Experimental study on dual recirculation of polymer electrolyte membrane fuel cell

Durability and start-up ability in sub-zero environment are two technical bottlenecks of vehicular polymer electrolyte membrane (PEM) fuel cell systems. With exhaust gas recirculation on the anode and cathode side, the cell voltage at low current density can be reduced, and the membrane can be humidified without external humidifier. They may be helpful to prolong the working lifetime and to promote the start-up ability. This paper presents an experimental study on a PEM fuel cell system with anodic and cathodic recirculation. The system is built up based on a 10 kW fuel cell stack, which consists of 50 cells and has an active area of 261 cm². A cathodic recirculation pump and a hydrogen recirculation pump are utilized on the cathode and anode side, respectively. Key parameters, e.g., stack current, stack voltage, cell voltage, air flow, relative humidity on the cathode side, oxygen concentration at the inlet and outlet of the cathode side, are measured. Results show that: 1) with a cathodic recirculation the system gets good self-humidification effect, which is similar to that with an external humidifier; 2) with a cathodic recirculation and a reduction of fresh air flux, the cell voltage can be obviously reduced; 3) with an anodic recirculation the cell voltage can also be reduced due to a reduction in the hydrogen partial pressure, the relative humidity on the cathode side is a little smaller than the case with only cathode recirculation. It indicates that, for our stack the cathodic recirculation is effective to clamp cell voltage at low current density, and a self-humidification system is possible with cathodic recirculation. Further study will focus on the dynamic model and control of the dual recirculation fuel cell system.     

Development of a PEM stack and performance analysis including the effects of water content in the membrane and cooling method

For the use of proton exchange membrane (PEM) fuel cell systems to become widespread, the components required to build one should be minimized. Because a PEM fuel cell has a limited operating temperature range, it requires some kind of cooling method. In this study, different cooling methods were investigated experimentally. A PEM fuel cell stack with an active area of 100 cm² and 8 cells in series was developed and used in this research. When 50% relative humidity inlet gases were supplied (at 15 A of current discharge and 70 °C), cell temperatures at the center increased from around 60 °C to 85 °C, and cell voltage dropped from 4.8 V to 3.2 V because of membrane drying (insufficient cooling). When fully hydrated inlet gases (100% relative humidity) were supplied to the PEM stack at the same test conditions, the cell temperature remained around 65 °C, and stack voltage remained around 5.7 V at 15 A of current discharge. Fully hydrated inlet gases play a positive role both for water transport (when the proton moves from the anode to the cathode) and to maintain the fuel cell stack temperature to prevent stack drying.     

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.     

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.     

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.     

The design and development of an HT-PEMFC test cell and test station

A proper system must be applied in order to have reliable power production and higher levels of durability. The functions of this system are to fully utilize the benefits of the fuel cell components and to deliver the required power. This paper presents the design for an HT-PEMFC single-cell test cell with the development of a test station to operate the cell. The single-cell test cell and the real-time monitoring test station were designed using LabVIEW software, and were implemented using NI's cFP hardware devices, the details of which are provided. The architecture of the test station was aimed at measuring and controlling the mass flow rate, pressure and temperature of the reactant gases, and the stack temperature and current. An electronic load, with a quick dynamic response, was used to test the fuel cell reaction. The start-up, shutdown and monitoring functions were managed by the test station, which monitored the critical parameters of the fuel cell, namely, the voltage, current loading, generated power, hydrogen/air inlet and outlet and stack temperature. The results showed that the designed and developed single-cell HT-PEMFC and test station were able to generate power. An in-depth research needs to be conducted into the innovative design and development of HT-PEMFC systems since these are the key factors for optimum performance.     

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.     

Impact of defective single cell on the operation of polymer electrolyte membrane fuel cell stack

Potential inversion of a single cell in a PEMFC stack reduces its performance and can lead to severe damage in the MEAs. Numerous causes of potential inversion can be considered, from MEA degradation to insufficient feed of reacting gases. The first part of the paper deals with experiments carried out to investigate whether the potential inversion could be suppressed by increasing the inlet gas flow. Then, the defective cell resistances were measured and compared to those exhibited by the other cells in the 23-cell stack. We also studied the influence of this cell on the whole stack resistances. Finally, the defective cell was dismantled and analysed, in order to explain where the defect comes from and whether it has affected each part of the cell.     

Study on voltage clamping and self-humidification effects of pem fuel cell system with dual recirculation based on orthogonal test method

Durability and reliability are still major challenges of vehicular polymer electrolyte membrane fuel cell (PEMFC) systems. With exhaust gas recirculation on both the anode and cathode sides, two important functions can be achieved: the voltage clamping in low current density, and the self-humidification without any external humidifiers. The former restrains catalyst decay in small load working conditions, and the latter is beneficial for improving the cold-start ability. In this study, dynamic performances and stable characteristics of a fuel cell system with dual exhaust gas recirculation are firstly experimentally studied using an orthogonal test method. System parameters, including humidification temperature of cathode external humidifier, fresh air stoichiometric ratio (SR), current density, cathode and anode recirculation pump speeds, are regarded as key factors in the experiments based on the testing conditions of the test-bench. Two four-factor and three-level orthogonal tables are designed, and the effects of key factors on system performance indices (average cell voltage, relative humidity (RH) at cathode inlet, high frequency resistance (HFR), oxygen concentrations at cathode inlet and outlet, and the concentration difference between these two positions) are investigated. Results show that: (1) with the cathode recirculation, the cell voltage can be reduced in low current densities by coordinately adjusting the recycled gas flow and reducing fresh air SR; (2) with the dual recirculation, the fuel cell membrane can be well hydrated, and system performance only shows 3% reduction compared with a system with an external humidifier; (3) the difference between the oxygen molar concentration at the inlet and outlet of cathode gas channels becomes small using dual recirculation.     

Characterization of flooding and two-phase flow in polymer electrolyte membrane fuel cell stacks

A partially flooded gas diffusion layer (GDL) model is proposed and solved simultaneously with a stack flow network model to estimate the operating conditions under which water flooding could be initiated in a polymer electrolyte membrane (PEM) fuel cell stack. The models were applied to the cathode side of a stack, which is more sensitive to the inception of GDL flooding and/or flow channel two-phase flow. The model can predict the stack performance in terms of pressure, species concentrations, GDL flooding and quality distributions in the flow fields as well as the geometrical specifications of the PEM fuel cell stack. The simulation results have revealed that under certain operating conditions, the GDL is fully flooded and the quality is lower than one for parts of the stack flow fields. Effects of current density, operating pressure, and level of inlet humidity on flooding are investigated.     

Humidity of reactant fuel on the cell performance of PEM fuel cell with baffle-blocked flow field designs

The objective of this work is to examine the effects of humidity of reactant fuel at the inlet on the detailed gas transport and cell performance of the PEM fuel cell with baffle-blocked flow field designs. It is expected that, due to the water management problem, the effects of inlet humidity of reactant fuel gases on both anode and cathode sides on the cell performance are considerable. In addition, the effects of baffle numbers on the detailed transport phenomena of the PEM fuel cell with baffle-blocked flow field are examined. Due to the blockage effects in the presence of the baffles, more fuel gas in the flow channel can be forced into the gas diffuser layer (GDL) and catalyst layer (CL) to enhance the chemical reactions and then augment the performance of the PEMFC systems. Effect of liquid water formation on the reactant gas transport is taken into account in the numerical modeling. Predictions show that the local transport of the reactant gas, the local current density generation and the cell performance can be enhanced by the presence of the baffles. Physical interpretation for the difference in the inlet relative humidity (RH) effects at high and low operating voltages is presented. Results reveal that, at low voltage conditions, the liquid water effect is especially significant and should be considered in the modeling. The cell performance can be enhanced at a higher inlet relative humidity, by which the occurrence of the mass transport loss can be delayed with the limiting current density raised considerably.     

Thermodynamic modeling and exergy analysis of proton exchange membrane fuel cell power system

The proton exchange membrane (PEM) fuel cell (PEMFC) is equipped with a series of auxiliary components which consume considerable amount of energy. It is necessary to investigate the design and operation of the PEMFC power system for better system performance. In this study, a typical PEMFC power system is developed, and a thermodynamic model of the system is established. Simulation is carried out, and the power distribution of each auxiliary component in the system, the net power and power efficiency of the system are obtained. This power system uses cooling water for preheating inlet gases, and its energy-saving effect is also verified by the simulation. On this basis, the exergy analysis is applied on the system, and the indexes of the system exergy loss, exergy efficiency and ecological function are proposed to evaluate the system performance. The results show that fuel cell stack and heat exchanger are the two components that cause the most exergy loss. Furthermore, the system performance under various stack inlet temperatures and current densities is also analyzed. It is found that the net power, energy efficiency and exergy efficiency of the system reach the maximum when the stack inlet temperature is about 348.15 K. The ecological function is maintained at a high level when the stack inlet temperature is around 338.15 K. Lower current density increases the system ecological function and the power and exergy efficiencies, and also helps decrease the system exergy loss, but it decreases the system net power.     

车用PEMFC氢气系统建模及其排放特性研究

建立基于尾氢再循环的车用PEMFC氢气系统的集总参数模型和质子交换膜燃料电池堆的二维CFD模型,瞬态模拟研究额定功率工况下尾氢排放对系统及电堆工作特性的影响。结果表明：排放过程中,阳极进气压力和进气流量等参数出现显著的波动现象,且波动幅度和波动时间与排放持续时间存在直接关系;电堆性能在排放过程中有所下降,排放结束后能迅速恢复到排放前的水平;阳极内部的水气分布在排放过程中得到明显改善。     

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.     

Rapid humidity regulation by mixing of dry and humid gases with feedback control for PEM fuel cells

Humidity regulation in fuel cells utilizing bulk mass transfer and membrane-based methodologies have drawbacks and limitations, which are undesirable during fuel cell operation. In this work, we demonstrate a rapid and nearly accurate humidity control by mixing of dry and humidified gas streams for tracking predefined relative humidity set points. The humidity responses from the mixed stream could track various relative humidity profiles when coupled with feedback to PI and PID control algorithms. A discussion on the usefulness of this method for controlling the humidification of fuel cell inlet gases, its effectiveness and limitations under high frequency operation are provided.     

A parametric comparison of three fuel recirculation system in the closed loop fuel supply system of PEM fuel cell

Anodic fuel recirculation system has a significant role on the parasitic power of proton exchange membrane fuel cell (PEMFC). In this paper, different fuel supply systems for a PEMFC including a mechanical compressor, an ejector and an electrochemical pump are evaluated. Furthermore, the performances of ejector and electrochemical pump are studied at different operating conditions including operating temperature of 333 K–353 K, operating pressure of 2 bar–4 bar, relative humidity of 20%–100%, stack cells number from 150 to 400 and PEMFC active area of 0.03 m²–0.1 m². The results reveal that higher temperature of PEMFC leads to lower power consumption of the electrochemical pump, because activation over-potential of electrochemical pump decreases at higher temperatures. Moreover, higher operating temperature and pressure of PEMFC leads to higher stoichiometric ratio and hydrogen recirculation ratio because the motive flow energy in ejector enhances. In addition, the recirculation ratio and hydrogen stoichiometric ratio increase, almost linearly, with increase of anodic relative humidity. Utilization of mechanical compressor leads to lower system efficiency than other fuel recirculating devices due to more power consumption. Utilization of electrochemical pump in anodic recirculation system is a promising alternative to ejector due to lower noise level, better controllability and wide range of operating conditions.     

Design of an air humidifier for a 5 kW proton exchange membrane fuel cell stack operated at elevated temperatures

Air humidification is a crucial issue for superior performance of proton exchange membrane fuel cell (PEM fuel cell) stacks. In this work, an air humidifier is proposed for a 5 kW PEM fuel cell stack working at elevated temperatures, e.g., 90–95 °C. The high temperature coolant exiting the stack is utilized to pre-heat the air in the heat exchanging tubes of the humidifier, and the heated air is humidified with deionized water supplied by a nozzle fixed in a top cavity. Both the tubes and the nozzle are properly designed to ensure sufficient heat transfer and superior atomization. Humidification performance is evaluated under different operation conditions. The nozzle is able to inject well-atomized water with uniform droplet diameter. With the variation of inlet air flow rate, the relative humidity (RH) of the outlet air increases at the beginning, then decreases gradually due to the attenuation of dew point (DP) temperature. However, the humidification performance can be improved when higher temperature deionized water is injected or high temperature coolant is supplied. At a coolant temperature of 95 °C, the outlet air DP temperature is maintained over 80 °C with 25 °C injection water. Moreover, better humidification performance is achieved when the injection water flow rate is controlled according to the working conditions of the stack.     

Modeling and control of air stream and hydrogen flow with recirculation in a PEM fuel cell system—II. Linear and adaptive nonlinear control

In this part of the paper, linear and nonlinear multivariable controllers are designed for the air stream and hydrogen flow with recirculation in a proton exchange membrane (PEM) fuel cell system. The focus of the model is to obtain the desired transient performance of air stoichiometric ratio, cathode inlet pressure, and pressure difference between the anode and the cathode. Based on linearization of the nonlinear dynamic model in the first part of this paper, the coupling between control inputs and performance is analyzed first. The phase relationship between the stack voltage and water transport in frequency domain is meaningful to the future humidity estimation and active purge operation. Then, linear quadratic Gaussian (LQG) algorithm based on observer feedback is used for set-point tracking, and a model-predictive controller (MPC) with an on-line neural network identifier is also designed to improve robustness. Compared with decentralized PI controllers, the multivariable controllers improve the transient response and shows better disturbance rejection capability.