Thermal conductivity measurements of copper-coated metal hydrides (LaNi5, Ca0.6Mm0.4Ni5, and LaNi4.75Al0.25) for use in metal hydride hydrogen compression systems

This study was performed to supplement the limited data on metal hydrides by experimentally determining the thermal conductivity of various metal hydride powder pellets that underwent a copper coating process. Three different metal hydride powders were tested (LaNi₅, Ca_0.6Mm_0.4Ni₅, and LaNi_4.75Al_0.25) for their thermal conductivity, with variations in testing parameters, and with copper coating processes that varied from 30 to 60 s A testing apparatus was specifically designed for this experiment, and the study reported thermal conductivity values of 3–6 W/m-K.     

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 LaNi₅ 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_0.6Ni₅, 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 LaNi₅ for over 3000 cycles and the cooling power of the reactor gradually decreased by approximately 55%.     

Thermal analysis of the Ca0.4Mm0.6Ni5 metal–hydride reactor

Experimental results are presented for the coupled metal-hydride reactors of Ca_0.4Mm_0.6Ni₅ (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_0.6Ni₅ 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.     

Measurement and analysis of reaction kinetics of La – based hydride pairs suitable for metal hydride – based cooling systems

The reaction kinetics of metal hydride pairs consisting of La_0.9Ce_0.1Ni₅, La_0.8Ce_0.2Ni₅, LaNi_4.7Al_0.3 and LaNi_4.6Al_0.4 were measured at different temperatures to determine their suitability for metal hydride – based cooling systems (MHCSs). The effect of operating conditions and compositional changes on driving potential and reaction rates during cooling and regeneration processes was studied. The reaction rates were increased with Ce content and decreased with Al content. The cooling and regeneration time of MHCS, for working temperature range of 140 °C (heat source), 25 °C (heat sink) and 10 °C (cooling), were measured for each pair. The estimated cycle time took the following trend (La_0.8Ce_0.2Ni₅ – LaNi_4.7Al_0.3) < (La_0.9Ce_0.1Ni₅ – LaNi_4.7Al_0.3) < (La_0.8Ce_0.2Ni₅ – LaNi_4.6Al_0.4) < (La_0.9Ce_0.1Ni₅ – LaNi_4.6Al_0.4). Two reaction kinetics models namely Jander diffusion model (JDM) and Park – Lee model (PLM) were studied and employed for reaction kinetics analyses. The activation energies (E) of these hydrides were calculated using the Arrhenius plot. Estimated values of activation energies from these models were compared by substituting in the hydriding expression and accurate values of activation energies established for these hydrides.     

Investigations of AB5-type hydrogen storage materials with enhanced hydrogen storage capacity

The present study deals with investigations on synthesis, characterization and hydrogenation behavior of the MmNi₅-type hydrogen storage alloys Mm_0.9Ca_0.1Ni_4.9−xFe_xAl_0.1 (x = 0, 0.1, 0.2 and 0.3). All the alloys are synthesized by radio frequency induction melting following the composite pellet route. The X-ray diffraction pattern of dehydrogenated alloy without iron, detects peaks corresponding to calcium hydride, which are absent in the XRD pattern of the alloy with iron. The hydrogenation behavior is monitored by means of activation curves, absorption-desorption pressure-composition isotherms, hysteresis factors and desorption kinetic curves. The substitution of Iron at the place of nickel in the alloys Mm_0.9Ca_0.1Ni_4.9−xFe_xAl_0.1 (x = 0, 0.1, 0.2 and 0.3) gives an increase in the hydrogen storage capacity as 1.82, 1.90, 2.2 and 1.95 wt% corresponding to x = 0, 0.1, 0.2 and 0.3 respectively. The correlation between structural characteristics and hydrogenation behavior is described and discussed.     

Effects of partial substitutions of cerium and aluminum on the hydrogenation properties of La(0.65−x)CexCa1.03Mg1.32Ni(9−y)Aly alloy

Hydrogen in metal hydrides could be one of the promising energy storage mediums to address the intermittent nature of renewable energy. To convert the hydrogen energy to electricity, the storage system has to be coupled with a fuel cells system. Hence, it is important to design a hydrogen storage system that meets the operating requirements for a fuel cell system. In this work, the effects of partial substitution of both cerium and aluminum on the hydrogenation properties of La_(0.65−x)Ce_xCa_1.03Mg_1.32Ni_(9−y)Al_y alloys were investigated simultaneously using factorial design. Both Ce and Al additions greatly improved the reversibility of hydrogen storage capacity. However, the maximum hydrogen storage capacity and absorption kinetics can be reduced by the additions. As Ce and Al gave opposite effects on the absorption and desorption plateaus, they could be used to tune the properties of the alloys to the desired operating conditions for fuel cell applications.     

Experimentally tuned dual stage hydrogen compressor for improved compression ratio

An experiment-driven design procedure for optimizing the combination of stages of a dual stage hydrogen compressor with enhanced compression ratio is presented herein. Three different combinations of reactors were used using LaNi₅, Ca_0.6Mm_0.4Ni₅ and Ca_0.2Mm_0.8Ni₅ as hydrogen storage materials. Compression ratios were found to be similar for low supply pressure conditions, which improved significantly for high supply pressure conditions in single stage experiments. A dual stage compressor system with LaNi₅ in first stage and Ca_0.2Mm_0.8Ni₅ in the second stage was proposed based on single stage results, which was found to be very effective for enhancing compression ratio. Results show that 53% higher compression ratio can be attained by selecting appropriate storage materials for stages, compared to LaNi₅ based economic dual stage system.     

Comparative analysis of the hydriding kinetics of LaNi5, La0.8Nd0.2Ni5 and La0.7Ce0.3Ni5 compounds

In two-phase domains, the plateau pressure of hydride forming materials (such as intermetallic compounds or IMCs) depends markedly on the operating temperature (Van’t Hoff relationships). Therefore, for practical applications, it is necessary to select hydrogen storage materials by considering the thermal environment of the hydride tank. The thermodynamic properties (absorption and desorption pressure plateaux) of IMCs can be adjusted to some extend by chemical alloying with foreign metals and substitution on different crystallographic sites. In this paper, we report on the hydriding kinetics of substituted AB₅ compounds. Isotherms have been measured at different temperatures on La_xNd_1−xNi₅ (x ≈ 0.2) and La_xCe_1−xNi₅ (x ≈ 0.3) compounds. Pneumato-chemical impedance spectroscopy has been used to analyze the hydriding kinetics and to determine microscopic rate parameters associated with surface dissociation of molecular hydrogen, diffusion-controlled transport of atomic hydrogen to bulk regions and hydride formation. Results have been compared to those measured on LaNi₅ and the interest of using such substituted compounds for application in auxiliary power units is discussed.     

Static and dynamic measurement‐based thermodynamic analysis of solid sorption refrigeration system

Pairs of low‐temperature (La_0.9Ce_0.1Ni₅ and La_0.8Ce_0.2Ni₅) and high‐temperature (LaNi_4.7Al_0.3 and LaNi_4.6Al_0.4) LaNi₅ hydrides were used for thermodynamic analysis of metal hydride‐based solid sorption refrigeration system (SSRS). In general, static pressure–concentration isotherm (PCI) properties were considered for the thermodynamic analysis of solid sorption thermodynamic cycles. But, the hydrogen transfer processes involved in the thermodynamic cycle are dynamic in nature. Therefore, in the present study, the dynamic PCIs of these metal hydrides were measured at different temperatures and compared with their static PCIs. Significant variation in PCI and thermodynamic properties were observed in dynamic conditions compared with static conditions. Later, the performance of SSRS was determined by using static and dynamic PCI properties. Refrigeration effect and coefficient of performance of SSRS decreased by 60% and 35% respectively, when dynamic (at 80 mi/min) PCI data were considered. Copyright © 2016 John Wiley & Sons, Ltd.     

Hydrogen compression characteristics of a dual stage thermal compressor system utilizing LaNi5 and Ca0.6Mm0.4Ni5 as the working metal hydrides

The research performed herein consisted of the design, construction, and testing of a dual stage metal hydride hydrogen compression system intended to be used with lower grade geothermal or waste energy sources. The metal hydrides used in this study were LaNi₅ and Ca_0.6Mm_0.4Ni₅. A Finite Time Thermodynamics (FTT) model was also developed and the model proved useful in determining how the compression results and energy requirements for the system change with variations in the system parameters. Dual stage system results showed a final compression ratio of approximately 12 when using cooling and heating temperatures of 10 °C and 90 °C, respectively. The final output pressures and compression ratios were found to follow an upward trend when increasing the heating bath temperatures. It can be concluded from the experimental results, that though the dual stage hydrogen compression system has room for improvement, it is an effective way of compressing the hydrogen from low initial pressures while using low grade energy sources.     

Thermodynamic and structural properties of LaNi5−yAly compounds and their related hydrides

Among the intermetallic compounds able to absorb reversibly large amounts of hydrogen in convenient conditions, LaNi₅ presents large possibility of substitution involving a change of the plateau pressure of the related hydrides in a wide range of pressure. The results presented concern the metallurgical, structural and thermodynamic properties of the compounds LaNi_5?yAl_y and their related hydrides:The limit of existence of pseudobinary intermetallic compounds with hexagonal structure P6/mmm is determined (Y_A1 = 1.25).The lattice parameters and cell volume of the intermetallic compounds are determined in terms of Y_A1. The enthalpy of formation of LaNi₅ and LaNi₄Al are measured by a calorimetric method.The lattice parameters and cell volume of the hydride are determined by X-ray diffraction.The study of the absorption and desorption hydrogen isotherms show a decrease of the plateau pressures with the increasing rate of aluminium and decrease of the capacity. The enthalpy and entropy of formation of hydrides are deduced from the study of the change of their equilibrium pressures as a function of temperature. Correlations are established between structural and thermodynamical properties of intermetallic compound of this series and their related hydrides.     

Enthalpy change (ΔH) and entropy change (ΔS) measurement of CeMn1−xAl1−xNi2x (x=0.00, 0.25, 0.50 and 0.75) hydrides by electrochemical P–C–T curve

The thermodynamic properties of CeMn_1−xAl_1−xNi_2x (x=0.00, 0.25, 0.50 and 0.75) hydrides have been investigated in this paper. With increasing Ni substitution content, the hydrogen concentration (H/M) in CeMn_1−xAl_1−xNi_2x (x=0.00, 0.25, 0.50 and 0.75) hydride increases from 0.129 wt% for x=0.00 to 0.421 wt% for x=0.75 at 293 K. The pressure–concentration isotherm (P–C–T) curves show that no hydrogen equilibrium pressure plateau has been observed for CeMnAl hydride while the slope of the plateau become flatter and longer with increasing Ni content. Meanwhile, the enthalpy change (ΔH⁰) and the entropy change (ΔS⁰) of the hydrides for dehydrogenization shift from −67.44 kJ mol⁻¹ (x=0.00) to 21.16 kJ mol⁻¹ (x=0.75) and from −0.24 kJ mol⁻¹ K⁻¹ (x=0) to −0.03 kJ mol⁻¹ K⁻¹ (x=0.75), respectively. With increasing Ni content, both ΔH⁰ and ΔS⁰ for dehydrogenization shift to the positive direction and make alloy hydrides more stable and hydrogen desorption much easier.     

Design of a hydrogen compressor for hydrogen fueling stations

Hydrogen compressors dominate the hydrogen refueling station costs. Metal hydride based thermally driven hydrogen compressor (MHHC) is a promising technology for the compression of hydrogen. Selection of metal hydride alloys and reactor design have a great impact on the performance of the thermally driven MHHC. A thermal model is developed to study the performance characteristics of the two-stage MHHC at different operating conditions. The effects of heat source temperature and hydrogen supply pressure on the compression ratio and isentropic efficiency are investigated. Finite volume method is used for discretizing the reaction kinetics, continuity, momentum and energy equations. Metal hydrides selected for this analysis are Mm_0.2La_0.6Ca_0.2Ni₅ and Ti_1.1Cr_1.5Mn_0.4V_0.1. The thermal model was validated with the results extracted from an experimental study. Validation results demonstrated that the numerical results are in good agreement with the data reported in literature.     

Measurement of thermodynamic properties of some hydrogen absorbing alloys

In this paper, the effect of hydrogen concentration on the reaction enthalpies of some metal hydride alloys during hydriding and dehydring is presented. Pressure–concentration–temperature characteristics of the metal hydride alloys are measured under nearly isothermal condition during both absorption and desorption. Reaction enthalpies and entropies of LaNi₅, LaNi_4.7Al_0.3, LmNi_4.91Sn_0.15, Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5 and MmCo_0.72Al_0.87Fe_0.04Ni_3.91 are estimated by constructing van't Hoff plots at different hydrogen concentrations. It is observed that the effect of hydrogen concentration on reaction enthalpies is more significant for the alloys having larger plateau slopes. At the initial stage of hydrogenation, metal hydrides are found to have larger reaction enthalpies which decrease gradually by about 5–15% at the end of the hydrogen absorption. At any given temperature, desorption enthalpies of LaNi₅, LmNi_4.91Sn_0.15, MmCo_0.72Al_0.87Fe_0.04Ni_3.91, LaNi_4.7Al_0.3 and Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5 are found to be higher by about 5, 8, 10, 28 and 32% than their respective absorption enthalpies. Reaction enthalpies of the selected metal hydride alloys are expressed as a function of hydrogen concentration by a fourth order polynomial equation obtained from fitting with the experimental data.     

Electrochemically synthesized large area network of CoxNiyAlz layered triple hydroxides nanosheets: A high performance supercapacitor

A network of Co_xNi_yAl_z layered triple hydroxides (LTHs) nanosheets was prepared by the potentiostatic deposition process at −1.0 V (vs. Ag/AgCl) onto stainless steel electrodes. X-ray diffraction patterns show that the Co_xNi_yAl_zLTHs belong to the hexagonal system with layered structure. Cyclic voltammetry and charge discharge measurements in the potential range of −0.1 to 0.5 V and 0.0–0.4 V, respectively, vs. Ag/AgCl in 1 M KOH electrolyte indicate that Co_xNi_yAl_zLTHs have excellent supercapacitive characteristics. The maximum specific capacitance of ∼1263 F g⁻¹ was obtained for Co_0.59Ni_0.21Al_0.20LTH. The impedance studies indicated highly conducting nature of the Co_xNi_yAl_zLTHs.     

Metal hydrides for energy applications – classification,PCI characterisation and simulation

This short review discusses the classification, pressure concentration isotherm (PCI) characterisation and applications of metal hydrides. Special attention is paid to describe the design criterion and fabrication of Sievert's apparatus and PCI characterisation methods of metal hydrides. Different classes of metal hydrides and their properties are broadly discussed. Also, some of the important applications of metal hydrides are presented. The absorption and desorption PCIs of four Lanthanum based alloys namely La_0.9Ce_0.1Ni₅, La_0.8Ce_0.2Ni₅, LaNi_4.7Al_0.3 and LaNi_4.6Al_0.4 are measured using volumetric method in the temperature range of 20 to 80 °C. Later, different PCI simulation models are compared, and Zhou's model is used to simulate the experimentally measured PCIs at different temperatures. It is found that the simulation results are in good agreement with the experimentally measured results. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of substituting Al for Co on the hydrogen-storage performance of La0.7Mg0.3Ni2.6AlxCo0.5−x (x = 0.0–0.3) alloys

La_0.7Mg_0.3Ni_2.6Al_xCo_0.5−x (x = 0.0–0.3) alloys were prepared by induction melting, and the effects of partially substituting Al for Co on the structure and hydrogen-storage properties of the alloys were investigated systematically. It is found that La(Ni, Co, Al)₅ phase with hexagonal CaCu₅-type structure, LaNi₃ phase with PuNi₃ structure and MgNi₂ phase exist as the main phases in La_0.7Mg_0.3Ni_2.6Al_xCo_0.5−x (x = 0.0–0.3) alloys, and the cell volume of the La(Ni, Co, Al)₅ phase increases with the amount of Al added. The results show that the substitution of Al for Co can reduce the plateau pressure and the hysteresis between hydrogen absorption and desorption, and improve the hydrogen-absorption capacity and thermal stability of the hydride. Moreover, the addition of Al can delay the oxidation of the surface layer of the alloy electrodes in electrolyte, slow down the capacity degradation and prolong the cycling lifetime, and enhance the electrocatalytic activity of the hydrogen-storage electrodes for hydrogen oxidation.     

Selection of metal hydrides for a thermal energy storage device to support low-temperature concentrating solar power plants

Metal hydrides have become more and more significant both as hydrogen storage devices and as basic elements in energy conversion systems. Besides the well-known rare earth hydrides, magnesium alloys are very promising in the field of thermal energy storage for concentrating solar power plants. There is interest in analysing the performances of such materials in this context; for this purpose, a numerical model to describe hydrogen absorption and desorption processes of a metal hydride has been connected to a model elaborated with the help of Cycle-Tempo software to simulate a CSP plant operation. The integration of this plant with four metal hydride systems, based on the combination of two low-temperature hydrides (LaNi₅, LaNi_4.8Al_0.2) and two high-temperature hydrides (Mg, Mg₂Ni) has been studied. The investigation has taken into account CSP overall performances, transfer surfaces and storage efficiencies, to determine the feasibility of designed plants. Results show that the selection of the optimal hydrides must take into account hydride operation temperatures, reaction enthalpies, storage capacities and kinetic compatibility. In the light of the calculated parameters, a solar ORC plant using R134a as the working fluid is a valuable choice if matched to a storage system composed of LaNi₅ and Mg₂Ni hydrides.     

Effect of measurement parameters on thermodynamic properties of La-based metal hydrides

Pressure–concentration isotherms (PCIs) of LaNi_5−xAl_x (x = 0.3 and 0.4) hydrides were measured using a volumetric method. Two important thermodynamic properties, enthalpy of formation (ΔH) and entropy of formation (ΔS), were calculated using the van't Hoff equation. The effects of the Al content on the hydrogen storage capacity, plateau pressure and thermodynamic properties were studied. Additionally, the effects of the charging/discharging pressure difference (ΔP_s) during each step of the absorption/desorption PCI measurement on the hydrogen storage capacity (wt%), equilibrium pressure (P_e), plateau slope, reaction enthalpy (ΔH) and entropy (ΔS) were studied for LaNi_4.6Al_0.4 hydride. All of these properties (P_e, ΔH, ΔS, etc.) showed a significant variation with ΔP_s. The effect of the temperature range on the estimation of the enthalpy of formation was investigated. It was observed that ΔH depends on the experimental temperature range.