Experimental and Numerical Study of Serpentine Flow Fields for Improving Direct Methanol Fuel Cell Performance

Methanol crossover is an important issue as it affects direct methanol fuel cell (DMFC) performance. But it may be controlled by selecting a proper flow field design. Experiments were carried out to investigate the effect of single, double and triple serpentine flow field configurations on a DMFC with a 25 cm² membrane electrode assembly (MEA) with a constant open ratio. A three dimensional model was also developed for the anode of the DMFC to predict methanol concentration and cell current density distributions. Experimental and model results show that at lower methanol concentrations (0.25–0.5M), single serpentine flow field (SSFF) provides high peak power density, while a double serpentine flow field (DSFF) gives high peak power density at a high methanol concentration (1–2M). Single and double serpentine flow fields exhibit the same peak power density (33 mW cm⁻²) at 1M. But the cell efficiency of double serpentine flow field is 12.5% which is 3.5% point greater than single serpentine flow field. This is attributed to reduced mixed potential. triple serpentine flow field (TSFF) shows the lowest peak power density and cell efficiency, which is attributed to high mass transfer resistance.     

Numerical and Experimental Verification of the Polymer Electrolyte Fuel Cell Performances Enhanced by Under‐Rib Convection

Understanding the current density distributions in polymer electrolyte fuel cells (PEFCs) is crucial for designing cell components, such as the flow field of bipolar plates. A new serpentine flow field equipped with sub‐channels and by‐passes (SFFSB) was numerically and experimentally confirmed to enhance the reactant transport rates and liquid removal efficiency compared with a conventional advanced serpentine flow field (CASFF). Consequently, the maximum current and the power densities of the SFFSB were increased due to the promotion of under‐rib convection. In this study, current density distributions are measured under transient conditions to verify the PEFC performances enhanced by under‐rib convection. The current density distributions of the SFFSB are compared with those of the CASFF. The results show that the SFFSB has a higher local current density and a more uniform distribution than the CASFF, therefore, the PEFC performances with the new flow field of SFFSB is enhanced by the better current density distributions.     

Thermo‐mechanical Coupling Properties of Proton Exchange Membrane in Liquid Water

In this study, the effects of the combination and interaction of thermal and mechanical loading on mechanical behaviors of proton exchange membranes (PEMs) under immersed condition were investigated. Experiments under two kinds of loading path were performed: the quasi‐simultaneous loading path where mechanical loads and temperature occurred simultaneously and the rectangular loading path where thermal and mechanical loads were inserted in turn. The quasi‐simultaneous loading path is composed of in‐phase, 90° and 180° out‐of‐phase loading paths. Comparison between in‐phase and 180° out‐of‐phase loading paths showed that strain accumulation under in‐phase loading path mainly resulted from creep strain while that under 180° out‐of‐phase loading path were mostly induced by cyclic stress. Accumulation of strain was comprised of tensile strain and creep strain, and these two strain components showed different evolution trends with thermo‐mechanical cycles. In addition, the modulus of the membrane was history‐dependent as indicated from results of rectangular loading path. Through comparing creep strain with strain energy density which is an indicator of damage in the membrane, we came to the conclusion that creep strain contributed to the damage of the membrane in the first three cycles.     

Modelling and Analysis of Electro‐chemical,Thermal, and Reactant Flow Dynamics for a PEM Fuel Cell System

In this paper an approach for the dynamic modelling of polymer electrolyte membrane fuel cells is presented. A mathematical formulation based on empirical equations is discussed and several features, exhibiting dynamic phenomena, are investigated. A generalized steady state fuel cell model is extended for the development of a method for dynamic electrochemical analysis. Energy balance and reactant flow dynamics are also explained through physical and empirical relationships. A well‐researched system (Ballard MK5‐E stack based PGS‐105B system) is considered in order to understand the operation of a practical fuel cell unit. Matlab‐SIMULINK^TM has been used in simulating the models. The proposed method appears to be relatively simple and consequently requires less computation time. Simulation results are compared with available experimental findings and a good match has been observed.     

Flow‐Field Designs of Bipolar Plates in PEM Fuel Cells: Theory and Applications

A generalized model developed by Wang was modified for flow field designs of the most common layout configurations with U‐type arrangement, including single serpentine, multiple serpentine, straight parallel, and interdigitated configurations. A direct and quantitative relationship was established among flow distribution, pressure drop, configurations, structures, and flow conditions. The model was used for a direct, systematic, and quantitative comparison of flow distributions and pressure drops among the most common layout configurations of interest. The straight parallel configuration had the lowest pressure drops but suffered the most possibility of the uneven flow distribution across the channels. The single serpentine had the best flow distribution but had the highest pressure drops. The flow distribution and the pressure drop in the multiple serpentine was between the straight parallel and the single serpentine. Finally, we suggested basic criteria of the flow field designs of bipolar plates for the industrial applications. This provides a practical guideline to evaluate how far a fuel cell is from design operating conditions, and measures how to improve flow distribution and pressure drop.     

Transport Properties,Local Coordination,and Thermal Stability of the Water/Diethylmethylammonium Methanesulfonate Binary System

Ammonium based protic ionic liquids are highlighted for their great potential to sustain proton transport in proton exchange membrane (PEM) fuel cells. Yet, there remain questions concerning the effect of water produced by the fuel cell at the cathode side on the performance of the ionic liquid. In this contribution we report the effect of water on the transport properties and the local coordination in the binary system of the protic ionic liquid diethylmethylammonium methanesulfonate ([DEMA][OMs]) and water, employing ¹H NMR, Raman, and infrared spectroscopy. We observe that the self‐diffusion of cations and anions increases with the water content and that cations and anions diffuse at the same rate at all concentrations investigated. ¹H NMR and vibrational spectroscopy, on the other hand, indicate that added water interacts primarily with the anion and slightly affects the ionicity of the ionic liquid. In addition, by investigating the thermal stability of the binary system we find that although [DEMA][OMs] displays a continuous loss of water upon increasing temperature a fraction of water molecules can be retained even above 120 °C, and that the complete loss of water is immediately followed by decomposition, which is observed to occur at about 185 °C.     

Three Dimensional Model of a High Temperature PEMFC. Study of the Flow Field Effect on Performance

A three‐dimensional isothermal model of a high temperature polymer membrane fuel cell equipped with polybenzimidazole membrane is described. All major transport phenomena were taken into account except the species crossover through the membrane. The cathode catalyst layer was treated as spherical catalyst agglomerates with porous inter‐agglomerate spaces. The inter‐agglomerate spaces were filled with a mixture of electrolyte (hot phosphoric acid) and polytetrafluoroethylene (PTFE). This approach proved to be an essential requirement for accurate simulation. In this particular paper, the influence of different flow field designs and dimensions on performance was intensely study. Traditional configurations were tested (straight, serpentine, pin‐in, and interdigitated), and new designs were proposed. With these new designs, we tried to maximize performance by providing homogeneous reactants distribution over the active area keeping low‐pressure drop and relatively high velocity. The dimension and position of the inlet and outlet manifolds were also analyzed. From the obtained results a massive influence of the manifolds position and dimension on performance was observed. This fact can provide important guidelines for future bipolar plates optimization.     

Performance Assessment of a Combined PEM Fuel Cell and Triple‐Effect Absorption Cooling System for Cogeneration Applications

In this paper, a parametric study of a combined proton exchange membrane (PEM) fuel cell and triple‐effect absorption cooling system (TEACS) is undertaken to investigate the effect of different operating conditions and system parameters on the COPs, efficiency of the fuel cell and the integrated system's overall utilisation factor. It is found that the fuel cell efficiency increases from 40% to 44.5% as the operating temperature of the fuel cell increases. However, as the fuel cell's temperature and current density increase, the COPs decrease from 2.4 to 0.9 as a result of the increase in the energy output of the fuel cell ranging from 7.4 to 10.7 kW. The efficiency of the fuel cell decreases from 41% to 32% with an increase in both fuel cell's current density and membrane thickness. The overall utilisation factor of the integrated system decreases from 84% to 35% with an increase in the current density and molar flow rate. Finally, this study reveals that the present integrated PEM fuel cell unit with a TEACS can be considered as an attractive and environmentally benign option for cogeneration purposes in sustainable buildings.     

In situ Synchrotron X‐ray Radiography Investigations of Water Transport in PEM Fuel Cells

Water transport in an operating PEM fuel cell was investigated with synchrotron X‐ray radiography with a spatial resolution of 3 μm and a temporal resolution of 5 s. This method allows for the detection of water accumulations with less than 10 μm diameter. We demonstrate that synchrotron X‐ray imaging can dramatically expand the possibilities of imaging with high spatial and time resolution, especially as a complement to neutron radiography. Water transport processes from the first appearance of small water accumulations in the gas diffusion layer to their transport into the channel system were analysed in situ. Correlations between local effects such as water formation and operating conditions of the whole system, e.g. power variations, were found. A recently described eruptive water transport mechanism is analysed in detail.     

Enhanced Performance of Solid Oxide Fuel Cell by Manipulating the Orientation of Cylindrical Pores in Anode Substrate

Effect of the orientation of cylindrical pores within an anode has been studied on the performance of anode‐supported solid oxide fuel cell (SOFC). Paper‐fibers are used as pore‐former and highly oriented cylindrical pores are formed within the anode prepared by uniaxial compaction. A thick anode brick is fabricated followed by cutting in different directions to obtain anode substrates with desirable orientation of pores. When the orientation of cylindrical pores is perpendicular to the anode surface, the gas transport is significantly improved so that the reduction rate of the NiO/YSZ anode is considerably accelerated and the cell concentration polarization is minimized. The corresponding single cell exhibits a maximum power density as high as 1.54 W cm^–2 in hydrogen and 0.90 W cm^–2 in nitrogen diluted methane at 800 °C. The result indicates that the output performance of anode‐supported cells could be significantly improved by manipulating the orientation of pores.     

Effects of GDL Structure with an Efficient Approach to the Management of Liquid water in PEM Fuel Cells

A model fuel cell with a single transparent straight flow channel and segmented anode was constructed to measure the direct correlation of liquid water movement with the local currents along the flow channel. Water drops emerge through the largest pores of the GDL with the size of the droplets that emerge on the surface determined by the size of the pore and its location under the gas flow channel or under the land. Gravity, surface tension, and the shearing force from the gas flow control the movement of liquid in the gas flow channel. By creating a single large diameter pore in the GDL, liquid water flow emergent from the GDL was forced to be in specific locations along the length of the channel and either under the land or under the channel. The effects of gravity were amplified when the large pore was under the channel, but diminished with the large pore under the land. Current fluctuations were minimised when the dominant water transport from the GDL pore was near the cathode outlet. The results show that it is possible to engineer the water distribution in PEM fuel cells by modifying the pore sizes in the GDL.     

Mathematical Model of Proton Exchange Membrane Fuel Cell with Consideration of Water Management

One‐dimensional model on the membrane electrode assembly (MEA) of proton exchange membrane fuel cell is proposed, where the membrane hydration/dehydration and the possible water flooding of the respective cathode and anode gas diffusion layers are considered. A novel approach of phase‐equilibrium approximation is proposed to trace the water front and the detailed saturation profile once water emerges in either anode or cathode gas diffusion layer. The approach is validated by a semi‐analytical method published earlier. The novel approach is applicable to the polarization regime from open circuit voltage to the limiting current density under practical operation conditions. Oxygen diffusion is limited by water accumulation in the cathode gas diffusion layer as current increases, caused by excessive water generation at the cathode catalyst layer and the electro‐osmotic drag across the membrane. The existence of liquid water in the anode gas diffusion layer is predicted at low current densities if high degrees of humidification in both anode and cathode feeds are employed. The influences of inlet relative humidity, imposed pressure drop, and cell temperature are correlated well with the cell performance. In addition, the overpotentials attributed from individual components of the MEA are delineated against the cell current densities.     

Effect of the geometry and pattern of the flow channel on the performance of polymer electrolyte membrane fuel cell

This research focuses on the effect of the geometry and patterns of the gas flow channel on the PEM fuel cell performance. Simulation was conducted and the results were verified by experiments. Three-dimensional, single phase, compressible and isothermal models of 5 cm² electrodes, anode and cathode, were developed and studied by utilizing a commercial Computational Fluid Dynamics (CFD) software, FLUENT 4.5. Two types of gas flow channel were investigated: conventional and interdigitated. The results showed that the flow channel pattern does not have a significant effect on the anode cell performance, whereas it has a strong effect/influence on the cathode cell performance. The interdigitated design provides a higher limiting current density and cell performance than the conventional design on the cathode side. Moreover, the cell performance does not depend on the inlet and outlet channel widths. On the contrary, for the interdigitated design, it was influenced by the shoulder width. Finally, experiments were conducted to validate the simulation results.     

Behavior of Pressure Gradient and Transient Pressure Signals during Liquid‐Liquid Two‐Phase Flow

Liquid‐liquid two‐phase flows are encountered in several process industries, multiphase reactors and oil industries. In each of these applications, identification of flow patterns poses a challenging problem and many efforts are directed towards developing suitable devices for this purpose. In the present work, attempts have been made to use pressure gradient and transient pressure signals to study flow patterns during the simultaneous flow of two liquids through a horizontal pipe. It is observed that the slope of the pressure gradient curves as a function of fluid superficial velocities is a weak function of the flow pattern. However, the variation of the slope with the pattern transition is much more significant when the pressure gradient is normalized with respect to only kerosene flow through the pipe (Δp_TP/Δp_KO). Further attempts have been made to identify flow patterns from transient pressure signals and the statistical analysis of these random signals has been undertaken. The PDF analysis and the wavelet multiresolution technique have been adapted to explain the signals in detail. The flow regimes identified are smooth stratified, wavy stratified, plug flow, ‘three‐layer' flow, ‘oil dispersed in water and water' and ‘oil and water in oil' flow patterns. The signal characteristics are depicted for each flow pattern.     

Current Advances in Polymer Electrolyte Fuel Cells Based on the Promotional Role of Under‐rib Convection

Literature data on the promotional role of under‐rib convection for polymer electrolyte fuel cells (PEFCs) fueled by hydrogen and methanol are structured and analyzed, thus providing a guide to improving fuel cell performance through the optimization of flow field interaction. Data are presented for both physical and electrochemical performance showing reactant mass transport, electrochemical reaction, water behavior, and power density enhanced by under‐rib convection. Performance improvement studies ranging from single cell to stack are presented for measuring the performance of real operating conditions and large‐scale setups. The flow field optimization techniques by under‐rib convection are derived from the collected data over a wide range of experiments and modeling studies with a variety of components including both single cell and stack arrangements. Numerical models for PEFCs are presented with an emphasis on mass transfer and electrochemical reaction inside the fuel cell. The models are primarily used here as a tool in the parametric analysis of significant design features and to permit the design of the experiment. Enhanced flow field design that utilizes the promotional role of under‐rib convection can contribute to commercializing PEFCs.     

Analysis of the Ripple Current in a 5 kW Polymer Electrolyte Membrane Fuel Cell Stack

Polymer electrolyte fuel cell systems are increasingly being used in applications requiring an inverter to convert the direct current (DC) output of the stack to an alternating current (AC). These inverters, and other time‐varying inputs to the stack such as the anode feed pressure, cause deviations from the average stack current, or ripple currents, which are undesirable for reasons of performance and durability. A dynamic fuel cell model has been developed and validated against experimental data for a 5 kW fuel cell stack, examining in detail the ripple current behaviour. It was shown that the ripple currents exceed the 2% maximum recommended value, and may lead to long‐term degradation of the fuel cell stack.     

On the Catalytic Degradation in Fuel Cell Power Supplies for Long‐Life Mobile Field Sensors

Important tasks such as environment monitoring require field devices such as sensors that can operate for long durations. Current power supply technologies such as batteries limit many applications. Fuel cells are a promising alternative to batteries because they can have much higher energy densities. However, their lives may be short due to catalyst degradation. Here, a simplified model of proton exchange membrane (PEM) fuel cell catalyst degradation is applied to small fuel cells. The model focuses on the combined effects of catalyst dissolution and migration. The effect of migration on catalyst degradation is found to be substantial and this has not been accounted for in previous models. The model considers the effect of field conditions such as varying power demands, temperature and humidity, and predicts the catalyst life of the fuel cell and its power output. The predicted life is a proposed metric that can quantify the relative importance and effect of field conditions on the catalyst particularly for the design and control of fuel cell power supplies. Experiments are presented that support the model. This model is applied to a study on field sensors and results suggests unless PEM fuel cells are isolated from damaging field conditions, they will have short lives.     

Modelling and Performance Evaluation of Solid Oxide Fuel Cell for Building Integrated Co‐ and Polygeneration

Models of fuel cell based combined heat and power systems, used in building energy performance simulation codes, are often based on simple black or grey box models. To model a specific device, input data from experiments are often required for calibration. This paper presents an approach for the theoretical derivation of such data. A generic solid oxide fuel cell (SOFC) system model is described that is specifically developed for the evaluation of building integrated co‐ or polygeneration. First, a detailed computational cell model is developed for a planar SOFC and validated with available numerical and experimental data for intermediate and high temperature SOFCs with internal reforming (IT‐DIR and HT‐DIR). Results of sensitivity analyses on fuel utilisation and air excess ratio are given. Second, the cell model is extended to the stack model, considering stack pressure losses and the radiative heat transfer effect from the stack to the air flow. Third, two system designs based on the IT‐DIR and HT‐DIR SOFCs are modelled. Electric and CHP efficiencies are given for the two systems, as well as performance characteristics, to be used in simulations of building integrated co‐ and polygeneration systems.     

Performance Evaluation and Parametric Optimization of a Proton Exchange Membrane Fuel Cell/heat‐driven Heat Pump Hybrid System

With the help of the current models of proton exchange membrane (PEM) fuel cells and three‐heat‐source heat pumps, a generic model of a PEM fuel cell/heat‐driven heat pump hybrid system is established, so that the waste heat produced in the PEM fuel cell may be availably utilized. Based on the theory of electrochemistry and non‐equilibrium thermodynamics, expressions for the efficiency and power output of the PEM fuel cell, the coefficient of performance and rate of pumping heat of the heat‐driven heat pump, and the equivalent efficiency and power output of the hybrid system are derived. The curves of the equivalent efficiency and power output of the hybrid system varying with the electric current density and the equivalent power output versus efficiency curves are represented through numerical calculation. The general performance characteristics of the hybrid system are analyzed. The optimally operating regions of some important parameters of the hybrid system are determined. The influence of some main irreversible losses on the performance of the hybrid system is discussed in detail. The advantages of the hybrid system are revealed.