Dynamic response of a unitized regenerative fuel cell under various ways of mode switching

Mode switching is an important process in unitized regenerative fuel cells. The complex interactions of heat and mass transfer during the operation of mode switching have a significant effect on cell performance. Twelve different ways of mode switching were proposed by controlling inlet boundary conditions of supplies and operating voltage. Numerical simulations were applied to analyze the dynamic response of heat and mass transfer as well as electrochemical signals under the different ways of mode switching. Current density increased with mass fraction of reactants. Cell heat source had an instant response to current density, but the temperature was slow to respond to the heat source. Hydrogen‐side inlet velocity had minimal impact on mode switching. The time for cell reaching stability increased with the increase of voltage change time, and the time for current density, mass transfer, and temperature reaching stable values increased in order. Unitized regenerative fuel cell had similar dynamic response in the 2 period: cell temperature increased in the fuel cell mode and decreased in the water electrolysis mode after mode switching.     

Theory,modelling and performance measurement of unitised regenerative fuel cells

Unitised Regenerative Fuel Cells (URFCs) based on Proton Exchange Membrane (PEM) technology provide a promising opportunity for reducing the cost of the hydrogen subsystem used in renewable-energy hydrogen systems for remote area power supply. A general theoretical relationship between cell potential and current density of a single-cell URFC operating in both fuel-cell and electrolyser modes is derived using the Butler–Volmer equation for both oxygen and hydrogen electrodes, and accounting for membrane resistance and mass transport losses. Modifying the standard Butler–Volmer equation with a denominator term containing two additional ‘saturation’ parameters to reflect mass transport constraints generates voltage–current curves that are much closer to experimentally obtained polarisation curves in both modes. The theoretical relationship is used to construct a computer model based on Excel and Visual Basic to generate voltage–current curves in both electrolyser and fuel cell modes for URFCs with a range of membrane electrode assembly characteristics. Hence the influence of key factors such as exchange current densities and charge transfer coefficients on cell performance is analysed. Experimental results for voltage–current curves from singe-cell URFCs with a number of different oxygen-side catalysts are reported, and compared to the theoretically modelled curves. Generally values have been found for exchange current densities, charge transfer coefficients, and saturation current densities that give a close fit between the empirical and theoretically generated curves. The values found conform well to expectations based on the catalyst loadings, in partial confirmation of the validity of the modelling approach. The model thus promises to be a useful tool in identifying electrodes with materials and structures, together with optimal catalyst types and loadings that will improve URFC performance.     

A three‐dimensional heat transfer model for thermal performance evaluation of ZrCo‐based hydride bed with embedded circular‐shaped cooling tubes

Nowadays, metal hydrides are generally deemed as one of the most potential materials that are in favor of compact hydrogen storage for industrial applications. This work was committed to evaluate the thermal performance of a circular‐shaped‐tube thin double‐layered hydrogen storage reactor using a three‐dimensional model. Finite element simulations were conducted to systematically study the influences of structural geometries, cooling patterns, and material thermophysical properties on the heat diffusion behavior under the framework of convection heat transfer. The results indicate that the proposed model effectively characterizes temperature evolutions during hydrogen absorption process. Moreover, a statistical analysis was performed to reveal the sensitivity sequence of these factors on the total thermal performance, suggesting that decreasing the hydride layer thickness, increasing the number of U‐shaped cooling tubes, accelerating the cooling fluid flow rate, and enhancing the thermal conductivity are more beneficial to the thermal performance improvement. Detailed analysis confirms the possibility of developing the present hydrogen storage tank utilizing metal hydrides for engineering practice.     

Thermal analysis of high-pressure hydrogen during the discharging process

The transient temperature and pressure of hydrogen are measured during the hydrogen discharging process through an orifice in a high-pressure vessel. The initial pressures of hydrogen in the vessel are set to approximately 30, 60, and 100 MPa. The mass flow rate and heat flux between hydrogen and the inner wall of the vessel are theoretically estimated using fundamental equations and experimental results with accurate thermophysical properties of hydrogen. The generation of temperature distribution and flow due to heat transfer in the vessel during discharge is verified by numerical analysis. Further, the relationship between reference gas temperature and heat flux in the vessel during the release of high-pressure hydrogen is studied. The average heat flux in the vessel is calculated using experimental and numerical analysis. The appropriate reference temperature is obtained using the comparison of the average heat flux in the vessel. In addition, the dominant heat transfer mode during hydrogen discharge is investigated. Numerical analysis shows that natural convection is formed inside the vessel due to a decrease in temperature. The Nusselt numbers in this process are presented as a function of Rayleigh numbers which are obtained by the experimental results and mass and energy conservations. The relationship between the Nusselt and Rayleigh numbers agrees with the heat transfer correlations of natural convections.     

Review of metal hydride hydrogen storage thermal management for use in the fuel cell systems

Thermal management of metal hydride (MH) hydrogen storage systems is critically important to maintain the hydrogen absorption and release rates at desired levels. Implementing thermal management arrangements introduces challenges at system level mostly related to system's overall mass, volume, energy efficiency, complexity and maintenance, long-term durability, and cost. Low effective thermal conductivity (ETC) of the MH bed (~0.1–0.3 W/mK) is a well-known challenge for effective implementation of different thermal management techniques. This paper comprehensively reviews thermal management solutions for the MH hydrogen storage used in fuel cell systems by also focusing on heat transfer enhancement techniques and assessment of heat sources used for this purpose. The literature recommended that the ETC of the MH bed should be greater than 2 W/mK, and heat transfer coefficient with heating/cooling media should be in the range of 1000–1200 W/m²K to achieve desired MH's performance. Furthermore, alternative heat sources such as fuel cell heat recovery or capturing MH heat during charging and releasing it back during discharging have also been thoroughly reviewed here. Finally, this review paper highlights the gaps and suggests directions accordingly for future research on thermal management for MH systems.     

Investigation of three‐dimensional heat and mass transfer in a metal hydride reactor

A mathematical model for three‐dimensional heat and mass transfer in metal–hydrogen reactor is presented. The model considers three‐dimensional complex heat, and mass transfer and chemical reaction in the reactor. The main parameter in hydriding processes is found to be the equilibrium pressure, which strongly depends on temperature. Hydride formation enhanced at regions with lower equilibrium pressure. Hydriding processes are shown to be two dimensional for the system considered in this study. Effects of heat transfer rate and R/H (radius to height) ratio on hydride formation are investigated. Hydride formation increases significantly with larger heat transfer rate from the boundary walls, however after a certain heat transfer rate, the increase in formation rate is found to be not significant, due to the low thermal conductivity of the metal‐hydride systems. The estimated results agree satisfactorily with the experimental data in the literature. Copyright © 2002 John Wiley & Sons, Ltd.     

Performance investigation of a two-stage sorption hydrogen compressor

Metal hydrides (MH) are widely investigated for several thermodynamic applications; sorption hydrogen compressor (SHC) is one among them. In this study, the thermodynamic performance and heat – mass transfer behaviour of a two-stage sorption hydrogen compressor (TSSHC) are investigated with the employment of La_0·9Ce_0·1Ni₅ and MmNi_4.8Al_0.2 alloys in series. The hydrogen supply and the discharge temperatures are chosen as 20 °C and 80 °C, respectively. The thermodynamic performance data, i.e. compressor work and efficiency, are evaluated using the experimentally measured pressure-concentration-isotherm (PCI) and thermodynamic properties. In contrast, the heat and mass transfer behaviour is predicted by solving governing equations through the finite volume method (FVM). The numerical model is validated with experimental PCIs, and the results are in close agreement. The predicted cycle time is 75 min, comprising hydrogen supply, sensible heating and cooling, and hydrogen delivery. The TSSHC possessed a compression ratio of 9.5 and a cycle efficiency of 11.4% in which the hydrogen supply pressure is 9 bar using 0.5 kg of each alloy. Later, the influence of mass transfer on overall compressor work, heat input and efficiency is also presented.     

CFD simulation of heat transfer and phase change characteristics of the cryogenic liquid hydrogen tank under microgravity conditions

The heat transfer and phase change processes of cryogenic liquid hydrogen (LH₂) in the tank have an important influence on the working performance of the liquid hydrogen-liquid oxygen storage and supply system of rockets and spacecrafts. In this study, we use the RANS method coupled with Lee model and VOF (volume of fraction) method to solve Navier-stokes equations. The Lee model is adopted to describe the phase change process of liquid hydrogen, and the VOF method is utilized to calculate free surface by solving the advection equation of volume fraction. The model is used to simulate the heat transfer and phase change processes of the cryogenic liquid hydrogen in the storage tank with the different gravitational accelerations, initial temperature, and liquid fill ratios of liquid hydrogen. Numerical results indicate greater gravitational acceleration enhances buoyancy and convection, enhancing convective heat transfer and evaporation processes in the tank. When the acceleration of gravity increases from 10^?2 g0 to 10^?5 g0, gaseous hydrogen mass increases from 0.0157 kg to 0.0244 kg at 200s. With the increase of initial liquid hydrogen temperature, the heat required to raise the liquid hydrogen to saturation temperature decreases and causes more liquid hydrogen to evaporate and cools the gas hydrogen temperature. More cryogenic liquid hydrogen (i.e., larger the fill ratio) makes the average fluid temperature in the tank lower. A 12.5% reduction in the fill ratio resulted in a decrease in fluid temperature from 20.35 K to 20.15 K (a reduction of about 0.1%, at 200s).     

Electrode structure optimization combined with water feeding modes for Bi-Functional Unitized Regenerative Fuel Cells

Bi-functional Unitized Regenerative Fuel Cells (URFCs) based on proton-exchange membrane (PEM) electrolyte are promising in the field of energy storage. Their performance was investigated with catalyst loadings, carbon paper hydrophobicity and water feeding modes in both fuel cell (FC) and water electrolysis (WE) operations. The optimum membrane electrode assembly (MEA) is with 0.20 mg cm⁻² Pt (in 50% Pt/C catalyst) for the hydrogen electrode (HE), and a total catalyst loading of 0.80 mg cm⁻² for the oxygen electrode (OE). And its performance can reach 2.18 A cm⁻² at 1.80 V in WE test and 0.64 A cm⁻² at 0.60 V in FC test with air oxidant. Comparison of different carbon papers reveals that suitable hydrophobicity of OE carbon papers is obtained when the PTFE content accounts for 26.95 wt.%. Choice of water feeding modes, which has an influence on URFC performance in WE mode, is proven to have relations with current density and carbon paper hydrophobicity in a sophisticated way.     

Development and preliminary testing of a unitized regenerative fuel cell based on PEM technology

Results related to the development and testing of a unitized regenerative fuel cell (URFC) based on proton-exchange membrane (PEM) technology are reported. A URFC is an electrochemical device which can operate either as an electrolyser for the production of hydrogen and oxygen (water electrolysis mode) or as a H₂/O₂ fuel cell for the production of electricity and heat (fuel cell mode). The URFC stack described in this paper is made of seven electrochemical cells (256 cм² active area each). The nominal electric power consumption in electrolysis mode is of 1.5 kW and the nominal electric power production in fuel cell mode is 0.5 kW. A mean cell voltage of 1.74 V has been measured during water electrolysis at 0.5 A cm⁻² (85% efficiency based on the thermoneutral voltage of the water splitting reaction) and a mean cell voltage of 0.55 V has been measured during fuel cell operation at the same current density (37% electric efficiency based on the thermoneutral voltage). Preliminary stability tests are satisfactory. Further tests are scheduled to assess the potentialities of the stack on the long term.     

Enhancement of hydrogen charging in metal hydride-based storage systems using heat pipe

Heat transfer in metal hydride bed significantly affects the performance of metal hydride reactors (MHRs). Enhancing heat transfer within the reaction bed improves the hydriding rate. This study presents performance analysis in terms of storage capacity and time of three different cylindrical MHR configurations using storage media LaNi₅: a) reactor cooled with natural convection, b) reactor with a heat pipe on the central axis, c) reactor with finned heat pipe. This study shows the impact of using heat pipes and fins for enhancing heat transfer in MHRs at varying hydrogen supply pressures (2–15 bar). At any absorption temperature, hydrogen absorption rate and hydrogen storage capacity increase with the supply pressure. Results show that using a heat pipe improves hydrogen absorption rate. It was found that finned heat pipe has a significant effect on the hydrogen charge time, which reduced by approximately 75% at 10 bar hydrogen supply pressure.     

Complete and reduced models for metal hydride reactor with coiled-tube heat exchanger

Thermal effects during hydriding/dehydriding have a significant influence on the performance of metal hydride hydrogen storage system. The heat exchanger is widely used in the metal hydride reactor in order to improve the efficiency of system. In this work, based on mass balance, momentum balance, energy balance equations, equation of reaction kinetics and equilibrium pressure equation, a two dimensional axisymmetric model of metal hydride reactor packed with LaNi₅ is developed on Comsol platform. The model is validated by comparing its simulation results with the experiment data and the simulation results from other works. Then, the straight pipe heat exchanger and the coiled-tube heat exchanger are taken into consideration in order to improve heat transfer from metal hydride reactor to ambient environment. The complete three dimensional model is developed for the metal hydride reactor equipped with the coiled-tube heat exchanger. The case with coiled-tube heat exchanger shows better efficiency than the other. In general, the temperature in central area is higher than others. In order to cool central area effectively, two designs of heat exchangers, including the combination of coiled-tube heat exchanger and straight pipe heat exchanger and the concentric dual coiled-tube heat exchanger, are studied. The results show that it is an effective method to improve the efficiency of metal hydride reactor by equipping dual coiled-tube heat exchangers. Reduced two dimensional model is applied to metal hydride reactor with coiled-tube heat exchanger to reduce computing time. The simulation results of reduced model generally agree with those of complete three dimensional model.     

Effect of mode switching on the temperature and heat flux in a unitized regenerative fuel cell

Unitized regenerative fuel cells operate in not only fuel cell but also water electrolyzer mode. Heat management is important for the stable operation of unitized regenerative fuel cells. In this work, temperature and heat flux on the surface of the gas diffusion layer at the hydrogen side of a unitized regenerative fuel cell are experimentally measured using thin film sensors. Four pairs of sensors with good linear relation coefficient are inserted in the unitized regenerative fuel cell. The variation of temperature and heat flux on the gas diffusion layer surface during mode switching is obtained. The effect of mode switching on temperature and heat flux in the unitized regenerative fuel cell is analyzed. Experimental results show that reactant switching significantly affects temperature and heat flux. Reactant switching also causes decreased temperature and variation in heat flux. Despite of the decrease of temperature caused by the low-temperature water, the temperature increases with the operation of the URFC. When the effect of reactant switching is ignored, temperature is further found to increase in fuel cell and water electrolyzer modes, and heat flux remains relatively stable.     

Influence of PTFE coating on gas diffusion backing for unitized regenerative polymer electrolyte fuel cells

Gas diffusion backings (GDBs) with various PTFE loadings for unitized regenerative polymer fuel cells (URFCs) were prepared and the relations between the PTFE loading amount and the URFC performance were examined. As for the GDB of the hydrogen electrode, both the fuel cell and water electrolysis performances were not affected by the amount of PTFE loading on the hydrogen side GDB. However, the URFC performances significantly depended on the PTFE loading amount of the GDB for the oxygen electrode; during the fuel cell and water electrolysis operations, URFC showed higher performances with smaller PTFE loadings but the cell with no PTFE-coated GDB showed a very deteriorated fuel cell performance. Cycle properties of the URFC revealed that the efficiency of the URFC decreased with the increasing cycles when the PTFE loading on oxygen side GDB was too low, however, a stable operation can be achieved with the appropriate PTFE loading on the GDB.     

Analysis of heat transfer characteristics of an unsaturated soil bed: a simplified numerical method

This paper is a continuation of a study reported in this Journal in February 1999. The paper presents a summary of the two‐dimensional macroscopic continuity, momentum and energy equations in a cylindrical co‐ordinate system that describe heat and mass transfer through unsaturated soil. The hydrodynamic laws governing flow of water through unsaturated soil are also presented. The explicit numerical procedure and the method to solve the equations are described. Characteristics of the corresponding computer program are also discussed. The results obtained with the current cylindrical governing equations are compared with the previously reported results based upon the Cartesian system of equations. It is observed that the results obtained with cylindrical formulations are in closer agreement with the experimental results. The effects of various heat transfer processes as well as the motion of fluids on heat transfer in a clay bed coupled to a heat pump are discussed. Heat diffusion into the soil by conduction is shown to be predominant through the early stage of heating, while the liquid water motion contributes to heat transfer during the intermediate times and the gas motion is shown to become significant during the last stages of drying. The contribution of the convective transport increases with the temperature and becomes equal to the contribution by conduction at moderately high temperatures. Copyright © 2001 John Wiley & Sons, Ltd.     

Influence of properties of gas diffusion layers on the performance of polymer electrolyte-based unitized reversible fuel cells

Polymer electrolyte-based unitized reversible fuel cells (URFCs) combine the functionality of a fuel cell and an electrolyzer in a single device. In a URFC, titanium (Ti)-felt is used as a gas diffusion layer (GDL) of the oxygen electrode, whereas typical carbon paper is used as a GDL of the hydrogen electrode. Different samples of Ti-felt with different structural properties (porosity and fiber diameter) and PTFE content were prepared for use as GDLs of the oxygen electrode, and the relation between the properties of the GDL and the fuel cell performance was examined for both fuel cell and electrolysis operation modes. Experimental results showed that the cell with a Ti-felt GDL of 80 μm fiber diameter had the highest round-trip efficiency due to excellent fuel cell operation under relatively high-humidity conditions despite degradation in performance in the electrolysis mode.     

Numerical modelling and heat transfer optimization of large-scale multi-tubular metal hydride reactors

Heat management during the absorption/desorption process is a key aspect in improving the performance of large-scale hydrogen storage systems. In this article, the absorption and desorption performance of a multi-tubular hydride reactor is numerically investigated and optimized for 60 kg mass of LaNi₅ alloy. The 90% absorption with 7, 14, and 19 tubes is achieved in 985, 404, and 317 s with an overall reactor weight of 78.46, 88, and 88.2 kg, respectively. The 14-tube reactor performance is investigated by introducing the longitudinal fins inside the tubes. The reactor performance is enhanced by allocating fins into different pairs of half and full fins constrained by overall fin volume. A thermal resistance network model is presented to investigate the effect of fin distribution and coolant velocity on equivalent resistance of the metal hydride reactor. Storage performance obtained from numerical model validates the thermal resistance analysis from heat transfer viewpoint. With six full fins, 90% hydrogen absorption is achieved in 76 s. However, tubes with 6, 8, and 12 pairs of half and full fins require 74, 58, and 54 s, respectively. The 14-tube reactor with 8 pairs of half and full fins is used for quantifying the augmentation in the absorption performance in response to operating conditions (supply pressure and heat transfer fluid temperature). A design methodology is outlined for the development of a large-scale multi-tubular hydride reactor based on a heat transfer optimization strategy.     

Numerical simulation of laminar flow development with heat and mass transfer in PEM fuel cell flow channels having oxygen and hydrogen suction at one channel wall

Numerical simulation has been carried out of the fluid flow, heat and mass transfer for the developing laminar flow in polymer electrolyte membrane (PEM) fuel cell cathode and anode flow channels, respectively. Each flow channel is considered to be composed of two parallel walls, one porous (simulating electrode surface) and one non‐porous, or impermeable, wall (simulating bipolar plate surface). Various flow situations have been analyzed, and the local and the averaged friction coefficient, Nusselt number for heat transfer and Sherwood number for mass transfer are determined for various flow conditions corresponding to different stoichiometries, operating current densities and operating pressures of the cell. The effect of suction or injection (blowing) wall boundary condition has also been investigated, corresponding to the oxygen consumption in the cathode and hydrogen consumption in the anode. Correlations for the averaged friction coefficient, Nusselt and Sherwood numbers are developed, which can be useful for PEM fuel cell modeling and design calculations. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of electric field on heat transfer performance of automobile radiator at low frontal air velocity

The effect of electric field on the performance of automobile radiator is investigated in this work. In this experiment, a louvered fin and flat tube automobile radiator was mounted in a wind tunnel and there was heat exchange between a hot water stream circulating inside the tube and a cold air stream flowing through the external surface. The electric field was supplied on the airside of the heat exchanger and its supply voltage was adjusted from 0 kV to 12 kV.From the experiment, it was found that the unit with electric field pronounced better heat transfer rate, especially at low frontal velocity of air. The correlations for predicting the air-side heat transfer coefficient of the automobile radiator, with and without electric field, at low frontal air velocity were also developed and the predicted results agreed very well with the experimental data.     

Research on low-temperature heat exchange performance of hydrogen preheating system for PEMFC engine

The durability of membrane electrodes, which are the core components of the Proton Exchange Membrane fuel cell (PEMFC), seriously affects the service life of the stack. Under the action of long-term low temperature, the gas diffusion layer at the entrance will be aged and its micro-porous layer structure will be destroyed, which will hinder the removal of liquid water and gas transport, so it is necessary to preheat the anode hydrogen. In the present study, the influence of different pitch ratio and diameter ratio on heat transfer of corrugated tube heat exchanger is simulated by means of thermal-fluid coupled numerical simulation and periodic unit model, the effects of coolant flow rate and temperature on the overall heat transfer performance were also studied. The validity of the simulation results is verified by experiments, and the effect of hydrogen preheating on the stack performance is also tested. The simulation results show that the corrugated joint will disturb the flow of hydrogen, which increases the temperature gradient along the radial direction of the main flow and enhances the heat transfer. When Re is lower than 4000, the friction factor decreases quickly and then gradually flattens out. Compared with 0.5 for bellows with a pitch ratio of 1, the friction factor increases by 17%. With the increase of Re, the j values of different pitch ratios differ greatly and decrease linearly. For every 5 cm increase in the length of the corrugated tube, the total heat exchange capacity is increased by about 20%，and the total heat transfer increases about 100 W with the increase of the coolant flow 0.04 kg/s.