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 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.     

Transient characteristics investigation of the integrated ejector-driven hydrogen recirculation by multi-component CFD simulation

The hydrogen supply of the fuel cell system is realized by the cooperation of multiple components. Transient characteristics of a single component can affect the performance of other components. In this study, a three-dimensional multi-component computational fluid dynamics (CFD) model was developed to investigate the synergistic transient characteristics of the hydrogen recirculation components such as hydrogen injector, ejector, and purge valve in an 80 kW PEMFC. The results show that the entrainment performance of the ejector is reduced under unsteady purge conditions compared with steady conditions. The pressure fluctuation of the secondary flow is significant even under purge closed durations. There are drastic changes in velocity and pressure in the ejector, especially in the mixing chamber. Moreover, an abundant hydrogen supply capacity of the injector is necessary to deal with the excessive anode pressure fluctuation. The feedforward-feedback integrated control of the injector is a more efficient strategy to reduce pressure fluctuations compared with the feedback 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.     

Optimisation of the fuelling of hydrogen vehicles using cascade systems and ejectors

The present study investigates the replacement of expansion valves, used in the cascade system of hydrogen fuelling stations, by a series of ejectors. The major advantage of using ejectors is to recover part of the kinetic energy lost during the expansion of a high-pressure primary flow, in order to entrain a lower pressure secondary flow; thus resulting in a more efficient fuelling.Firstly, a quasi-steady 1-D simulation model of the ejector was calibrated using computational fluid dynamics in terms of the main geometry and pressure conditions.Secondly, the quasi-steady 1-D model of the ejector was used in a dynamic model of the hydrogen fuelling station, in order to investigate the influence of its geometry on the transient fuelling performances. Different fuelling scenarios were explored with varying number of buffer tanks in the cascade system of the fuelling station, and different initial pressures in the vehicle's tank. The results show that the replacement of the expansion valve by an ejector may reduce the energy consumption for hydrogen compression by up to 6.5% using two buffer tanks in the cascade system. On the other hand, increasing the number of buffer tanks reduces the energy savings as the driving pressure ratio decreases.     

Effect of addition of hydrogen and exhaust gas recirculation on characteristics of hydrogen gasoline engine

The effects of exhaust gas recirculation (EGR) on combustion and emissions under different hydrogen ratios were studied based on an engine with a gasoline intake port injection and hydrogen direct injection. The peak cylinder pressure increases by 9.8% in the presence of a small amount of hydrogen. The heat release from combustion is more concentrated, and the engine torque can increase by 11% with a small amount of hydrogen addition. Nitrogen oxide (NOx) emissions can be reduced by EGR dilution. Hydrogen addition offsets the blocking effect of EGR on combustion partially, therefore, hydrogen addition permits a higher original engine EGR rate, and yields a larger throttle opening, which improves the mechanical efficiency and decreases NOx emissions by 54.8% compared with the original engine. The effects of EGR on carbon monoxide (CO) and hydrocarbon (HC) emissions are not obvious and CO and HC emissions can be reduced sharply with hydrogen addition. CO, HC, and NOx emissions can be controlled at a lower level, engine output torque can be increased, and fuel consumption can be reduced significantly with the co-control of hydrogen addition and EGR in a hydrogen gasoline engine.     

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.     

Experiment and simulation investigations for effects of flow channel patterns on the PEMFC performance

Experiments and simulations are presented in this paper to investigate the effects of flow channel patterns on the performance of proton exchange membrane fuel cell (PEMFC). The experiments are conducted in the Fuel Cell Center of Yuan Ze University and the simulations are performed by way of a three‐dimensional full‐cell computational fluid dynamics model. The flow channel patterns adopted in this study include the parallel and serpentine flow channels with the single path of uniform depth and four paths of step‐wise depth, respectively. Experimental measurements show that the performance (i.e. cell voltage) of PEMFC with the serpentine flow channel is superior to that with the parallel flow channel, which is precisely captured by the present simulation model. For the parallel flow channel, different depth patterns of flow channel have a strong influence on the PEMFC performance. However, this effect is insignificant for the serpentine flow channel. In addition, the calculated results obtained by the present model show satisfactory agreement with the experimental data for the PEMFC performance under different flow channel patterns. These validations reveal that this simulation model can supplement the useful and localized information for the PEMFC with confidence, which cannot be obtained from the experimental data. Copyright © 2007 John Wiley & Sons, Ltd.     

Numerical analysis of transport phenomena for designing of ejector in PEM forklift system

In the present study, Computational Fluid Dynamics (CFD) technique is used to design an ejector for anode recirculation in an automotive PEMFC system. A CFD model is firstly established and tested against well-documented and relevant solutions from the literature, and then used for different ejector geometries under different operating conditions. Results showed that a single ejector with optimized geometry cannot cover the required recirculation in the entire range of the fuel cell current. Having two ejectors for different ranges of currents is thus proposed as an alternative solution in which the system can better take the advantage of ejectors for recirculation purpose. In addition, the operating mode of one variable nozzle ejector has been investigated and compared with aforementioned cases. The results showed that the variable nozzle ejector can work in the same operational mode as in the case with two ejectors. However, in practice, the latter one needs a more complicated control system and it is more difficult to manufacture.     

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.     

Experimental study of engine performance and emissions for hydrogen diesel dual fuel engine with exhaust gas recirculation

With an alarming enlargement in vehicular density, there is a threat to the environment due to toxic emissions and depleting fossil fuel reserves across the globe. This has led to the perpetual exploration of clean energy resources to establish sustainable transportation. Researchers are continuously looking for the fuels with clean emission without compromising much on vehicular performance characteristics which has already been set by efficient diesel engines. Hydrogen seems to be a promising alternative fuel for its clean combustion, recyclability and enhanced engine performance. However, problems like high NOx emissions is seen as an exclusive threat to hydrogen fuelled engines. Exhaust gas recirculation (EGR), on the other hand, is known to overcome the aforementioned problem. Therefore, this study is conducted to study the combined effect of hydrogen addition and EGR on the dual fuelled compression ignition engine on a single cylinder diesel engine modified to incorporate manifold hydrogen injection and controlled EGR. The experiments are conducted for 25%, 50%, 75% and 100% loads with the hydrogen energy share (HES) of 0%, 10% and 30%. The EGR rate is controlled between 0%, 5% and 10%. With no substantial decrement in engine's brake thermal efficiency, high gains in terms of emissions are observed due to synergy between hydrogen addition and EGR. The cumulative reduction of 38.4%, 27.4%, 33.4%, 32.3% and 20% with 30% HES and 10% EGR is observed for NOx, CO2, CO, THC and PM, respectively. Hence, the combination of hydrogen addition and EGR is observed to be advantageous for overall emission reduction.     

CFD investigating the effects of different operating conditions on the performance and the characteristics of a high-temperature PEMFC

The effects of different operating conditions on the performance and the characteristics of a high-temperature proton exchange membrane fuel cell (PEMFC) are investigated using a three-dimensional (3-D) computational fluid dynamics (CFD) fuel-cell model. This model consists of the thermal-hydraulic equations and the electrochemical equations. Different operating conditions studied in this paper include the inlet gas temperature, system pressure, and inlet gas flow rate, respectively. Corresponding experiments are also carried out to assess the accuracy of this CFD model. Under the different operating conditions, the PEMFC performance curves predicted by the model correspond well with the experimentally measured ones. The performance of PEMFC is improved as the increase in the inlet temperature, system pressure or flow rate, which is precisely captured by the CFD fuel cell model. In addition, the concentration polarization caused by the insufficient supply of fuel gas can be also simulated as the high-temperature PEMFC is operated at the higher current density. Based on the calculation results, the localized thermal-hydraulic characteristics within a PEMFC can be reasonably captured. These characteristics include the fuel gas distribution, temperature variation, liquid water saturation distribution, and membrane conductivity, etc.     

Numerical investigation on the effect of flow field and landing to channel ratio on the performance of PEMFC

Three-dimensional numerical investigation of PEMFC with landing to channel ratio (L:C) of 2:2 for 25-cm² serpentine-parallel channel has been simulated, and the obtained results have been validated with the polarization curve obtained through experiments. It is found that the maximum error in the polarization curve is less than 4%, and thus a very good deal exists between the simulation study and experimentation. Upon validation, the study has been extended for various flow path designs with different L:C ratio numerically. The prediction reveals that the L:C ratio of 2:2 exhibits the better performance for all the flow channels considered, and it is found that the straight-zigzag flow field with L:C ratio of 2:2 attributes the maximum power density of 0.3250 W/cm² for an optimum open circuit voltage of 0.4 Volts with minimal pressure drop. Oxygen consumption in the cathode flow channels of serpentine-parallel, serpentine-zigzag, and straight-parallel are 77.08%, 10.41%, and 42.70% lesser than that of straight-zigzag PEMFC, respectively. The pressure drop in the flow channel of serpentine-parallel, serpentine-zigzag, and straight-parallel with landing to channel ratio 2:2 are 78.18%, 95.81%, and 48.33% higher than that of straight-zigzag flow field, respectively. The polarization curve, hydrogen (H₂), oxygen (O₂), water content along the flow channel and the proton conductivity, H₂O content across the membrane electrolyte, and current density contour at the GDL/catalyst interface of the anode side for all flow channel configurations have been presented and discussed.     

Investigating the transport characteristics and cell performance for a micro PEMFC through the micro sensors and CFD simulations

The local transport characteristics and the global polarization curve for a self-made micro proton exchange membrane fuel cell (PEMFC) have been experimentally and numerically investigated in this paper. The micro-sensors are developed to measure the local fluid temperature, cell voltage, and current density and the fuel cell test system is used to measure the polarization curve. A three-dimensional (3-D) non-isothermal compressible computational fluid dynamics (CFD) full-cell model is also adopted to simulate the test micro PEMFC. This CFD model has been validated with these global and local data. The ionic conductivity is increased as the water content in the membrane increases, enhancing the cell performance. This positive effect of inlet fuel humidity on the cell performance is also captured by the CFD simulation model.     

Investigation of the effect of humidity at both electrode on the performance of PEMFC using orthogonal test method

Under the vehicle working conditions, operating temperature and relative humidity (RH) of gas supplies have an important influence on the performance and durability of PEMFC. In this paper, the influence of hydrogen RH, air RH, operating temperature and air stoichiometric ratio on the performance of PEMFC was studied with the help of 5 factors and 4 levels orthogonal test table. The fifth factor was set to be empty to carry out the variance analysis. Using variance analysis method, the influence degree of random errors on the experiments was analyzed. The credibility of the experimental results was verified along with the polarization curve and the electrochemical impedances spectroscopy (EIS) results. The experiment results show that the most important factor which affects the performance of PEMFC is air stoichiometric ratio, followed by air RH, and then the operating temperature. The effect of hydrogen RH on PEMFC performance is minimal and negligible. Low hydrogen RH can be used during operation to reduce energy consumption of auxiliary system.     

A three-dimensional full-cell CFD model used to investigate the effects of different flow channel designs on PEMFC performance

A three-dimensional “full-cell” computational fluid dynamics (CFD) model is proposed in this paper to investigate the effects of different flow channel designs on the performance of proton exchange membrane fuel cells (PEMFC). The flow channel designs selected in this work include the parallel and serpentine flow channels, single-path and multi-path flow channels, and uniform depth and step-wise depth flow channels. This model is validated by the experiments conducted in the fuel cell center of Yuan Ze University, showing that the present model can investigate the characteristics of flow channel for the PEMFC and assist in the optima designs of flow channels. The effects of different flow channel designs on the PEMFC performance obtained by the model predictions agree well with those obtained by experiments. Based on the simulation results, which are also confirmed by the experimental data, the parallel flow channel with the step-wise depth design significantly promotes the PEMFC performance. However, the performance of PEMFC with the serpentine flow channel is insensitive to these different depth designs. In addition, the distribution characteristics of fuel gases and current density for the PEMFC with different flow channels can be also reasonably captured by the present model.     

Design and characterization of an electronically controlled variable flow rate ejector for fuel cell applications

A variable flow ejector is presented to address the challenge of providing cost-effective recirculation of hydrogen in fuel cell systems. The ejector uses supersonic flow to provide sufficient pressure rise for the Ballard Mark 9 SSL stack used in the University of Delaware’s fuel cell hybrid buses. Details of geometry optimization via computational fluid dynamic simulation, control system design, electronic control implementation, and mechanical design are discussed. Results from testing in the final application are included, showing the ejector’s excellent performance compared to Ballard’s specifications for recirculation flow rate.     

Development of a methodology to optimize the air bleed in PEMFC systems operating with low quality hydrogen

The use of hydrogen with lower quality than that specified in current regulation is an attractive option for stationary PEMFC power production. In this paper, the effect of CO is mitigated using air bleed levels up to 2% in an H₂ PEMFC fed with CO concentrations below 20 ppm. A methodology to optimize the air bleed levels is developed using a novel arrangement of cells coupled to a gas chromatograph. The methodology relies on evaluating the distributed performance of the cell and on determining the CO and CO₂ molar flow rates at the anode outlet. Furthermore, the amount of CO adsorbed onto the catalyst and the fraction of catalytic sites covered by CO are estimated. The results show that different parameters, such as the H₂ volumetric flow rate, CO concentration and air bleed level, influence both the steady state and dynamics of PEMFCs operated with low quality hydrogen.     

Simulation analysis of hydrogen recirculation rates of fuel cells and the efficiency of combined heat and power

The objective of this study was to simulate a proton-electrolyte membrane fuel cell (PEMFC) system, namely a PEMFC stack, an anode gas supply subsystem, an anode gas-recovery subsystem, a cathode gas supply subsystem, and a tail gas exhaustion subsystem. In addition, this paper presents an analysis of the efficiency of combined heat and power (CHP) systems. MATLAB and Simulink were employed for dynamic simulation and statistical analysis. The rates of active and the passive anode hydrogen recirculation were considered to elucidate the mechanism of hydrogen circulation. When recovery involved diverse recovery mechanisms, the recirculation rate was affected by the pressure at the hydrogen outlet of the PEMFC system. The greater the pressure was at that outlet, the higher the recovery rate was. In the hydrogen recovery system, when the temperature of the hydrogen supply end remained the same, increasing the temperature of the gas supply end increased the efficiency of the fuel cells; fixing the flow of the hydrogen supply end and increasing the temperature of the hydrogen supply end increased the efficiency of the PEMFC system. A calculation of the efficiency of the recovery system indicated that the thermal efficiency of the fuel cells exceeded 35%, the power generation efficiency exceeded 45%, and the efficiency of the CHP system exceeded 80%.