Performance investigations of a single-stage metal hydride heat pump

This paper discusses the performance investigations of a single-stage metal hydride heat pump (SS-MHHP) working with five different alloy pairs, namely, MmNi_4.6Al_0.4/MmNi_4.6Fe_0.4, LaNi_4.61Mn_0.26Al_0.13/La_0.6Y_0.4Ni_4.8Mn_0.2, LmNi_4.91Sn_0.15/Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5, LaNi_4.6Al_0.4/MmNi_4.15Fe_0.85 and Zr_0.9Ti_0.1Cr_0.9Fe_1.1/Zr_0.9Ti_0.1Cr_0.6Fe_1.4. The performance of the system is predicted by solving the unsteady, two-dimensional coupled heat and mass transfer processes in metal hydride bed of cylindrical configuration using a fully implicit finite volume method. The influences of operating temperatures such as heat source (T_H), heat sink (T_M) and refrigeration (T_C) temperatures on the coefficient of performance (COP) and specific cooling power (SCP) of the system are presented. The predicted hydride bed temperature profiles are compared with the experimental data reported in the literature and a reasonably good agreement is observed between them. The optimum operating temperature ranges of each pair of alloys are suggested. For the selected operating temperatures, a maximum COP of 0.66 is predicted for Zr_0.9Ti_0.1Cr_0.9Fe_1.1/Zr_0.9Ti_0.1Cr_0.6Fe_1.4 hydride pair, while LmNi_4.91Sn_0.15/Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5 hydride pair produces the highest SCP of 53.25 W/kg of total mass of the system.     

Study of heat and mass transfer in MmNi4.6Al0.4 during desorption of hydrogen

In this paper, a numerical investigation of two-dimensional coupled heat and mass transfer during desorption of hydrogen in a cylindrical metal hydride reactor containing MmNi_6.4Al_0.4 is presented. By considering the variation in heat transfer fluid temperature along the axial direction (variable wall temperature boundary condition), the changes in hydride bed temperature at different axial locations are presented. The effect of variable wall temperature boundary condition on hydrogen desorption rate for different hot fluid temperatures and hydride bed thicknesses is investigated. The rate of hydrogen desorption at different hot fluid temperatures showed good agreement with the experimental data reported in the literature. As the desorption progresses, the change in heat transfer fluid temperature along the axial direction is found to decrease with time and becomes unchanged at the end of the process. The effect of variable wall temperature boundary condition on desorption time is found to be significant for the hydride bed thicknesses of about 7.5 mm and more. For a given bed thickness of 17.5 mm, the maximum difference in desorption time between variable wall and constant wall temperature convective boundary conditions is about 375 s at 303 K.     

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.     

Simulation of double-stage double-effect metal hydride heat pump

This paper presents the operational characteristics of a double-stage double-effect metal hydride heat pump (DSDE-MHHP) working with LaNi_4.1Al_0.52Mn_0.38/LmNi_4.91Sn_0.15/Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5 as high/medium/low temperature alloys. The performances of the DSDE-MHHP are predicted by solving the transient, two-dimensional, conjugate heat and mass transfer characteristics between the paired metal hydride reactors of cylindrical configuration using the finite volume method (FVM). The designed operating temperatures chosen for the present analysis are 568, 361, 296, and 289 K as heat driven (T_D), heat rejection (T_H), heat sink (T_M) and refrigeration (T_C) temperatures, respectively. The variations in hydrogen concentrations, hydride equilibrium pressures, and temperatures within the hydride beds, and the heat exchange between the hydride beds with the heat transfer fluids are presented for a complete cycle. The operating cycle of a DSDE-MHHP is explained on dynamic pressure–concentration–temperature (PCT) plot. The variation of temperatures in the reactors during hydriding and dehydriding processes is compared with experimental data and a good agreement was observed between them. For given operating temperatures of 568/361/296/289 K, the average coefficient of performance (COP) and the specific cooling power (SCP) of the system are found to be 0.471 and 28.4 W/kg of total hydride mass, respectively.     

Kinetics and electrochemical characteristics of Mg2NiH4-x wt.% MmNi3.8Co0.75Mn0.4Al0.2 (x = 5, 10, 20, 40) composites for Ni-MH battery

The structure, kinetics and electrochemical characteristics of Mg₂NiH₄-x wt.% MmNi_3.8Co_0.75Mn_0.4Al_0.2 (x = 5, 10, 20, 40) composites prepared by mechanical milling have been investigated in this paper. XRD results indicate that the as-milled Mg₂NiH₄ shows nanocrystalline or amorphous-like structure, and it does not react with MmNi_3.8Co_0.75Mn_0.4Al_0.2 during mechanical milling. As the amount of MmNi_3.8Co_0.75Mn_0.4Al_0.2 increases, the maximum discharge capacity decreases initially from 508 mAh/g (x = 5) to 440 mAh/g (x = 10) and then increases to 509 mAh/g (x = 40). Meanwhile, the capacity retention (R₁₀) increases from 12.8% (x = 5) to 23.4% (x = 40), and the corrosion potential of electrode (E_corr) increases from −0.930 V to −0.884 V (vs. Hg/HgO). Especially, the more MmNi_3.8Co_0.75Mn_0.4Al_0.2 content the composite contains, the higher high rate dischargeability (HRD) the electrode exhibits, which could be attributed to the catalytic reaction and reduction of the Mg₂NiH₄ grain size brought by MmNi_3.8Co_0.75Mn_0.4Al_0.2. The improvement in electrode kinetics has been depicted from the bulk hydrogen diffusion coefficient (D), the exchange current density (I₀) and the charge transfer resistance (R_ct) on the alloy surface.     

Parametric studies on a metal hydride based hydrogen storage device

In this paper, a two-dimensional computational investigation of coupled heat and mass transfer process in an annular cylindrical hydrogen storage device filled with _MmNi4.6Al0.4${\mathrm{MmNi}}_{4.6}{\mathrm{Al}}_{0.4}$ is presented using a commercial software FLUENT 6.1.22. Hydrogen storage performance of the device is studied by varying the operating parameters such as hydrogen supply pressure and absorption temperature. Further, the effects of various bed parameters such as hydride bed thickness and overall heat transfer coefficient on the storage performance of the device are also studied. The average temperature of the hydriding bed and hydrogen storage capacity at different supply pressures showed good agreement with the experimental data reported in the literature. It is observed that as the hydriding process is initiated, the absorption of hydrogen increases rapidly and then it slows down after the temperature of the hydride bed increases due to the heat of the reaction. At any given absorption temperature, the hydrogen absorption rate and hydrogen storage capacity are found to increase with the supply pressure. The variation in the hydrogen absorption capacity, rate of reaction and temperature profiles at different supply pressures from 5 to 35 bar in steps of 5 bar are presented. Further, the effects of overall heat transfer coefficients from 750 to 1250 W/m² K and cooling fluid temperatures from 288 to 298 K on hydrogen storage capacity are also investigated. It is shown that the heat transfer rate enhances the hydriding rate by accomplishing a rapid reaction. At the supply condition of 35 bar and 298 K, _MmNi4.6Al0.4${\mathrm{MmNi}}_{4.6}{\mathrm{Al}}_{0.4}$ stores about 13.1 g of hydrogen per kg of alloy.     

An air–metal hydride battery using MmNi3.6Mn0.4Al0.3Co0.7 in the anode and a perovskite in the cathode

Hydrogen storage alloy MmNi_3.6Mn_0.4Al_0.3Co_0.7 (MH) was tested as anode material in a metal hydride–air cell. The cathode was a non-noble metal air electrode based on a mixture of perovskite and pyrolyzed macrocycles on carbon. Polarization and discharge capacities of the electrodes were measured and compared at 22 °C and 40 °C using air or oxygen at the cathode. Discharge capacity reaching 330 mAh/g MH with pure oxygen at 40 °C and 305 mAh/g MH with air at 22 °C were obtained. Power densities and/or energy densities were found to significantly depend on the increase of the electrode kinetics on both the ORR (oxygen reduction reaction) and HOR (hydrogen oxidation reaction). However, for air electrode, an increase of oxygen concentration by using pure oxygen gas plays a more important role than an 18 °C temperature increase.     

Investigations of synthesis and characterization of MmNi4.3Al0.3Mn0.4 AND MmNi4.0Al0.3Mn0.4Si0.3, hydrogen storage materials through thermal and spin melting processes

The present study deals with investigations on the synthesis and characterization of negative electrode material for high energy density Ni-MH battery. The hydrogen storage material (MH) has been synthesized through thermal and spin melting techniques. A comparative study of materials synthesized by these two techniques with emphasis on the characteristics relevant to battery electrode applications has been carried out. In the present study, the modified composition of AB₅-type corresponds to the spin as well as thermal melted versions of MmNi_4.3Al_0.3Mn_0.4 and MmNi_4.0Al_0.3Mn_0.4Si_0.3.Structural characterization has revealed that, whereas for the spin melted MmNi_4.3Al_0.3Mn_0.4 the dominant growth is perpendicular to the c-axis, it is parallel to the c-axis for MmNi_4.0Al_0.3Mn_0.4Si_0.3. The hydrogenation behaviour of these materials has been monitored through P-C-T and kinetic curves. Attempts have been made to establish a correlation between the structure and hydrogenation behaviour. The spin melted material (MmNi_4.3Al_0.3Mn_0.4) exhibits reduced pulverization and hence is expected to have increased cycle life. This version of the material also exhibits higher storage capacity, faster kinetics and faster activation as compared to the conventionally prepared bulk form. The bulk version of the alloy with silicon has been found to undergo easy activation (2nd cycle) as compared to the bulk version of the alloy without silicon (6th cycle). The spin melted version of the material with silicon leads to smaller (finer) particle size material compared to the alloy form without silicon.     

X-ray diffraction and H-storage in ultra-small palladium particles

In situ X-ray diffraction (XRD) and gravimetric hydrogen uptake measurements of d ∼ 2–3 nm spherical PdH_x particles have been studied in the temperature and pressure range of 323 < T < 428 K and 0 < P < 10 bar. The Pd particles were protected from sintering with a hydrogen-permeable carbon coating. While only containing ∼300–1000 atoms, the Pd particles were found to exhibit the same fcc structure and lattice constant as the bulk. Our isothermal studies show that, with increasing x, these highly crystalline PdH_x nanoparticles also exhibit a complete transformation from the dilute α solid solution phase to the more concentrated β hydride phase. However, we observed that the character of the α–β phase transition in these nanoparticles is very different from that in the bulk. Indeed, the hydrogen uptake isotherm exhibits a noticeable positive slope in the α + β co-existence region. Furthermore, we also observed a noticeable narrowing of the α + β co-existence region (δx) in the nanoparticles. Also, a significant suppression of the critical temperature T_c for the phase boundary was observed: T_c(nano) ≈ 430 K while T_c(bulk) ≈ 570 K. These results signal a significant change in the thermodynamic behavior of very small hydride nanoparticles that may be common to many other nano-scale metal hydride systems as well.     

MmNi3.55Co0.75Mn0.4Al0.3B0.3 hydrogen storage alloys for high-power nickel/metal hydride batteries

Non-stoichiometric La-rich MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 hydrogen storage alloys using B–Ni or B–Fe alloy as additive and Ce-rich MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 one using pure B as additive have been prepared and their microstructure, thermodynamic, and electrochemical characteristics have been examined. It is found that all investigated alloys show good activation performance and high-rate dischargeability though there is a certain decrease in electrochemical capacities compared with the commercial MmNi_3.55Co_0.75Mn_0.4Al_0.3 alloy. MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 alloys using B–Ni alloy as additive or adopting Ce-rich mischmetal show excellent rate capability and can discharge capacity over 190 mAh/g even under 3000 mA/g current density, which display their promising use in the high-power type Ni/MH battery. The electrochemical performances of these MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 alloys are well correlated with their microstructure, thermodynamic, and kinetic characteristics.     

Effect of boron additive on the cycle life of low-Co AB5-type electrode consisting of alloy prepared by cast and rapid quenching

In order to modify the cycle stability of low-Co AB₅-type alloy, a trace of boron was added in MmNi_3.8Co_0.4Mn_0.6Al_0.2 hydrogen storage alloy. The low-Co AB₅-type alloys MmNi_3.8Co_0.4Mn_0.6Al_0.2B_x(x=0, 0.1, 0.2, 0.3, 0.4) were prepared by cast and rapid quenching. The cycle lives and microstructures of the as-cast and quenched alloys were measured and analyzed. The effects of boron additive on the microstructures and cycle lives of as-cast and quenched alloys were investigated comprehensively. The obtained results showed that the addition of boron could dramatically enhance the cycle lives of the as-cast and quenched alloys. When boron content x increases from 0 to 0.4, the cycle lives of the as-cast alloys were increased from 118 to 183 cycles, and for as-quenched alloys with quenching rate of 38 m/s from 310 to 566 cycles.     

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.     

Analysis of crossed van't Hoff metal hydride based heat pump

In this paper, the operating feasibility of a single-stage metal based hydride heat pump (SS-MHHP) working on the principle of crossed van't Hoff line concept is presented. The performance of the system is predicted by solving the unsteady, two-dimensional mathematical model in an annular cylindrical configuration employing two different hydride alloy pairs, namely, V_0.846Ti_0.104Fe_0.05/Fe_0.9Mn_0.1Ti and V_0.855Ti_0.095Fe_0.05/MmNi_4.7Al_0.3 (regeneration alloy/refrigeration alloy). The influences of heat source (T_H) and refrigeration (T_C) temperatures on the amount of hydrogen transferred between the paired reactors, coefficient of performance (COP) and specific cooling power (SCP) of the crossed van't Hoff SS-MHHP system are studied. Within the selected ranges of operating temperatures, the COP of the crossed van't Hoff SS-MHHP is about 60% higher than the conventional single-stage MHHP. The optimum operating temperatures of V_0.846Ti_0.104Fe_0.05/Fe_0.9Mn_0.1Ti and V_0.855Ti_0.095Fe_0.05/MmNi_4.7Al_0.3 combinations are found to be 373/303/291 K and 400/303/283 K (heat source/heat sink/refrigeration temperatures), respectively. At the optimum operating temperatures, the COP and SCP of the V_0.846Ti_0.104Fe_0.05/Fe_0.9Mn_0.1Ti and V_0.855Ti_0.095Fe_0.05/MmNi_4.7Al_0.3 combinations are 0.89 and 30.8 W/kg of total mass and 0.86 and 30.3 W/kg of total mass, respectively.     

Correlation between microstructure and hydrogen storage capacity in MmNi5 alloys with Al,Mn and Sn substitutions

The morphology and compositions of the phases present in the microstructures of annealed hydrogen storage MmNi_5−xAl_x (x = 0.4 and 0.8), MmNi_5−xMn_x (x = 0.4 and 0.8) and MmNi_4.6Sn_0.4 alloys (Mm-Indian Mischmetal) have been studied. Al, Mn and Sn substitutions in MmNi₅ produce two-phase microstructures. The precipitates tend to become globular upon annealing. The hydrogen storage capacity of these alloys correlates with the volume fraction of the matrix phase. The higher the matrix volume fraction, the higher the hydrogen storage capacity. The elements that constitute Mm are present in greater proportion in the major constituent matrix phase of the two phase structure. Only trace amounts of these elements could be found in the precipitate phase in the Al- and Mn- substituted alloys. However, for the Sn-substituted composition, the proportion of these elements is nearly the same in matrix and precipitate phase. © 1999 International Association for Hydrogen Energy.     

Electrochemical PCT isotherm study of hydrogen absorption/desorption in AB5 type intermetallic compounds

The enthalpy (ΔH) and entropy (ΔS) of hydride formation/decomposition could be determined either experimentally or theoretically based on models proposed in the literature. The experimental pathway includes gas/solid-phase measurement of pressure–composition–temperature (PCT) isotherms at different temperatures. This measurement is followed by plotting of van't Hoff dependences and evaluation of the ΔH and ΔS from their slopes and intersects, respectively. In this study we demonstrate the applicability of electrochemical PCT isotherm measurements as an advanced method for thermodynamic analysis of hydrogen adsorption/desorption process. Experimentally this is done by electrochemical charging/discharging of an electrode, prepared from AB₅ type alloy with MmNi_4.6Co_0.6Al_0.8 composition (Mm – mischmetal). In addition, the hydride formation as a result of the electrochemical charging is independently confirmed by ex-situ XRD diffraction. Our work demonstrates that not only the electrochemical approach is a viable alternative of PCT gas/solid-phase measurement but it also represents a safer, cost-effective and faster protocol than its hydrogen gas–solid phase equivalent.     

Destabilization of LiAlH4 by nanocrystalline MgH2

In this work, we report the synthesis, characterization and destabilization of lithium aluminum hydride by ad-mixing nanocrystalline magnesium hydride (e.g. LiAlH₄ + nanoMgH₂). A new nanoparticulate complex hydride mixture (Li–nMg–Al–H) was obtained by solid-state mechano-chemical milling of the parent compounds at ambient temperature. Nanosized MgH₂ is shown to have greater and improved hydrogen performance in terms of storage capacity, kinetics, and initial temperature of decomposition, over the commercial MgH₂. The pressure–composition isotherms (PCI) reveal that the destabilized LiAlH₄ + nanoMgH₂ possess ∼5.0 wt.% H₂ reversible capacity at T ≤ 350 °C. Van't Hoff calculations demonstrate that the destabilized (LiAlH₄ + nanoMgH₂) complex materials have comparable enthalpy of hydrogen release (∼85 kJ/mole H₂) to their pristine counterparts, LiAlH₄ and MgH₂. However, these new destabilized complex hydrides exhibit reversible hydrogen sorption behavior with fast kinetics.     

Mixing effects of metal hydrides on equilibrium behavior and reaction kinetics

Mixing effects of hydriding alloys on equilibrium and kinetic properties are studied, and experimental results are presented for the binary mixtures of LaNi₅/TiMn_1.5, MmNi₅/TiMn_1.5, MmNi_4.8Al_0.2/TiMn_1.5, and LaNi₅/Ti_0.8Zr_0.2Cr_0.8Mn_1.2 under isothermal conditions of 30–60°C with 10° intervals. Major known advantages of mixing are to give desirable P-T-C properties, to enhance reaction rate, and to increase the available temperature and pressure ranges. The authors' objective is to provide a method of designing a particular pair of hydride mixtures with increased hydride composition ranges for transferring hydrogen fairly quickly, increasing availability, improving unit performance, and increasing operational flexibility for a given application.