Porous Metal Hydride Research Articles

Permeability of a deformable metal hydride bed during hydrogen absorption

Heat transfer in porous metal hydride (MH) beds is the major bottleneck in scaling up of the metal hydride technology, while a role of mass transfer limitations in operation is generally overlooked in lab-scale experiments with relatively small beds. We present relations for gas permeability within the Kozeny-Carman approach and a dimensionless parameter to compare heat and mass transfer limitations in metal hydride devices. From experiments on hydrogen absorption and desorption in a flow-through reactor containing 1 kg of La0.9Ce0.1Ni5 alloy we experimentally determined an apparent permeability and showed, that the “breathing’ of the bed can lead to appearance of dual porosity due to fracturing of the bed, which leads to increased permeability at absorption of hydrogen (∼20 μm2) comparing with desorption (∼0.3 μm2). Particle swelling leads to decrease of porosity both due to decrease of pores and due to closure of fractures, thus deformation of the metal hydride bed leads to decrease of permeability during absorption. The effect can be important for flow-through reactors with thick layers of metal hydride.

Evaluation of the potential for distributed generation of green hydrogen using metal-hydride storage methods

This study presents methodology for the evaluation of appropriateness of a hydrogen generator for gas production in multiple distributed plants based on renewable energy sources. The general idea is to form hydrogen clusters integrated with storage and transportation. The paper focuses on the financial viability of the plants, presenting the results of economic evaluation together with sensitivity analysis for various economic factors. The analyzed case study proves that over a wide range of parameters alkaline electrolyzers show favorable economic characteristics, however, a PEM-based plant is more resilient to changes in the price of electricity, which is the main cost component in hydrogen generation. The study is enriched with an experimental investigation of low-pressure storage methods, based on porous metal hydride tanks. The effectiveness of the tanks (β) compared to pressurized hydrogen tanks, in the same volume and pressure is equal to β = 10.2. A solution is proposed whereby these can be used in a distributed hydrogen generation concept due to their safe and simple operation, without additional costly equipment, e.g., compressors. A method for evaluation of the avoided energy consumption as a function of the effectiveness of the tanks is developed. Avoided energy consumption resulting from implementing MH tanks equals 1.33 – 1.37 kWh per kilogram of hydrogen depending on the number of stages of a compressor. The methods proposed in this paper are universal and can be used for various green hydrogen facilities.

Hydrogen recovery from big-scale porous metal hydride (MH) reactor: impact of pressure and MH-thermophysical properties

An advanced ANSYS FLUENT-based model was developed for hydrogen recovery from a multi-tubular fixed-bed metal hydride (MH) reactor of large-scale design. The model was firstly validated by comparing its results to specific experimental data. Mass and heat transfer processes inside the fixed bed were investigated for various pressures and thermochemical characteristics of the MH (thermal conductivity, porosity and reaction parameters). The findings were reported as average, local and spatial changes in the metal’s bed temperature and hydrogen content. During the initial stage of the endothermic desorption (t<100 s), the bed temperature dropped dramatically in all cases . During this time, there was a massive emission of hydrogen. The bed temperature was then raised due to the reactor’s external convective heating, while the hydrogen release continued until the MH was completely dehydrided. The dehydrogenation rate of the MH was enhanced when the discharge pressure was raised. Furthermore, some other characteristics of the MH, i.e., porosity, thermal conductivity, desorption rate constant and activation energy, significantly impacted the resulting mass and heat fluxes inside the bed material.

Performance and energy efficiency of a solid-state hydrogen storage system: An experimental study on La0.7Ce0.1Ca0.3Ni5

This study presents an experimental investigation conducted on an annular porous metal hydride reactor equipped with radial fins. The reactor was filled with 9 kg of La0.7Ce0.1Ca0.3Ni5, and water was considered as heat transfer fluid. The absorption characteristics were studied under different supply pressures (5–20 bar) and desorption characteristics under different inlet temperatures of heat transfer fluid (30–50 °C). An energy efficiency based on the higher heating value of hydrogen was evaluated for the developed solid-state hydrogen storage device. Further, the effect of pre-sensible heating on desorption performance and energy efficiency was analyzed. Finally, the experimental results were compared with the extensively studied LaNi5 alloy. The results showed that the alloy exhibited a constant desorption rate despite slower desorption kinetics. The energy efficiency of the developed system was found to be 76.76%. The comparison results showed that La0.7Ce0.1Ca0.3Ni5 exhibited higher storage capacity than LaNi5 after 20 cycles and approximately similar absorption and desorption rates. The desorption process without pre-sensible heating was more efficient and reduced overall desorption time by 46.5% than the desorption with pre-sensible heating. However, the desorption process without pre-sensible heating has not significantly affected the energy efficiency.

Heat and mass transfer in a metal hydride reactor: combining experiments and mathematical modelling

Heat transfer in porous metal hydride (MH) beds determines efficiency of MH devices. We present a COMSOL Multiphysics numerical model and experimental investigation of heat and mass transfer in a MH reactor filled with 4.69 kg of AB5 type alloy (Mm0.8La0.2Ni4.1Fe0.8Al0.1). To achieve an agreement between the model and experiments it is necessary to include a flow control device (inlet valve or flow regulator) into the model. We propose a simplified and easy-to-calculate boundary condition based on a porous domain with variable permeability at reactor inlet. The permeability of the domain is connected with hydrogen mass flow by a PID controller. Thus, boundary conditions for the inlet pressure and mass flow are coupled and heat transfer inside the reactor could be calculated without additional assumptions applied to heat and mass transfer in the MH bed.

A novel porous metal hydride tank for hydrogen energy storage and consumption assisted by PCM jackets and spiral tubes

Hydrogen energy storage through Metal Hydride (MH) reactors has various applications in concentrated solar powers and fuel cells for stationary applications in renewable energy systems. Hydrogen storage performance and consumption of these systems are strongly dependent on heat and mass transfer characteristics. Incorporating Phase Change Materials (PCMs) and spiral tube heat exchangers into metal hydride reactors improves storage performance significantly. The present paper includes a numerical investigation on the storage performance of a novel Porous Metal Hydride Tank (PMHT) integrated with PCM as a passive heat transfer system. Moreover, a heat exchanger with newly-offered Flat Spiral Tube Planes (FSTPs) is mounted into a reactor for heat transfer enhancement. The air as Heat Transfer Fluid (HTF) flows into the metal reactor through a spiral tube to enhance the heat and mass transfer. The effects of various spiral configurations, different shapes, and thicknesses of PCM jackets on the absorption/desorption process of hydrogen are investigated in detail. Also, various inlet temperatures are applied to the system to examine its impact on the absorption/desorption process. According to the results, increasing the number of FSTPs along the PMHT reduces the absorption duration by 8%. Moreover, compared with PMHT without PCM, the absorption and desorption performance of PCM-based PMHT is improved by 44% and 20%, respectively. Furthermore, increasing and decreasing the air inlet temperature leads to a 43% and 47% reduction in total desorption and absorption duration, respectively. Finally, conical-shaped PCM-jackets are found to be capable of demonstrating better absorption and desorption performances. They offer the advantage of less PCM usage to provide even a higher performance than annulus PCM-jackets. In this regard, the conical-shaped PCM provides the means for the PMHT to reach its maximum hydrogen absorption by only 85% usage of its PCM volume. In comparison, annulus-shaped PCM uses 98% of its PCM volume to help the PMHT reach its maximum hydrogen absorption capability.

Advances in mathematical modeling of hydrogen adsorption and desorption in metal hydride beds with lattice Boltzmann method

This study presents numerical modeling and simulations of thermal fluid flows in high-volumetric-density metal hydride beds during hydrogen (H2) adsorption and desorption within the lattice Boltzmann method (LBM) framework. A novel LBM is developed for predicting the flow and conjugate heat transfer in a practical lab apparatus involving a combination of solid chamber, free expansion zone, and porous media metal hydride that have not been addressed to date. With a correction term in the collision operator and a new equilibrium distribution function, the present model has a consistent expression of the heat capacity ratio for different fluid regions and derives the correct form of macroscopic energy and generalized momentum equations (including Darcy, Brinkman, and Forchheimer terms). The model is then validated through comparisons of the simulated results with previous experimental data under different initial pressure and temperature conditions for LaNi5–H2 storage systems as well Mg–H2 reactors, achieving excellent agreement. In addition, accounting for conjugate heat transfer and other porous forces in the present LBM yields improved predictions over prior numerical approaches.

Electrodeposition and Stacking to Form 3D Hierarchically Porous Structures

Rapid transport between fluid and solid phases can yield high power density in electrochemical energy storage systems, and efficient chemical separations in chromatography. Transport is often slower in the solid phase, and in long, narrow channels of the fluid phase. Careful architecture can lead to significant improvement of overall transport rates. A known architectural theme that can achieve this is hierarchical porosity, where there are a few long, wide channels connecting to many shorter, narrower channels that provide greater access to the solid phase. We have fabricated and tested hierarchically porous metal hydride electrodes and chromatography columns with overall dimensions in the millimeter range. The fluid channels are tens or hundreds of micrometers wide. The solid layers are tens of micrometers thick, and penetrated by pores that are about 10 nanometers wide. We achieve this through electrodeposition of thick layers of block copolymer-templated nanoporous palladium onto a fine wire mesh or photoresist-patterned wafer. Pieces of these materials are then stacked and fixed in place to form a defined geometry that is macroscopic in three dimensions. Palladium forms a hydride under mild chemical or electrochemical conditions. We have measured hydrogen transport rates into or out of these structures electrochemically and by mass spectrometry. Transport rates are significantly faster than for cases where one or both scales of porosity are absent. We expect that similar improvements could be demonstrated for other multiphase chemical systems, such as electrodes that store lithium ions, or organic solutes for chemical separations. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND2015-10657A

Modeling of heat and mass transfer in LaNi5 matrix during hydrogen absorption-desorption cycle

Packed bed reactors using metal hydride are attracting a lot of attention as potential hydrogen storage systems. Some operational and design variables are major constraints to obtain a proper infl ow/outfl ow of hydrogen into a metal hydride reactor. These variables include packed bed thermal conductivity, porosity, pressure and temperature distributions in the reactor during the absorption/desorption cycle. They also cause a mechanical stress induced by temperature gradient. In this paper, two dimensional models are implemented in COMSOL multiphysics to simulate the hydrogen fl ow, pressure and temperature distributions in the packed bed reactor during absorption/desorption cycle. Also, stresses in porous metal hydride induced by temperature variation in the heating/cooling cycle were evaluated. A possible effect of stress induced, porosity changes on diffusion and heating of hydrogen in both radial and axial direction in packed bed is discussed. The model consists of a system of partial differential equations (PDE) describing structural mechanics of stress, heat and mass transfer of hydrogen in the porous matrix of the packed bed reactor.

Modelling and experimental results of heat transfer in a metal hydride store during hydrogen charge and discharge

A numerical model for the transient hydrogen charge/discharge rates and thermal behaviour of metal hydride stores was developed and verified against experiments using a cylindrical reactor filled with AB5-type metal hydride. The model assumes local thermal equilibrium between the gas and solid phases, and incorporates the pressure and temperature-dependent hydrogen reaction rates, as well as heat transfer in the porous metal hydride bed. The model was verified through experimental data. The experiments were performed using a unit with hydrogen storage capacity of 130 Nl H2; the store was submerged in an isothermal water bath. Experiments at different water bath temperatures and charge/discharge hydrogen pressures indicated a relation between charge/discharge time and these parameters. The reactor’s ability to deliver a constant hydrogen flow at different water bath temperatures was experimentally investigated. During simulations it was found that the model applied is sensitive to perturbations of some of its parameters; activation energy of absorption, effective conductivity and heat of reaction were found to be the most important ones. The charge and discharge performances of the store are controlled by the reaction rate in the first half-part of the H absorption/desorption experiments and by a heat transfer in the second half-part of charge/discharge. a 2009 Published by Elsevier Ltd on behalf of International Association for Hydrogen Energy.

A hydrogen-compression system using porous metal hydride pellets of [formula omitted

The objective of this study is to examine the performance of a hydrogen compressor by using recently developed porous metal hydride compact technology exhibiting high thermal conductance. The inlet temperatures of the cooling medium for hydrogen storage (20 and 30 ∘ C ), the heating medium for compression (60, 70, 80, and 90 ∘ C ), and the kind of metal hydride ( LaNi 5 and LaNi 4.75 Al 0.25 ) were considered as key parameters. For each condition, transient temperature distributions and compressed pressures were monitored. Results showed that the hydrogen compressor fared better at lower storage temperatures. The research showed that LaNi 4.75 Al 0.25 had a faster absorption rate than LaNi 5 . In addition, the compression ratio was found to be primarily dependent on the temperature difference between cooling and heating; the compression work and the efficiency of the LaNi 5 reactor were slightly larger than those of the LaNi 4.75 Al 0.25 reactor. Under the considered conditions, the maximum work (2.44 kJ) and efficiency (5.91%) occurred when LaNi 5 operated at cooling temperature of 20 ∘ C and heating temperature of 90 ∘ C .

Studying the process of oxygen ionization on a porous metal hydride electrode

Problems that are connected with utilization of oxygen evolving during overcharge of the nickel oxide electrode in sealed nickel metal hydride batteries are considered. It is established experimentally that the rate of the process of oxygen reduction in conditions of forced gas supply into pores of a metal hydride electrode increases by two orders of magnitude as compared with the intensity of this process during natural convection. Up to 80% of evolved oxygen undergo ionization on a metal hydride electrode in these conditions even in a regime of forced (hour-long) charge of a model sealed nickel-metal hydride battery. The dependence of the current density of oxygen reduction at a metal hydride electrode on the filling of the electrode’s porous space by oxygen is estimated with the aid of manometric and potentiostatic methods. It is shown that practically all the oxygen ionization current is generated at the walls of gas-filled electrode pores, under thin electrolyte films, with a local current intensity of 1–3 mA cm−2.

Modeling of the impedance response of porous metal hydride electrodes

Porous metal hydride electrodes of the alloy MmNi 3.5–3.7Co 0.7–0.8 Mn 0.3–0.4Al 0.3–0.4 have been characterized by means of impedance spectroscopy. A mathematical model for the impedance response, including effects of diffusion of hydrogen, surface kinetics, conductivity in the metal phase and the solution phase, as well as a continuous, lognormal, particle size distribution, was implemented and fitted to the experimental results by application of a least square fitting routine. The model is based on physical parameters, thus avoiding problems related to the conventional interpretation of impedance spectra in terms of equivalent circuits. Very good agreement between experimental results and model results was obtained for a wide range of frequencies, indicating that physical parameters to a great extent can be determined under realistic operating conditions. The latter was confirmed by independent measurements of the variation in open circuit voltage with respect to the state of charge of the electrode. The model provides an improved methodology for the determination of diffusion coefficients based on electrochemical impedance data. Furthermore, the model can be applied for parametric studies of metal hydride electrodes.

Investigation of a Solar-Thermal Bio-mimetic Metal Hydride Actuator

Metal hydrides have been investigated for use in a number of solar thermal energy applications, such as heat regenerators or hydrogen storage technology, but rarely for thermal actuators. Preliminary experimental results from a prototype solar thermal metal hydride actuator, using copper-encapsulated porous metal hydride compacts of LaNi5, indicate that this thermal-mechanical system can produce high specific forces (over 100 (N/g)), with response times on the order of seconds. These operational characteristics, along with features such as being bio-mimetic, compact, operationally safe, lubricationless, noiseless, soft actuating, and environmentally benign, result in an actuator that is ideal for many industrial, space, defense, and biomedical applications. In this paper, we report recent work directed toward predicting and characterizing the performance bounds of the actuator, specifically concentrating on elements which might comprise an actuator driven by concentrated solar radiation. A complete solution of the 1-D governing heat and mass transfer equations with an ideally selective reactor surface are used to predict bounds on performance in terms of volume flow rates and realistic actuation times. The advantages and disadvantages of the design are discussed from this perspective. The preliminary data show a great potential for these metal hydride actuators to be used for solar thermo-mechanical applications.

An Electrochemical Impedance Spectroscopy Method Applied to Porous LiMn2 O 4 and Metal Hydride Battery Electrodes

An electrochemical impedance spectroscopy method utilizing the signals from reference electrodes positioned in front of and behind a porous electrode is investigated. The basis for the method is illustrated theoretically, and limiting values under different assumptions are presented. The method was applied to a porous metal hydride and a battery electrode. It was concluded that the method facilitates the separation of local impedance effects from the effects of the potential distribution in the porous electrode. Consequently, a more accurate determination of local kinetic parameters and the effective conductivity in the pore electrolyte could be obtained. © 2003 The Electrochemical Society. All rights reserved.

Formulation and Numerical Solution of Non-Local Thermal Equilibrium Equations for Multiple Gas/Solid Porous Metal Hydride Reactors

The assumption of local thermal equilibrium (LTE) is very common in the study of reacting flows in porous media. The assumption simplifies the structure of the solutions and places fewer constraints on computational methods for the domain and boundary conditions. However, in certain systems, such as gas/solid metal hydride reactors, the boundary conditions may impose high energy transfer rates which produce slowly evolving phase change fronts coupled with rapid kinetics. Overall performance of the systems is proportional to the release or absorption of hydrogen, and this is sensitively related to temperature. Thus, capturing local departures from LTE is required. This paper directly evaluates the influence of these effects by solving an NLTE (non-local thermal equilibrium) formulation for coupled reactors as a function of the interphase heat transfer coefficient, hsf. The reactor dynamics and overall energy balances are compared to solutions previously obtained from LTE calculations. The results appear to be the first NLTE results for coupled reactors. They confirm the existence of NLTE effects and suggest the magnitude of hsf for which they can be minimized.

Kinetic study of a porous metal hydride electrode

A physio-chemical approach has been used to describe the behaviour of a porous metal hydride electrode. The metal hydride alloy MmNi 3.6Co 0.8Mn 0.4Al 0.3, was studied. Electrochemical impedance spectroscopy (EIS) and a potential ramp were used to study the applicability of the model close to and far from its equilibrium state. An EIS model was developed and took into account a finite conductivity in the solid or liquid phase together with a distribution in characteristic diffusion length, for the hydrogen diffusion, caused by a distribution in particle size. It was shown that the impedance in the low frequency region could not be described by an average particle size, if the particle size was distributed. The EIS model was used to determine different parameters as a function of relative concentration of hydrogen. The change within the parameters could not be explained by a single phase model. The parameters were also tested by a comparison with the result of a single particle's EIS measurement, and a good agreement was found. The potential ramp showed that the present model could not fully describe the transient behaviour of the respective material, far from its equilibrium state. The reason for this could be that the expression describing the surface phase and its connection to the bulk phase is too simplified.

Thermal analysis of the Ca 0.4Mm 0.6Ni 5 metal–hydride reactor

Experimental results are presented for the coupled metal-hydride reactors of Ca 0.4Mm 0.6Ni 5 (Mm=misch metal) with hydrogen pumped by the compressor. In order to augment the heat transfer in the reactor, metal hydride powders were copper-coated and compressed into a porous metal hydride (PMH) compacts. The reactors, packaged with these PMH compacts, produced continuous cooling output of approximately 0.8 kW/kg of Ca 0.4Mm 0.6Ni 5 when the cycle time was set at 4 minutes. The experimental results indicate that optimized reactors of viable specific powers can be fabricated. In addition, comparisons between experimental and theoretical work are in a good agreement.

Performance of high power metal hydride reactors

Metal hydride reactors were built with porous powder metal hydride (PMH) compacts. An improved reactor built with copper coated PMH compacts of LaNi 5 with a 1.27 cm diameter produced a nominal specific cooling power of 1.5 kW/kg hydride. A similar reactor, built with copper coated PMH compacts of Ca 0.4Mm 0.6Ni 5, showed 2.2 kW/kg hydride. Results with copper coated PMH compacts showed improved thermal conductivity. The compacts are structurally strong and prevent migration of fine metal hydride particles. Life-cycle tests were performed on the reactor with LaNi 5 for over 3000 cycles and the cooling power of the reactor gradually decreased by approximately 55%.

Increase of specific surface area of metal hydrides by lixiviation

In this paper, we present a new and powerful method to increase the specific surface area of nanocrystalline ball milled materials. The method consists of using a lixiviation technique to increase the surface area. The element to be lixiviated is ball milled with the hydrogen-absorbing alloy. After complete mixing, the lixiviated element is leached in an appropriate solution. The end product is a porous metal hydride with high specific surface area. The system magnesium–lithium is presented as an example. The results show that before lixiviation, the alloys absorb hydrogen slowly and the hydrogen capacity is reduced due to the presence of lithium. After lixiviation, the specific surface area shows a ten-fold increase and the hydrogen sorption kinetics are faster.