Performance investigation on a coaxial-nozzle ejector for PEMFC hydrogen recirculation system

The ejector driven by the high-pressure gas potential energy from the hydrogen storage tank can reliably recirculate the unconsumed hydrogen in the proton exchange membrane fuel cell (PEMFC) system. However, the fixed-geometry ejector cannot maintain consistently high performance among the whole power output range in the PEMFC system due to its shortage of limited operating range. In this paper, a coaxial two-nozzle ejector, satisfying the requirements of the PEMFC system under different power outputs, is developed for hydrogen recirculation. The proposed ejector is investigated numerically based on an experimentally verified simulation model to reveal the flow distribution and predict its performance. The simulation results show that the proposed ejector can work in a wide power range of 17.90–84.00 kW within a suitable supply hydrogen pressure range of 4–7 bar. More importantly, the ejector can not only maintain a recirculation ratio above 0.9 in the wide output power range but a high recirculation ratio greater than 2.0 in the low power output. The proposed ejector broadens the working range of a single ejector used in the PEMFC system, which significantly promotes the development of fuel cell being widely adopted in automobiles.     

Design and investigation of multi-nozzle ejector for PEMFC hydrogen recirculation

In this study, a multi-nozzle ejector is proposed for hydrogen recirculation in proton exchange membrane fuel cell (PEMFC) systems. Numerical simulations are performed on the basis of an experimentally verified three-dimensional numerical model to investigate the performance and inner-flow distribution of the proposed multi-nozzle ejector. We show that the fuel cell system with the multi-nozzle ejector can achieve a wide output-power range without a significant change in the primary pressure by simply switching the operating mode of the nozzles; moreover, the recirculation ratios are acceptable. The output-power can cover the range of 20–25 kW and 35–100 kW theoretically; in addition, a relatively stable anodic inlet pressure can be achieved. The collaboration of nozzles can effectively restrain the vortex, and the operating mode with two nozzles can achieve the best recirculation capacity. The proposed method extends the operating range of the ejector and the results of this study may contribute to the further multi-nozzle ejector design.     

Performance improvement of proton exchange membrane fuel cells with compressed nickel foam as flow field structure

In order to improve the performance of proton exchange membrane fuel cell (PEMFC), the compressed nickel foam as flow field structure was applied to the fuel cell. The fuel cell test system was built and the performance of fuel cells with nickel foam flow field with different thicknesses were tested and analyzed by electrochemical active surface area (EASA), electrochemical impedance and polarization curve. And its operating parameters were optimized to improve the performance of PEMFC. Our results show that the membrane electrode assembly (MEA) can show a larger catalytic active area and uniformity of gas diffusion can be improved by using the nickel foam flow field instead of the conventional graphite serpentine flow field, and the impedance characteristic of 110PPI nickel foam can be improved by increasing the compression ratio of the original material. What's more, the polarization characteristic and power output performance of PEMFC with nickel foam flow field were improved by optimizing the operating parameters. Using the optimized operating parameters (cell temperature = 80 °C; humidification temperature = 75 °C; stoichiometric ratio = 2; back pressure = 0.23 Map), a peak power density of 1.89 W cm⁻² was obtained with an output voltage of 0.46 V.     

Phase change characteristics and their effect on the performance of hydrogen recirculation ejectors for PEMFC systems

The working fluid of the hydrogen recirculation ejector in proton exchange membrane fuel cell (PEMFC) systems is humid hydrogen containing water vapour. However, previous studies on the hydrogen recirculation ejector using computational fluid dynamics (CFD) were based on the single-phase flow model without considering the phase change of water vapour. In this study, the characteristics of the phase change and its effect on the ejector performance are analysed according to a two-phase CFD model. The model is established based on a non-equilibrium condensation phase change. The results show that the average deviation of the entrainment ratio predicted by a single-phase flow model is 25.8% compared with experiments involving a hydrogen recirculation ejector, which is higher than the 15.1% predicted by the two-phase flow model. It can be determined that droplet nucleation occurs at the junction of the primary and secondary flow, with the maximum nucleation rate reaching 4.0 × 10²⁰ m^?3s^?1 at a primary flow pressure of 5.0 bar. The higher temperature, lower velocity, and higher pressure of the gas phase can be found in the mixing region due to condensation, resulting in a lower entrainment performance. The nucleation rate, droplet number, and liquid mass fraction increase remarkably with an increasing primary flow pressure. This study provides a meaningful reference for understanding phase change characteristics and two-phase flow behaviour in hydrogen recirculation ejectors for PEMFC systems.     

Performance of anodic recirculation by a variable flow ejector for a solid oxide fuel cell system under partial loads

An anode gas recycle (AGR) system driven by a variable flow rate ejector was developed for use in small-scale solid oxide fuel cell (SOFC) systems. The partial load conditions were simulated through recycling power generation experiments to clarify the fundamental characteristics of the variable flow ejector by using actual 1 kW-class SOFC equipment at the steady state. We achieved power generation in a range of recirculation ratios under partial load conditions of 62.5%–80% by controlling the recirculation characteristics with the developed ejector by using a needle. Results showed that the recirculation ratio can be controlled in the range of 0.595–0.694 by adjusting the driving energy with the ejector even at a partial load where the fuel gas flow rate of the ejector changes. Furthermore, the effect of the recirculation ratio on SOFC output was discussed based on the results of gas analyses and temperature measurements. As the recirculation ratio increased, the fuel concentration at the SOFC inlet decreased and the water vapor concentration increased. However, the effect of the recirculation ratio on the stack temperature and output power was proposed to be small. In addition, it was confirmed that the operation was performed under safe conditions where no carbon deposition occurred by circulating the steam generated inside the SOFC without an external water supply. Ejector characteristics during power generation experiments were lower than those at room temperature, which indicates that an ejector upstream pressure of approximately 20–170 kPa gauge pressure was required. Variations in the fluid properties of the driver gas in the ejector motive nozzle heated by the hot suction gas were found to degrade the performance of the ejector installed in the SOFC system, as compared with the results of simulation experiments at room temperature. Nevertheless, the recirculation ratio range required for operation could be satisfied by adjusting the flow velocity of the driving gas through needle control.     

Optimal design of a novel nested-nozzle ejector for PEMFC's hydrogen supply and recirculation system

The ejector-based hydrogen supply and recirculation system (HSRS) for a Proton Exchange Membrane Fuel Cell (PEMFC) system has the advantages of compact size and zero power consumption, compared with the HSRS using a recirculation pump. However, the conventional ejector with a single venturi nozzle can only function within a narrow power range of the PEMFC system due to its restricted primary inlet pressure. This study proposed a novel ejector design with nested nozzles to solve this problem. The key geometric parameters, including the nozzle diameters of a large nozzle (BN), a small nozzle (SN), and the axial distance between two nozzles, were optimized using CFD simulations to obtain the maximum entrainment capability. The BN mode is responsible for the stack's higher load operations, while the SN mode supports the lower power operations. Additionally, a bypass was used parallel to the nested-nozzle ejector in the HSRS to extend the ejector operating range further. The consistent CFD simulation and testing results of the nested-nozzle ejector showed effective hydrogen entrainment capability between 9% and 100% of power output for a 150 kW PEMFC stack. Moreover, the new nested-nozzle ejector HSRS showed much-reduced anode inlet pressure fluctuation compared to the HSRS using two conventional ejectors.     

Performance enhancement in a H2/O2 PEMFC with dual-ejector recirculation

Water management in various components of the proton exchange membrane fuel cell (PEMFC) is a significant and challenging issue affecting output performance. PEMFC utilizing dual ejector-based recirculation has been developed to evaluate and improve the performance and water transport properties. A detailed investigation into the effects of ejector operating conditions, such as primary flow pressure and secondary flow relative humidity, on the performance of PEMFC is conducted. The results show that significant performance improvement of PEMFC can be achieved by increasing the operating pressure. The power density can be increased by 37.8% and 86.5% with ejector primary flow pressures of 300 and 400 kPa, respectively. Furthermore, an optimization strategy integrating PEMFC operating condition is proposed to ensure the stability and lifespan of performance. The water management and integration optimization strategy obtained in this paper can provide valuable insight into options for improving the performance of PEMFC with dead-ended anode and cathode.     

Numerical investigation of ejector transient characteristics for a 130-kW PEMFC system

Improvement of fuel utilization is an important issue for proton exchange membrane fuel cell (PEMFC) system. As a promising anode recirculation method, ejector has attracted great attention because it does not require additional power consumption. However, some transient processes such as the suck, diffusion, and mix of fluids are still not thoroughly revealed, which significantly influence ejector performances. In this study, a dynamic three-dimensional (3D) multicomponent ejector model for a 130-kW PEMFC system is developed. The model is validated against experimental data, including the entrainment ratio and mass flow rates. The effects of operating conditions (eg, pressure, water vapor, and nitrogen mass fraction) are investigated. The results show that the fuel supply can be controlled by the primary flow pressure. When the pressure difference between the primary and secondary flow is less than 10 kPa, the secondary flow cannot be sucked into the ejector. The transient response of ejector during stack power variations can be classified into two periods: the primary flow impact period and the mixed flow impact period. Under normal fuel cell system operating conditions, when the inlet relative humidity of the secondary flow is higher than 85%, the water vapor condensation is possible to happen at the ejector outlet region, leading to fuel supply instability. Besides, the hydrogen entrainment ratio decreases with the increase of nitrogen mass fraction. The effects of geometric parameters (eg, nozzle convergence angle, secondary flow tube diameter, mixing tube length, and diffuser angle) on ejector performances are also studied. It is found that the relatively short tube leads to pressure fluctuations in the vacuum region. Increasing the tube length is beneficial to creating a stable vacuum region. However, excessive tube length can increase the friction loss. Increasing the secondary flow inlet tube diameter is beneficial to the entrainment ratio. However, further enlarging the diameter contributes negligibly to the increase of entrainment ratio once the secondary flow mass rate depends on pressure.     

Experimental investigation of the effect of hydrogen recirculation on the performance of a proton exchange membrane fuel cell

The hydrogen recirculation in proton exchange membrane fuel cell (PEMFC) is recommended for the hydrogen supply of PEMFC, and hydrogen ejectors are gradually being used in fuel cell vehicles due to low noise and low energy consumption. However, there is a lack of discussion about the influence of recirculation rate on the stack. Due to passive regulating mechanism of the ejectors, a miniature speed-adjustable peristaltic pump is used to simulate the hydrogen ejector in this study to investigate the effect of hydrogen recirculation on the performance of PEMFC stack. Experiments are conducted under different pump flow rates. The stack with hydrogen recirculation is proven to have better performance, but over high pump flow rate can lead to hydrogen shortage. It is interesting to find that the flow rate fluctuation of hydrogen inlet affects the stability of stack performance, and pressure drop and recovery time during purge process are proposed as effective indicators for performance analysis. Finally, pump flow rates between 60 ml/min and 105 ml/min are defined as “effective area”. Based on the analysis of effective indicators, keeping at “effective area” is further proved to improve the performance of the stack, which is also useful to design hydrogen recirculation.     

Enhanced low-humidity performance of proton exchange membrane fuel cell by incorporating phosphoric acid-loaded covalent organic framework in anode catalyst layer

Developing self-humidifying membrane electrode assembly (MEA) is of great significance for the practical use of proton exchange membrane fuel cell (PEMFC). In this work, a phosphoric acid (PA)-loaded Schiff base networks (SNW)-type covalent organic framework (COF) is proposed as the anode catalyst layer (CL) additive to enhance the PEMFC performance under low humidity conditions. The unique polymer structure and immobilized PA endow the proposed COF network with not only excellent water retention capacity but also proton transfer ability, thus leading to the superior low humidity performance of the PEMFC. The optimization of the additive content, the effect of relative humidity (RH) and PEMFC operating temperature are investigated by means of electrochemical characterization and single cell test. At a normal operation temperature of 60 °C and 38% RH, the MEA with optimized COF content (10 wt%) showes the maximum power density of 582 mW cm^?2, which is almost 7 times higher than that of the routine MEA (85 mW cm^?2). Furthermore, a preliminary durability test demonstrates the potential of the proposed PEMFC for practice operation under low humidity environment.     

Studies on the cathode humidification by exhaust gas recirculation for PEM fuel cell

For high efficiency and long durability of proton exchange membrane fuel cells (PEMFCs), polymer electrolyte membranes should be kept wet. Reactant gases should be humidified on this account. For the humidification, the PEMFC system uses an external or internal humidifier as a part of balance of plants (BOPs). However, external humidifiers have many disadvantages such as parasitic power loss, system complexity, high cost and bulky volume. As such, efforts have been made to remove the external humidifier or replace it with an advanced humidifier. In this work, to remove a humidifier, humidification by exhaust gas recirculation is investigated by theoretical analysis and experiments with 5-cell stack of an active area 250 cm². In the theoretical analysis, species conservation equations and energy conservation equation are solved to obtain the O₂ concentration, stoichiometric ratio, humidity ratio, temperature, amount of condensed water and so on. With the theoretical results, experiments with 5-cell, 250 cm² fuel cell stack were carried out in order to analyze the stack performance at the theoretical conditions of the cathode process stream of exhaust gas recirculation.     

Description and characterization of an electrochemical hydrogen compressor/concentrator based on solid polymer electrolyte technology

Proton-exchange membrane (PEM) technology is commonly used for manufacturing water electrolysers, H₂/O₂ fuel cells and unitized regenerative fuel cells. It can also be used to develop electrochemical compressors, for the purpose of concentrating and/or pressurizing gaseous hydrogen. The aim of the work reported here was to evaluate the main operating characteristics of a laboratory scale (≈10 N liter/h) monocell compressor. The role of various operating parameters (current density, temperature of electrochemical cell, water vapor partial pressure in the hydrogen feed gas, anodic gas composition, etc.) has been evaluated and is discussed. It is shown that the relative humidity of hydrogen oxidized at the anode of the compressor should be adapted to the current density during operation to avoid mass transfer limitations or electrode flooding. A cell voltage of 140 mV is required at 0.2 A cm⁻² to compress hydrogen in one step from atmospheric pressure up to 48 bar, corresponding to an energy consumption of ca. 0.3 kW h/Nm³. Experiments have been performed up to 130 bar. Series connection of several compressors is recommended to reach output pressures higher than 50 bar. To reduce gas cross-permeation effects which can negatively impact the efficiency of the compressor, additional experiments have been made using Nafion membrane modified by addition of zirconyl phosphate. Finally, data related to the extraction of hydrogen from H₂-N₂ gas mixtures are also reported and discussed.     

Numerical studies on ejector in proton exchange membrane fuel cell system with anodic gas state parameters as design boundary

A comprehensive entrainment performance evaluation system of the ejector was built including four indexes. An ejector's Computational Fluid Dynamics (CFD) model was established, and the sensitivity analysis of the entrainment performance to four key geometry parameters of the ejector, namely, the nozzle diameter (D_n), the primary nozzle exit position (NXP), the mixing tube diameter (D_m), and the secondary flow inlet diameter (D_s) was performed. Based on the quantified boundary conditions obtained by the Simulink simulation, the ejector structure was optimized with a new method. It is found that the total recirculation ratio increases but the hydrogen recirculation ratio decreases with the increase of the relative humidity of the secondary flow. The hydrogen recirculation ratio shows a unidirectional increase tendency with the increase of D_s and the decrease of D_n and NXP. The hydrogen recirculation ratio increases firstly and then decreases with the increase of D_m. High hydrogen recirculation ratio with low primary hydrogen flow rate, corresponding to low current operation point of fuel cell system and low sensitiveness to the changing relative humidity are usually incompatible. The hydrogen recirculation ratio with low primary flow rate degrades significantly when D_n increases and D_m is larger than a certain value. The ejector with smaller D_s shows lower sensitiveness to the changes of relative humidity, while the hydrogen recirculation ratio with low primary hydrogen flow rate is not affected badly. When matching with a specific system, it is necessary to balance the ejection performance at low current density operating points and the sensitiveness to the changes of relative humidity in combination with the anodic gas state at each operating point, so as to find the optimal structural parameters. The optimization sequence of structural parameters should follow: D_n is selected firstly, then NXP and D_s are optimized, and finally D_m is chosen.     

Simulation and experimental measurements of 10-kW PEMFC passive hydrogen recovery system

Simulations are performed to examine the performance of a vacuum ejector in the hydrogen recovery loop of a 10-kW PEMFC system. The simulations commence by examining the effects of the primary flow fluid pressure and secondary flow temperature on the recirculation ratio and hydrogen stoichiometric ratio. Further simulations are then performed to investigate the temperature, pressure, velocity and Mach number distributions within the ejector for various values of the primary flow inlet pressure and temperature. A prototype ejector is fabricated using a 3D printing technique. Experiments are performed to evaluate the gas tightness and gas recovery performance of the ejector under realistic operating conditions. The simulation results show that the recirculation ratio and hydrogen stoichiometric ratio increase with a decreasing primary flow inlet pressure and secondary flow inlet temperature. As the primary flow inlet pressure increases, the pressure, velocity, and Mach number in the mixing chamber increase, and the hydrogen recovery performance decreases. Furthermore, as the temperature of the primary fluid flow increases, the stability of the isentropic flow condition within the ejector is enhanced. The experimental results show that the prototype vacuum ejector has a maximum gas leakage of just 0.7 psi and a minimum hydrogen recirculation rate of 59.3%. Consequently, it has significant potential for passive hydrogen recovery in large-scale fuel cell systems.     

The influence of graphitization on the thermal conductivity of catalyst layers and temperature gradients in proton exchange membrane fuel cells

As the proton exchange membrane fuel cell (PEMFC) has improved its performance and power density, the efficiency has remained unchanged. With around half the reaction enthalpy released as heat, thermal gradients grow. To improve the understanding of such gradients, PEMFC component thermal conductivity has been increasingly investigated over the last ten years, and the catalyst layer (CL) is one of the components where thermal conductivity values are still scarce. CLs in PEMFC are where the electrochemical reactions occur and most of the heat is released. The thermal conductivity in this region affects the heat distribution significantly within a PEMFC. Thermal conductivities for a graphitized and a non-graphitized CL were measured for compaction pressures in the range of 3 and 23 bar. The graphitized CL has a thermal conductivity of 0.12 ± 0.05 WK^–1m^–1, whilst the non-graphitized CL conductivity is 0.061 ± 0.006 WK^–1m^–1, both at 10 bar compaction pressure. These results suggest that the graphitization of the catalyst material causes a doubling of the thermal conductivity of the CL. This important finding bridges the very few existing studies. Additionally, a 2D thermal model was constructed to represent the impact of the results on the temperature distribution inside a fuel cell.     

Customized design for the ejector to recirculate a humidified hydrogen fuel in a submarine PEMFC

The customized design of an anode recirculation system that uses an ejector based on the humidified hydrogen is proposed for a submarine PEMFC. Generally, the ejector is useful to enhance its system performance and to easily be operated and maintained since it does not require any parasitic power and has very simple structure. However, the existing commercial ejectors do not meet the practical operating requirements of the PEMFC system with the humidified hydrogen recirculation since the included water raises the ejector performance reduction and accompanying operating limits. The subsonic flow ejector designed by the proposed approach has met the desired entrainment ratio through the whole operating range of the target system as well as it allows the additional advantages to improve the system efficiency and simplicity and to overcome the conventional operating limits.     

Integration of solid oxide fuel cell and palladium membrane reactor: Technical and economic analysis

This paper presents a technical and economic analysis of a solid oxide fuel cell system equipped with a palladium membrane reactor (PMR–SOFC) with the aim of determining the benefits of such an integrated unit over the conventional reformer module (CON-SOFC). The performance of both SOFC systems under the conditions for energetically self-sustaining operation (Q_NET = 0) was achieved by varying the fuel utilization for each operating voltage. Two types of fuels, i.e., methane and desulphurized biogas, are considered. The simulation results show that the maximum power density of the CON-SOFC fuelled by methane (0.423 W/cm²) is higher than that of the CON-SOFC fuelled by biogas (0.399 W/cm²) due to the presence of CO₂ in biogas. For the PMR–SOFC, it is found that the operation at a higher permeation pressure offers higher power density because lower fuel utilization is required when operating the SOFC at the energy self-sustained condition. When the membrane reactor is operated at the permeation pressure of 1 bar, the methane-fuelled and biogas-fuelled PMR–SOFCs can achieve the maximum power density of 0.4398 and 0.4213 W/cm², respectively. Although the PMR–SOFC can offer higher power density, compared with the CON-SOFC, the capital costs of supporting units, i.e., palladium membrane reactor, high-pressure compressor, and vacuum pump, for PMR–SOFC need to be taken into account. The economic analysis shows that the PMR–SOFC is not a good choice from an economic viewpoint because of the requirement of a large high-pressure compressor for feeding gas to the membrane reactor.     

Characterization of nitrogen gas crossover through the membrane in proton-exchange membrane fuel cells

Gas crossover phenomenon through a membrane is inevitable in a proton-exchange membrane fuel cell (PEMFC). For nitrogen, the concentration at the cathode side is usually higher than that at the anode side, so N₂ permeates to the anode side. Nitrogen gas crossover (NGC) may cause fuel starvation, if N₂ gas accumulates in the hydrogen recirculation loop. Thus, it is important to determine the NGC under various PEMFC operating conditions. In this study, characterization of NGC under both open circuit voltage (OCV) and power generation conditions is investigated using a mass spectrometer. Under OCV conditions with the PEMFC membrane fully hydrated, N₂ concentration in the anode exit stream increases as cell temperature increases. Nitrogen permeability coefficients (NPC) are calculated based on the obtained N₂ concentration data. The results show that NPC exhibits an Arrhenius type relationship. Under OCV conditions, the maximum NPC is 5.14 × 10^?13 mol m^?1 s^?1 Pa^?1 with an N₂ activation energy of 19.83 kJ. Under power generation conditions, the NGC increases with increasing current density, which is the result of elevated membrane temperature and increased water content. When the anode stoichiometric ratio (SR_A) is lowered, the N₂ concentration increases under all tested current densities. A low hydrogen flow rate, along with a low SR_A at low current density, significantly increases N₂ concentration at the anode outlet.     

A comparative study of single and dual ejector concepts for anodic recirculation system in high-performance vehicular proton exchange membrane fuel cells

The fuel supply system is a critical element in Proton Exchange Membrane Fuel Cell (PEMFC). In vehicular applications, an anode recirculation system (ARS) is required to return residual hydrogen to the fuel line. A 1D model of an ejector-based ARS was created and integrated with a vehicular PEMFC in this study. Therefore, two configurations, single-ejector-based and dual-ejector-based ARS, were developed to regulate the fuel supply and, thus, utilized to examine and conduct a comparative study of their advantages and disadvantages to the dynamic behavior of the stack. The use of ARS was found to improve stack performance and fuel utilization by delivering high stoichiometry, recirculation rate, and relative humidity. Contrary to a single-ejector-based ARS, whose use is restricted due to its limited working range, dual-ejectors can cover all fuel cell stack operating ranges. Sensitivity analysis revealed that primary pressure was the most significant parameter affecting ejector performance and flow characteristics.     

Discrete ejector control solution design,characterization, and verification in a 5 kW PEMFC system

An ejector primary gas flow control solution based on three solenoid valves is designed, implemented and tested in a 5 kW proton exchange membrane fuel cell (PEMFC) system with ejector-based anode gas recirculation. The robust and cost effective combination of the tested flow control method and a single ejector is shown to achieve adequate anode gas recirculation rate on a wide PEMFC load range.In addition, the effect of anode gas inert content on ejector performance in the 5 kW PEMFC system is studied at varying load and anode pressure levels. Results show that increasing the inert content increases recirculated anode gas mass flow rate but decreases both the molar flow rate and the anode inlet humidity.Finally, the PEMFC power ramp-rate limitations are studied using two fuel supply strategies: 1) advancing fuel supply and venting out extra fuel and 2) not advancing fuel supply but instead using a large anode volume. Results indicate that the power of the present PEMFC system can be ramped from 1 kW to 4.2 kW within few hundred milliseconds using either of these strategies.