulink^® as an initial means of providing insight and analysis capabilities for the dynamic performance of commercially available metal hydride systems. Following the original work of Mayer et al. [Mayer U, Groll M, Supper W. Heat and mass transfer in metal hydride reaction beds: experimental and theoretical results. Journal of the Less-Common Metals 1987;131:235–44], this model employs first principles heat transfer and fluid flow mechanisms together with empirically derived reaction kinetics. Energy and mass balances are solved in cylindrical polar coordinates for a cylindrically shaped tank. The model tank temperature, heat release, and storage volume have been correlated to an actual metal hydride tank for static and transient absorption and desorption processes. A sensitivity analysis of the model was accomplished to identify governing physics and to identify techniques to lessen the computational burden for ease of use in a larger system model. The sensitivity analysis reveals the basis and justification for model simplifications that are selected. Results show that the detailed and simplified models both well predict observed stand-alone metal hydride tank dynamics, and an example of a reversible fuel cell system model incorporating each tank demonstrates the need for model simplification.     

Hydrogen refuelling station with integrated metal hydride compressor: Layout features and experience of three-year operation

Hydrogen compression is a main contributor in the capital and operation costs of the H₂ refuelling infrastructure. The use of metal hydrides (MH) for thermally-driven H₂ compression can provide efficient solution to mitigate this challenge. MH compressors are particularly promising for industrial customers who possess necessary infrastructure including pipeline H₂, sources of low-grade heat, etc.Here we present the details about layout of the H₂ refuelling station and its operation at Impala Platinum refineries in Springs, South Africa, since its start-up in September 2015. The station provides H₂ dispensing at the pressure up to 185 bar and uses pipeline H₂ (P = 50–60 bar) available at the customer site. H₂ compression to P = 200 bar with productivity up to 13 Nm³/h is provided by the integrated 1-stage MH H₂ compressor which uses steam (T∼140 °C) for the heating and circulating water (T∼20 °C) for the cooling; both steam and water are also available from the customer infrastructure. The station also includes H₂ dispenser, buffer tank (standard gas cylinder pack), and control block on the basis of Siemens Program Logic Controller (PLC) which provides fully automated system operation. Switching H₂ and steam/water flows is carried out with the help of remotely controlled, pneumatically actuated valves and auxiliary check valves. Additionally, at P = 200 bar the control block switches the system into standby mode when MH compression modules are cooled down and their gas manifolds are connected to the H₂ supply line. The H₂ dispensing is independent on the compressor operation and takes from 6 to 15 min. The refuelling station complies with South African safety regulations for operation in a fire and explosion hazardous environment.     

Performance simulation of metal hydride based helical spring actuators during hydrogen sorption

Self sensing/actuation materials are known as smart/intelligent materials due to their changes in structure and functionality based on external stimuli. Even though, metal hydrides are studied extensively as potential materials for hydrogen storage, their applicability becomes limited due to low gravimetric storage capacity. However their significant volumetric dilatation upon hydrogenation can make them potential candidates for sensors/actuators. As hydrogenation performance of these alloys is controlled by heat transfer as the major factor, devices based on this can be employed as thermal sensors/actuators. However response characteristics of such devices need detailed investigation. A numerical study is conducted on the performance of these actuator elements with LaNi₅ as the hydrogen storage alloy. Effects of different operational and geometric parameters on hydrogenation and actuator displacement are studied.     

Tests on a metal hydride based thermal energy storage system

In this paper, the performance tests on Mg + 30% MmNi₄ based thermal energy storage device is presented. Experiments were carried out at different supply pressures (10–30 bar) and absorption temperatures (120–150 °C). The effects of hydrogen supply pressure and absorption temperature on the amount of hydrogen/heat stored and thermal energy storage coefficient are presented. The maximum hydrogen storage capacity of 2.5wt% is reported at the operating conditions of 20 bar supply pressure and 150 °C absorption temperature. For a given absorption temperature of 150 °C, the thermal energy storage coefficient is found to increase from 0.5 at 10 bar to 0.74 at 30 bar supply pressure. For the given operating conditions of 20 bar supply pressure and 150 °C absorption temperature, the maximum amount of heat stored is about 0.714 MJ/kg and the corresponding thermal energy storage coefficient is 0.74.     

Studies of off-stoichiometric AB2 metal hydride alloy: Part 1. Structural characteristics

In Part 1 of this two-part series of papers, phase abundances, lattice parameters, crystallite sizes, and microstructures of three series of AB₂-based metal hydride alloys were studied. The base alloys with B/A stoichiometry of 2.0 in series 177, 190, and 193 are rich in C14, equal in C14/C15, and rich in C15 phases, respectively. In each series of alloys, the B/A stoichiometry varies from 1.8, 1.9, 2.0, 2.1, to 2.2. The effects of varying B/A stoichiometry to microstructures are the same for these three series of alloys. As the alloy formula changes from AB_1.8, AB_1.9, AB_2.0, AB_2.1, to AB_2.2, the following events occur: C14-to-C15 phase ratio decreases, both C14 and TiNi secondary phase lattice parameters and unit cell volume reduce; the a/c aspect ratio of C14 phase first decreases and then increases; abundances of non-Laves secondary phases decrease; and the Zr/Ti ratio in AB phase decreases. The C14/C15 ratio is closely related to the average electron density with a threshold that first decreases from 7.13 (AB_1.8) to 7.08 (AB_1.9) and to 7.06 (AB_2.0) and then increases to 7.08 (AB_2.1) and 7.09 (AB_2.2) as the stoichiometry increases. The distributions of B-site elements are not uniform with most of the V, Cr, Mn, Co residing in AB₂ phase and Sn in Zr₇Ni₁₀ phase.     

Model-based design of an automotive-scale,metal hydride hydrogen storage system

Sandia and General Motors have successfully designed, fabricated, and experimentally operated a vehicle-scale hydrogen storage demonstration system using sodium alanates. The demonstration system module design and the system control strategies were enabled by experiment-based, computational simulations that included heat and mass transfer coupled with chemical kinetics. Module heat exchange systems were optimized using multi-dimensional models of coupled fluid dynamics and heat transfer. Chemical kinetics models were coupled with both heat and mass transfer calculations to design the sodium alanate vessels. Fluid flow distribution was a key aspect of the design for the hydrogen storage modules and computational simulations were used to balance heat transfer with fluid pressure requirements.     

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.     

Analysis of a metal hydride reactor for hydrogen storage

In this paper a two-dimensional model of an annular cylindrical reactor filled with metal hydride suitable for hydrogen storage is presented. Comparison of the computed bed temperatures with published experimental data shows a reasonably good agreement except for the initial period. Effects of hydrogen pressure and external fluid temperatures on heat transfer and entropy generation are obtained. Results show that the time required for hydrogen charging and discharging is higher when the thermal capacity of the reactor wall is considered. The time required for absorption and desorption can be reduced significantly by varying the hydrogen gas pressure and external fluid temperatures. However, along with reduction in time the entropy generated during hydrogen storage and discharge increases significantly. Results also show that for the given input conditions, heat transfer between the external fluid and hydride bed is the main source of entropy generation.     

Environmental and economic assessment of hydrogen compression with the metal hydride technology

A critical step in the hydrogen supply chains is the compression phase, which is often associated with high energy consumption and environmental impacts. An environmental and cost analysis of a metal hydride (MH) compressor and competing technologies (an air booster and a commercial hydrogen compressor), is performed for an application to fuel cell driven forklifts. The MH compressor shows limited environmental impacts only when a source of waste heat is available for hydrogen desorption. In these case, impacts would be similar to a generic compressor, but larger than those generated by an air booster. The equivalent economic cost is 6 € per kg of compressed hydrogen for the MH compressor, which is much higher than for the air booster, but lower than for a generic hydrogen compressor. Technical aspects to be improved for large-scale applications of MH compressors are identified.