Thermodynamic simulation of hydrogen based thermochemical energy storage system

The increasing energy demand needs the attention for energy conservation as well as requires the utilisation of renewable sources. In this perspective, hydrogen provides an eco-friendly and regenerative solution toward this matter of concern. Thermochemical energy storage system working on gas-solid interaction is a useful technology for energy storage during the availability of renewable energy sources. It provides the same during unavailability of energy sources. This work presents a performance analysis of metal hydride based thermal energy storage system (MH-TES), which can transform the waste heat into useful high-grade heat output. This system opens new doors to look at renewable energy through better waste heat recovery systems. Experimentally measured PCIs of chosen metal hydride pairs, i.e. LaNi_4.6Al_0.4/La_0.9Ce_0.1Ni₅ (A-1/A-3; pair 1) and LaNi_4.7Al_0.3/La_0.9Ce_0.1Ni₅ (A-2/A-3; pair 2) are employed to estimate the thermodynamic performance of MH-TES at operating temperatures of 298 K, 373 K, 403 K and 423 K as atmospheric temperature (T_atm), waste heat input temperature (T_m), storage temperature (T_s) and upgraded/enhanced heat output temperature (T_h) respectively. It is observed that the system with alloy pair A-1/A-3 shows higher energy storage density of 121.83 kJ/kg with a higher COP of 0.48 as compared to A-2/A-3 pair. This is due to the favourable thermodynamic properties, and the pressure differential between coupled MH beds, which results in higher transferrable hydrogen. Besides, the effect of operating temperatures on COP is studied, which can help to select an optimum temperature range for a particular application.     

Performance investigation of double-stage metal hydride based heat pump

In this paper, the performance investigation 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 and Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5 hydride alloys is presented. The effects of half cycle time (θ), hydride mass ratio (M_R), sensible heat exchange factor (?) and operating temperatures, viz. heat source (T_D), heat sink (T_M), 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 DSDE-MHHP system are investigated. For the present analysis the heat rejection temperature (T_H) is maintained constant at 373 K. Numerically predicted hydride bed temperatures are compared with experimental data and a good agreement is observed between them. It is observed from the numerical results that the COP and SCP of the DSDE-MHHP system increase with heat source and refrigeration temperatures, and decrease with heat sink temperature. For operating temperatures of 578, 373, 298 and 283 K (T_D, T_H, T_M and T_C), the average COP and SCP of the system are found to be 0.81 and 48.1 W/kg of total alloy mass, respectively.     

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.     

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.     

Dynamic hydrogen sorption and its influence on metal hydride heat pump operation

Hydride units containing MmNi_4.5Al_0.5 (Mm = Mischmetall; MNA), LaNi₅ (LN) and LaNi_4.7Al_0.3 (LNA) as hydrogen storage materials were combined to simple heat pumps after the investigation of their dynamic sorption behaviour. These heat pumps were tested in the “upgrading” mode without any external user. The combination LNA/LN showed a maximum temperature increase of 13.9 K at inlet water temperatures of T_i = 286 K, T_m = 338.5 K (cycle time: 12 min); for the LNA/MNA combination at inlet water temperatures of T_i = 286 K and T_m = 353 K, the observed temperature increase was 12.3 K (cycle time: 12 min) and 17.6 K (cycle time: 19 min), respectively. The operation of the devices was found to be sensitive to characteristics of the hydride materials (e.g. hysteresis, plateau slope) as to those of the hydride bed and the container design (heat conductivity, heat capacity).     

Performance analysis of LaNi5 added with expanded natural graphite for hydrogen storage system

This paper presents a comparative study of two cases of metal hydride hydrogen storage units working on (i) LaNi₅ (ii) Compacts of LaNi₅ incorporated with expanded natural graphite (ENG). It has been observed from the previous studies that the hydriding/dehydriding reactions eventually causes large strain changes, due to which the hydride forming metal alloys disintegrate and form a powder bed. Such reactor beds usually have a low thermal conductivity which minimizes the heat transfer phenomenon occurring during the absorption of hydrogen gas. Therefore, there is a need to implement heat augmentation methods to significantly enhance the thermal conductivity. The objective of this research is to present a 2-D numerical model using Finite Volume Method (FVM) and estimate the hydrogen storage performance of a cylindrical metal hydride bed for both the cases, i.e. powdered metal hydride bed and ENG compacts-based reactor bed at different values of inlet pressure and heat transfer fluid temperature. In this study, a detailed investigation on the absorption process reveals that reactor beds with compacted disks of LaNi₅ and ENG demonstrate an enhanced effective thermal conductivity and efficient mass transfer. The simulation results show that a remarkable improvement in the heat transfer and hydrogen storage capacity with reduced absorption time can be achieved by using LaNi₅ and ENG compacts. It was observed that the average reactor bed temperature dropped from 345.13 K to 337.37 K when the ENG based compacted disks was introduced into the reactor bed. Moreover, for supply pressure of 24 bar and fluid temperature of 293 K, the time taken to absorb hydrogen into the rector to achieve stabilized hydrogen storage capacity was estimated to be 446s and 232 s for the case of metal hydride and ENG compacts-based bed, respectively.     

Computational study of metal hydride cooling system

A computational study of a metal hydride cooling system working with MmNi_4.6Al_0.4/MmNi_4.6Fe_0.4 hydride pair is presented. The unsteady, two-dimensional mathematical model in an annular cylindrical configuration is solved numerically for predicting the time dependent conjugate heat and mass transfer characteristics between coupled reactors. The system of equations is solved by the fully implicit finite volume method (FVM). The effects of constant and variable wall temperature boundary conditions on the reaction bed temperature distribution, hydrogen concentration, and equilibrium pressures of the reactors are investigated. A dynamic correlation of the pressure–concentration–temperature plot is presented. At the given operating temperatures of 363/298/278 K (T_H/T_M/T_C), the cycle time for the constant and variable wall temperature boundary conditions of a single-stage and single-effect metal  hydride system are found to be 1470.0 s and 1765.6 s, respectively. The computational results are compared with the experimental data reported in the literature for LaNi_4.61Mn_0.26Al_0.13/La_0.6Y_0.4Ni_4.8Mn_0.2 hydride pair and a good agreement between the two was observed.     

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.     

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.     

Design methodology and thermal modelling of industrial scale reactor for solid state hydrogen storage

In this study, a novel set of comprehensive arithmetic correlations has been proposed to design an industrial scale cylindrical reactor with embedded cooling tubes (ECT) for metal hydride (MH) based hydrogen storage and thermal management applications. Based on ASME standards, different nominal pipe sizes were imparted into a cylindrical reactor design with ECT to accommodate 50 kg of LaNi_4.7Al_0.3 alloy. A three dimensional numerical model has been developed using COMSOL Multiphysics 4.3a to predict the hydriding performance of designed reactors, which was further experimentally validated as well. At an absorption condition of 30 bar supply pressure and 298 K absorption temperature with 60 lpm volumetric HTF flow rate, 6 inch reactor with 99 ECT portrayed better heat transfer characteristics. From the parametric investigation, it is observed that the variation of supply pressure has predominant effect followed by the variation of the HTF flow rate on hydriding (absorption) kinetics of the device. However, the variation of absorption temperature has minuscule influence on the hydriding performance. At a supply condition of 30 bar and 298 K with water flow rate of 30 lpm, a hydrogen storage capacity (HSC) of 1.29 wt% was achieved within 2060 s.     

Experimental and numerical study of the isotherms and determination of physicochemical parameters of the hydrogen absorption/desorption process by the metal hydrides

In this work, absorption/desorption isotherms of the LaNi_3·6Mn_0·3Al_0·4Co_0.7 alloy, have been determined from the experimental data at three temperatures (T_Fluid = 298 K, T_Fluid = 303 K, and T_Fluid = 313 K). However, the experimental isotherms are compared with a proposed theoretical model. The physicochemical parameters of the proposed model are determined from the experimental data. Using these parameters, the absorption and desorption processes of hydrogen by the LaNi_3·6Mn_0·3Al_0·4Co_0.7 alloy can be compared. During the absorption/desorption process, the behaviors of each parameter are studied under the effect of temperature and pressure. In addition, internal energy, entropy, and enthalpy are calculated by using the proposed model. On the other hand, the temperature and pressure effects on the variation of these functions have been studied. The calculated physicochemical parameters suggested that the hydrogen absorption/desorption process in the LaNi_3·6Mn_0·3Al_0·4Co_0.7 alloy was feasible, spontaneous and exothermic in nature.     

Development of a metal hydride refrigeration system as an exhaust gas-driven automobile air conditioner

Aiming at developing exhaust gas-driven automobile air conditioners, two types of systems varying in heat carriers were preliminarily designed. A new hydride pair LaNi_4.61Mn_0.26Al_0.13/La_0.6Y_0.4Ni_4.8Mn_0.2 was developed working at 120–200 °C/20–50 °C/−10–0 °C. P-C isotherms and reaction kinetics were tested. Reaction enthalpy, entropy and theoretical cycling coefficient of performance (COP) were deducted from Van’t-Hoff diagram. Test results showed that the hydride pair has flat plateau slopes, fast reaction dynamics and small hystereses; the reaction enthalpy of the refrigeration hydride is −27.1 kJ/mol H₂ and system theoretical COP is 0.711. Mean particle sizes during cycles were verified to be an intrinsic property affected by constitution, heat treatment and cycle numbers rather than initial grain sizes. Based on this work pair, cylindrical reactors were designed and a function proving metal hydride intermittent refrigeration system was constructed with heat conducting oil as heat source and water as heat sink. The reactor equivalent thermal conductivity is merely 1.3 W/(m K), which still has not meet practical requirement. Intermittent refrigeration cycles were achieved and the average cooling power is 84.6 W at 150 °C/30 °C/0 °C with COP being 0.26. The regulations of cycling performance and minimum refrigeration temperature (MRT) were determined by altering heat source temperature. Results showed that cooling power and system COP increase while MRT decreases with the growth of heat source temperature. This study develops a new hydride pair and confirms its application in automobile refrigeration systems, while their heat transfer properties still need to be improved for better performance.     

Heat Transfer and Thermodynamic Performance of LiBr/H2O Absorption Heat Transformer with Vapor Absorption Inside Vertical Spiral Tubes

In this paper, an absorption heat transformer (AHT) with falling film of aqueous LiBr solution inside vertical spiral tubes is installed and tested. The variations of coefficient of performance (COP), thermal efficiency (E_th ), and the heat transfer coefficient of the absorber at different falling film flow rates, hot water flow rates, and operating temperatures are investigated experimentally. The results demonstrated that the coefficient of performance and thermal efficiency of the system decrease with the increase in the flow rate of LiBr solution, and the influence of flow rate of hot water on COP and E_th is insignificant. The available COP in the experiments is higher than 0.4. The heat and mass transfer coefficients of the absorber increase with the increase of the flow rate of LiBr solution, up to 400W/m²/K and 0.013 kg/m²/s (temperature of waste heat is 90°C). The heat transfer coefficient of the absorber increases with the increase of the temperature of waste heat, and decreases with the increase of the cooling water temperature. Meanwhile, the computer code ABSIM (Absorption Simulation) is used to simulate the AHT systems, and the simulated results are compared with the experimental data.     

Thermodynamic and electrochemical hydrogenation properties of LaNi5 − xInx alloys

Hydrogenation properties of LaNi_5 − xIn_x alloys (x = 0.1, 0.2 and 0.5) were examined by their direct reaction with gaseous hydrogen and by cathodic charging in 6 M KOH solution. The gas phase measurements were carried out using Sievert's type apparatus in 300–400 K temperature range and at hydrogen pressures up to 40 bars. Indium substitution for Ni in LaNi₅ significantly modifies the hydrogenation behavior, decreasing the equilibrium pressure of hydrogen and limiting the hydrogen capacity as compared to LaNi₅. The LaNi_4.9In_0.1 revealed a distinct presence of two pressure plateaus on the high temperature isotherms. Apart from the α-phase (hydrogen solid solution) and β-phase (LaNi₅H₆ hydride), formation of a new σ*-hydride phase was postulated at the hydrogen content extended over the region of H/f.u. = 1.3–1.8. Thermodynamic functions: enthalpy and entropy of the hydrogen absorption process were calculated from the H₂-pressure/composition (p–c) isotherms at several temperatures, applying the Van't Hoff's (lnp − 1/T) dependence. Electrochemical galvanostatic hydrogenation experiments at 185 mA/g charge/discharge rate revealed the greatest discharge current capacity of 319 mAh/g for LaNi_4.9In_0.1 alloy after 4–5 cycles. The hydrogen discharge capacities decrease with further increase of indium content in the alloy.     

Analysis of a novel metal hydride cycle for simultaneous heating and cooling

Out of the many promising applications of metal hydrides, refrigeration, heat pumping and heat transformation are important. In order to achieve improved performance, a novel three-alloy cycle is proposed in which, for heat input at an intermediate temperature, heat outputs at high temperature and also at warm temperature are obtained in addition to refrigeration. The performance of this cycle using the alloys LaNi_4.6Sn_0.4, LaNi_4.7Al_0.3 and MmNi_4.5Al_0.5 is studied based on thermodynamics and reaction kinetics. Coefficients of performance, half-cycle times and specific alloy outputs are evaluated.     

Preparation of the composite LaNi3.7Al1.3/Ni–S–Co alloy film and its HER activity in alkaline medium

The composite LaNi_3.7Al_1.3/Ni–S–Co alloy film was prepared by molten salt electrolysis and aquatic electrodeposition orderly. With Na₃AlF₆–La₂O₃–Al₂O₃ (91:8:1) system as molten salt electrolyte, the LaNi_3.7Al_1.3 alloy film was obtained by galvanostatic electrolysis at 100 mA cm⁻². The results showed that the La³⁺ and Al³⁺ ions could be co-reduced on the nickel cathode to form LaNi_3.7Al_1.3 film, i.e. La³⁺ + 1.3Al³⁺ + 6.9e + 3.7Ni = LaNi_3.7Al_1.3 at c.a. −0.5 V, which is much lower than that of the theoretical decomposition potential of lanthanum and aluminum. With high HER activity, the composite LaNi_3.7Al_1.3/Ni–S–Co film (η₁₅₀ = 65 mV, 353 K) could absorb large amount of H atoms, which would be oxidized and therefore effectively avoid the dissolution of the Ni–S–Co film under the state of open-circuit and consequently prolong the lifetime of the cathode.     

Investigation of coupled AB5 type high-power metal hydride reactors

The objective of this study is to investigate the coupled AB₅ type high-power metal hydride reactors in thermal energy applications. A system to test the reactors was set up coupling two reactors containing either LaNi_xAl_5−x or Ca_yMm_1−yNi₅ alloys, used as low and high pressure metal hydrides, respectively. In order to develop the high-power reactor, the metal hydrides were thinly coated (about 1–2 μm thickness) with copper and compressed to form the Porous Metal Hydrides (PMH) compacts. During the experiments, the dynamic behaviors of the reactive kinetics of the system were monitored. Among the tested systems, the coupling of Ca_0.6Mm_0.4Ni₅ and LaNi_4.75Al_0.25 was the most effective for thermal energy applications. It took the least time to reach the equilibrium state in both hydriding and dehydriding processes (approximately 250 s) and had the highest amount of heat generation/absorption. The smaller the value of y in Ca_1−yMm_yNi₅ alloys, causing the alloy to contain more calcium, the faster the reaction kinetics. In the case of the LaNi_xAl_5−x reactor, the addition of aluminum enhanced the reaction kinetics. Moreover, the reactor with the low pressure metal hydride, LaNi_xAl_5−x, took a longer period of time to reach the equilibrium state than when the high pressure metal hydride, Ca_1−yMm_yNi₅, was employed.