Air Stoichiometry Research Articles

Investigation of consistency and uniformity for commercial-size proton-exchange membrane fuel cell stack based on multipoint impedance measurement

Abstract The proton-exchange membrane fuel cell is a highly attractive clean energy technology for the future. However, differences in the consistency and uniformity of commercial-size fuel cell stacks significantly impact their health status and lifespan. This study is based on a commercial-size 10-cell stack. First, the characteristic frequency was determined by multipoint voltage and impedance sweeping experiments. Subsequently, the consistency and uniformity of the output characteristics of the stack at various loads under standard operating conditions were comprehensively analyzed using fixed-frequency multipoint impedance. Finally, a sensitivity analysis was conducted to quantitatively assess the effects of key parameters, including the air stoichiometry ratio and operating temperature, on the consistency and uniformity of voltage and high-frequency resistance. The inconsistency is more pronounced at high current densities, especially at the cathode inlet side compared to the outlet side. At low-current-density operation, the high-frequency resistance is greater at the cathode inlet side compared to the outlet side due to uneven gas distribution and more severe membrane drying at the inlet side. Increasing the air stoichiometry ratio enhances the consistency and uniformity of the stack. The effect of temperature on stack consistency is not significant in the appropriate temperature interval, but higher temperatures can reduce cell uniformity differences. The analytical approach in this study can provide guidance for stack consistency and uniformity studies.

Thermal analysis of fuel cells in renewable energy systems using Generative Adversarial Networks (GANs) and Reinforcement learning

Thermal analysis of fuel cells in renewable energy systems using Generative Adversarial Networks (GANs) and Reinforcement learning

Numerical Investigation of Flow Distribution in PEMFC and Pemwe Stacks Considering Two-Phase Flow in the Stack Headers

PEM fuel cell (PEMFC) and PEM water electrolyzer (PEMWE) stacks are assembled by connecting a series of unit cells. The stack headers are conduits formed by the ports of these unit cells; see Fig. 1(a). The flow field in the headers affects the flow uniformity of the stack, which significantly impacts the stack’s performance and lifespan. This study aims to investigate the fluid flows in the stack headers of PEMFC and PEMWE. A specific objective of this study is to understand how two-phase flow (liquid water and gases) in the headers would impact the flow distribution inside the stacks.The present study investigates the flow inside a stack by two-dimensional (2D) and three-dimensional (3D) models, as shown in Figure 1(b) and (c) respectively. The 3D model was constructed with an inlet header and an outlet header with flow resistors in between to represent the unit cells.1,2 CFD simulations using ANSYS Fluent were performed over a 100-cell PEMFC stack based on the k-ε RNG turbulence model. The two-dimensional 100-cell PEMFC stack model was built with COMSOL Multiphysics and considers the diffusion and transport, proton and electron conduction, electrochemical reactions on the catalytic layer, and heat transfer in the unit cells. The flow distribution obtained from the 3D model was used as the boundary conditions for the 2D model to calculate the effects of flow distribution on the overall performance of the stack. It should be pointed out that in the 3D models of PEMFC and PEMWE headers, simulations using the volume of fluid (VOF) model to track the liquid water droplets in a PEMFC and bubbles in a PEMWE were carried out to gain insights into the impact of two-phase flow on stack header flow sharing.The simulation results show the existence of unstable and highly transient turbulence within the PEMFC outlet header, as seen in Figs. 1(d) and 1(e). Jet flows from individual cells cause vortices inside the header, reducing the effective flow area. Liquid water accumulates on the inner wall of the header and is swept out by the cross-flow. In the dead-end region, liquid water undergoes helical motion under the influence of vortices, resulting in a longer residence time in the header and making it less prone to be expelled. This study parametrically investigated the effects of header size and air stoichiometry. As the cross-sectional area of the inlet header decreases from A=1,225 mm2 to A=857.5 mm2, the coefficient of mass flow variation within the stack increases, indicating a decrease in the uniformity of flow distribution inside the stack, as shown in Fig. 1(f). The air stoichiometry also affects the flow distribution inside the stack; as the air stoichiometry λ decreases from 2.2 to 1.4, the uniformity of gas distribution in the stack deteriorates significantly.The methodology used in the present study provides a reference for the parameter design and operating conditions of PEMFC and PEMWE stacks. It helps to gain insights into the optimization of stack header design and energy management.

The effect of pin-type flow field plate design on the current distribution in a H2-fed polymer electrolyte fuel cell

The effect of pin-type flow field plate design on the current distribution in a H2-fed polymer electrolyte fuel cell

Analysis of Gas Characteristics Produced in an Updraft Type Gasification Reactor with Various Gas Agent Flow Rates

Gasification is a process of converting compounds containing carbon to change both liquid and solid materials into combustible gaseous fuel (CO, H2, CO2, CH4 and H2O) through a combustion process with a limited air supply, namely 20-40% of the air stoichiometry. The aim of this research is to obtain syngas that is renewable natural gas quality. The gasification reactor used in this research has a reactor diameter of 600 mm and a reactor height of 1500 mm. Research on the characterization of the gasification process in updraft gasifiers made from horse manure using the thermal decomposition method where the gas agent used is air. Variations in the flow rate of the gas agent used are 10, 15, 20 and 30 lt/minute. The results of the research that has been carried out show that the greater the flow rate of the gas agent, the CO gas levels will increase with an average increase of 15%, however the increase in CO gas is followed by a decrease in CH4 gas levels, namely an average of 13%. Likewise, CO2 gas levels decreased by an average of 14%.

Effect of manifold size on PEMFC performance with metal foam flow field

Effect of manifold size on PEMFC performance with metal foam flow field

Modelling and operation characteristics of air-cooled PEMFC with metallic bipolar plate used in unmanned aerial vehicle

Modelling and operation characteristics of air-cooled PEMFC with metallic bipolar plate used in unmanned aerial vehicle

Determination of an Optimal Parameter Combination for Single PEMFC Using the Taguchi Method and Orthogonal Array

In this study, the optimization of the operational parameters for a single proton exchange membrane fuel cell (PEMFC) was carried out using the Taguchi method and orthogonal array. The operating parameters were H2 stoichiometry, air stoichiometry, cell temperature, and back pressure of the anode∙cathode, each with three levels. The performance of the PEMFC, operated according to the L9 orthogonal arrangement, was evaluated through I–V curves at a step-up current loading ranging from 0.1 to 0.7 A/cm2. The results indicated that the anode∙cathode back pressure had the greatest sensitivity to the output voltage compared to the other operating parameters. Increasing the back pressure resulted in higher current output densities at higher values than those applied in the orthogonal arrangement. As the back pressure increased, the output voltage tended to increase at each current density. However, for operating conditions above 150 kPa, the improvement in cell performance was either not significant or tended to decrease. Therefore, it can be concluded that the Taguchi method and orthogonal array are effective tools for selecting the optimal operating conditions for PEMFC.

Analysis of Cell to Cell Voltage Variations in a 4-cell and 25- Cell Low Temperature PEM Stacks

An experimental study of cell to cell voltage variations on two different low temperature (LT) PEM stacks is presented. One is a‐4 cell‐membrane‐humidified stack. The second is a‐25 cell stack with an external humidifier. The test results taken from open circuit voltage to full load reveal that in both stacks, the first cell almost always produced the least voltage. Further tests at additional two levels of air stoichiometry (Sair) with the 4‐cell stack confirmed the previous observations. An analysis is conducted to unravel the influence of various factors on cell to cell voltage variations. Limitations of this investigation are noted and future work is suggested.

Effect of Operating Conditions on Voltage Transient Response of High‐Temperature Proton Exchange Membrane Fuel Cell Under Load Changing

The transient response of a high‐temperature proton exchange membrane fuel cell (HT‐PEMFC) is critical for control and optimization. The objective of this study is to investigate the effects of operating conditions and control parameters on the transient response of the HT‐PEMFC during load variations to optimize the operating conditions. The main causes of the voltage undershoot are investigated experimentally, and the effects of air stoichiometry, air humidity, operating temperature, and backpressure on the transient response of the HT‐PEMFC are evaluated in terms of internal resistance and voltage response. The results show that during load changes, the local gas starvation of the cell and the change in the hydration state of membrane‐electrode‐assembly may be the main cause of the voltage undershoot. The voltage drop is affected by the combination of ohmic resistance, charge transfer resistance, and mass transfer resistance. In addition, operating temperature and air stoichiometry can affect voltage drop and voltage undershoot during load changes. Proper humidification of the air can alleviate voltage undershoot problems. Increasing the backpressure can improve the transient performance of the cell, but does not alleviate the voltage undershoot problem. The study can provide guidelines for the design and control of HT‐PEMFC operating conditions.

Effect of operating conditions of unmanned aerial vehicle powered by PEMFC on water and heat distributions of cell

ABSTRACT The cell performance of a proton exchange membrane fuel cell (PEMFC)-driven unmanned aerial vehicles (UAV) is affected by environmental changes during high-altitude flights. Therefore, the distribution of water and heat in a PEMFC was investigated. The result showed that the uniformity of temperature distribution on the membrane could be improved by increasing the air stoichiometry ratio, but the temperature difference on the membrane was still large and the minimum temperature difference was 14.6°C. When the temperature distribution is uniform, the temperature on the membrane is too low. Consequently, increasing the inlet temperature to 60°C, 70°C and 80°C resulted in inlet and outlet temperature differences of 3.5°C, 0.5°C and 2.7°C for the membrane respectively. By analyzing the distribution of the water content and temperature on the membranes, 70°C and 20 were considered to be the optimal inlet temperature and air stoichiometry, respectively. Under this condition, as the altitude increases, the temperature and water content on the membrane gradually decrease.

Numerical Study on the Influence of Variable Section-Enhanced Mass Transfer Flow Field Design on PEMFC Performance Based on Underrib Convection Flow

The optimal design of variable section is a flow field optimization method that changes the pressure of the reacting gas in the flow channel by setting up a contraction surface in the flow channel, which is effective in improving many aspects of the proton exchange membrane fuel cell (PEMFC) performance. However, there are also problems such as uneven oxygen distribution in some of the flow channels, and the current density and net output power are not significantly improved. To find a balance among gas transfer efficiency, drainage capacity, uniformity of current density distribution, and output performance, this paper investigates the design of a variable cross-section-enhanced mass transfer flow field for PEMFC flow channel contraction by establishing a three-dimensional numerical model, starting from the sidewall width direction and the bottom height direction of the flow channel cross-section dimensions, in order to assess the effects of variable cross-section contraction at the bottom and sidewalls of the flow channel on the cell performance. The polarization curves, mass transfer capacity, current density distribution, and net output power of the variable cross-section contraction flow field are analyzed and compared with the straight flow field. On this basis, the optimum size parameters and operating parameter combinations are determined. Variable cross-sectional contraction flow field improves PEMFC output performance. The contraction of the variable cross-section contraction flow field at the sidewalls effectively improves reactive gas transfer efficiency and drainage capacity, improves current density distribution, and increases the net output power of the PEMFC. In particular, the sidewall contraction flow field has a 9.0% increase in power density, a 6.45% increase in the effective mass transfer coefficient (EMTC), a 6.52% increase in uniformity of current density distribution, and a 6.05% net power increment compared to the straight flow field, respectively. The best output performance, the highest oxygen concentration, and the lowest water concentration are achieved with a combination of parameters: number of sidewall contractions N = 6 , length L f = 6 mm , and width W f = 1.2 mm . And a wide range of air stoichiometry ratios are applied when the contraction of the variable cross-section contraction flow field is located at the sidewalls. Increasing the cathode stoichiometry ratio from 10 to 50 increased the cell power density by 25.5% and from 50 to 90 increased the cell power density by 9.5%. The simulation results can provide theoretical basis for the application of variable section flow field design for practical PEMFC.

Comparative and sensitivity analysis of operating conditions on the performance of a high temperature PEM fuel cell

Comparative and sensitivity analysis of operating conditions on the performance of a high temperature PEM fuel cell

Enhancing the dynamic performance of proton exchange membrane fuel cells through two-phase flow experimental investigation

Efficient water transfer is crucial for enhancing the rapid response and durability of proton exchange membrane fuel cells (PEMFCs). In this paper, ex situ observation experiments and in situ dynamic performance tests of fuel cells are conducted and correlated with each other by two-phase flow pressure drop. The experimental results show that slug flow within the cathode gas flow channels (GFCs) is unfavorable to mass transfer and results in loading failure under high current densities, as identified in Design Failure Mode and Effects Analysis (DFMEA). However, slug flow in GFCs of PEMFCs can be converted into film flow by reducing the hydraulic diameter of GFC, increasing the air stoichiometry ratio, and extending the loading resistance time to an appropriate value. This facilitates a successful and efficient load change process of PEMFCs under high current densities. This study provides a methodology, combines ex situ observation and in situ evaluations, identifying potential risks, which can shed light on improving the dynamic performance and development process of PEMFCs.

Simultaneous measurement of local current and temperature distributions in proton exchange membrane fuel cells with dead-ended anode

The dead-ended anode (DEA) operation could cause local flooding and local hydrogen starvation in proton exchange membrane fuel cells (PEMFCs), resulting in drastic changes in fuel cell performance. In order to study the detailed local dynamic characteristics of DEA operation, simultaneous measurements of the dynamic variation of local current and local temperature in a PEMFC with DEA have been performed using a local current measurement gasket and micro thermocouples. The results show that the local fuel cell performance distribution gradually becomes uneven during DEA operation and the evolutions of local current and temperature strongly depend on the locations along the flow channel. The highest local current and local temperature are observed near the anode inlet, posing serious threats for the local fuel cell. Furthermore, with anode purging, the current distribution becomes uniform quickly due to the fast mass transfer capacity of hydrogen, but the local temperatures take much longer time to become uniform and stable because of the thermal capacity of fuel cell components. The anode purging also causes local temperature undershoots and the amplitude is over 4℃ in this study. Finally, the effects of operating conditions are discussed. The local current and temperature distributions become more uneven as overall operating current increases and operating temperature decreases. The local temperatures are all reduced as anode inlet pressure and air stoichiometry increase due to the enhanced convection cooling effect. It is demonstrated that measuring local current and temperature is effective in investigating local dynamic performance of PEMFCs with DEA.

Thermal management of bypass valves for temperature difference elimination in a 5 kW multi-stack solid oxide fuel cell system

Thermal management of bypass valves for temperature difference elimination in a 5 kW multi-stack solid oxide fuel cell system

System analysis of a protonic ceramic fuel cell and gas turbine hybrid system with methanol reformer

The performance of a methanol-fed protonic ceramic fuel cell (PCFC)/gas turbine (GT) hybrid system is investigated in this work. To build the system, Thermolib software is employed with input parameters obtained from references. Effects of air stoichiometry on system performance are analyzed. Results show that, as air stoichiometry is increased, the reformer temperature and CO concentration decrease, while H2 concentration increases. High air stoichiometry decreases PCFC temperature and performance. GT output power increases with increasing air flow. But, the power consumption by compressor also increases. Overall, to achieve higher system efficiency for this hybrid system, the optimum values of air stoichiometry are from 2.7 to 2.9. An additional heat recovery steam generator can also improve the overall system efficiency from 66.5% to 71.7%. This work helps in understanding the modeling and optimum functioning parameters of high power generation systems.

Unraveling the NO reduction mechanisms occurring during the combustion of NH3/CH4 mixtures

The interaction between NH3, CH4 and NO under different conditions of interest for combustion applications is analyzed, from both experimental and kinetic modeling points of view. Reduction of NO by reburn and by SNCR (selective non-catalytic reduction) strategies is evaluated, through an extense systematic study of the influence of the main variables of interest for NO reduction, by means of laboratory flow-reactor experiments at atmospheric pressure. Variables analyzed include: temperature in the 700 to 1500 K range, air stoichiometry from fuel-rich (λ = 0.31) to fuel-lean conditions (λ = 2.21), NH3/CH4 ratio in the 0.4 to 10.78 range, NH3/NO ratio in the 0.49 to 2.60 range the, and CH4/NO ratio in the 0.37 to 1.98 range, dilution level, and bath gas by using nitrogen and argon, the latter to allow the precise determination of nitrogen balances. Results are interpreted using a literature reaction mechanism, together with reaction pathway analysis tools, and the main findings are discussed. Results indicate that ammonia promotes the conversion of methane, while methane inhibits the conversion of ammonia, due to the competition for radicals of both components in the mixture. The interaction of ammonia and methane implies that the reduction of NO by NH3/CH4 mixtures is comparatively lower than the reduction obtained by NH3 and CH4 independently. Implications for practical applications of the reduction of NO by the studied mixtures are discussed.

Oxidant starvation under various operating conditions on local and transient performance of proton exchange membrane fuel cells

Oxidant starvation under various operating conditions on local and transient performance of proton exchange membrane fuel cells

Inconsistency evaluation of vehicle-oriented fuel cell stacks based on electrochemical impedance under dynamic operating conditions

Inconsistency evaluation of vehicle-oriented fuel cell stacks based on electrochemical impedance under dynamic operating conditions