Disclosure of the internal transport phenomena in an air-cooled proton exchange membrane fuel cell— part II: Parameter sensitivity analysis

The operating and designing parameters have significant influences on the performance of an air-cooled proton exchange membrane fuel cell. Figuring out the parameter sensitivity helps select the appropriate operating point and the geometry size for a fuel cell. In this paper, parameter sensitivity analysis is conducted for the performance and the internal transport phenomena of an air-cooled proton exchange membrane fuel cell based on different air stoichiometries, air relative humidities and air flow field designs. The numerical results show that large air stoichiometry helps lower the single cell temperature, keeps the membrane better hydrated, and improves cell performance. Especially, the fluctuation of water content always exists periodically for the case of different air stoichiometry, where the minimum value of water content appears underneath the cathode channel in contrast to the maximum value appearing underneath the cathode rib. Furthermore, the maximum periodic fluctuation amplitude of water content is even more than 8 for the case of air stoichiometry of 150. The water flooding phenomenon becomes severe with the increase of air stoichiometry. Air with larger relative humidity also increases the single cell performance by improving the hydration of the membrane. However, water flooding becomes worse with the increment of air relative humidity. The narrower channel design for the cathode flow field not only leads to a more uniform current density distribution but also keeps the membrane better hydrated and thus enhances the cell performance.     

Analysis of the water and thermal management in proton exchange membrane fuel cell systems

Water and thermal management is essential to the performance of proton exchange membrane (PEM) fuel cell system. The key components in water and thermal management system, namely the fuel cell stack, radiator, condenser and membrane humidifier are all modeled analytically in this paper. Combined with a steady-state, one-dimensional, isothermal fuel cell model, a simple channel-groove pressure drop model is included in the stack analysis. Two compact heat exchangers, radiator and condenser are sized and rated to maintain the heat and material balance. The influence of non-condensable gas is also considered in the calculation of the condenser. Based on the proposed methodology, the effects of two important operating parameters, namely the air stoichiometric ratio and the cathode outlet pressure, and three kinds of anode humidification, namely recycling humidification, membrane humidification and recycling combining membrane humidification are analyzed. The methodology in this article is helpful to the design of water and thermal management system in fuel cell systems.     

Experimental study of a novel piezoelectric proton exchange membrane fuel cell with nozzle and diffuser

The newly designed proton exchange membrane fuel cell with a piezoelectric actuation structure, called a PZT-PEMFC, can force air into an air-breathing PEMFC system. Previous studies indicated the PZT-PEMFC may solve the water-flooding problem and improve cell performance. In this experimental study, a PZT-PEMFC with nozzle and diffuser, PZT-PEMFC-ND, is built to verify the previous theoretical study. This innovative design may direct air flow into the cathode channel through the diffuser and prevent air backflow without valves. The performance test includes an analysis of PZT vibration frequencies, cell operation temperatures, gravity effect, and designs of the nozzle and diffuser. The optimal operating temperature for the PZT-PEMFC-ND is 323 K to avoid the risk of higher temperatures drying out the membrane electrode assembly (MEA). The optimal vibration frequency of the PZT-PEMFC-ND is 180 Hz, which may pump in enough air and solve the water-flooding problem in the cathode channel. This study also concludes that the innovative design of the PZT-PEMFC-ND, may reach the performance of an open cathode stack configuration, 0.18 W cm⁻², without an external air supply device.     

Design and implementation of LQR/LQG strategies for oxygen stoichiometry control in PEM fuel cells based systems

This paper presents the oxygen stoichiometry control problem of proton exchange membrane (PEM) fuel cells and introduces a solution through an optimal control methodology. Based on the study of a non-linear dynamical model of a laboratory PEM fuel cell system and its associated components (air compressor, humidifiers, line heaters, valves, etc.), a control strategy for the oxygen stoichiometry regulation in the cathode line is designed and tested. From a linearised model of the system, an LQR/LQG controller is designed to give a solution to the stated control problem. Experimental results show the effectiveness of the proposed controllers design.     

Exergoeconomic analysis of a vehicular PEM fuel cell system

In this study, we deal with the exergoeconomic analysis of a proton exchange membrane (PEM) fuel cell power system for transportation applications. The PEM fuel cell performance model, that is the polarization curve, is previously developed by one of the authors by using the some derived and developed equations in literature. The exergoeconomic analysis includes the PEM fuel cell stack and system components as compressor, humidifiers, pressure regulator and the cooling system. A parametric study is also conducted to investigate the system performance and cost behaviour of the components, depending on the operating temperature, operating pressure, membrane thickness, anode stoichiometry and cathode stoichiometry. For the system performance, energy and exergy efficiencies and power output are investigated in detail. It is found that with an increase of temperature and pressure and a decrease of membrane thickness the system efficiency increases which leads to a decrease in the overall production cost. The minimization of the production costs is very crucial in commercialization of the fuel cells in transportation sector.     

On-line implementation of model free controller for oxygen stoichiometry and pressure difference control of polymer electrolyte fuel cell

This paper proposes and validates a model free controller to improve the real time operating conditions of Proton Exchange Membrane Fuel Cells (PEMFC). This approach is based on an ultra-local model that does not depend on a precise knowledge of the system. It is perfectly adapted to a complex system such as the fuel cell, while benefiting from the ease of online implementation and low computational cost. The designed controller is used to regulate both the oxygen stoichiometry and the membrane inlet pressure, which are crucial operating conditions for the fuel cell's lifetime. The objectives of the proposed control strategy are twofold: preventing the starvation failure, and limiting the potential for mechanical degradation of the membrane during a large pressure difference. The performance of the proposed control strategy is initially evaluated by a simulation environment for both oxygen stoichiometry and inlet pressure difference control of fuel cell stack. An online validation on 1.2 KW fuel cell stack is conducted to control the membrane pressure drop. Two case studies are comprehensively investigated in relation to stoichiometry control: set point tracking and rejection of unmeasured disturbances caused by current variations. Simulations and experimental results reveal that the proposed controller provides significantly better performance in terms of fast trajectory tracking, and ensures less overshoot compared to the Fuzzy PID and PID controller. This efficiency is proven using the Integral Absolute Error (IAE), Integral Squared Error (ISE) and Integral of the Square input (ISU) performance indexes.     

High performance PEMFC stack with open-cathode at ambient pressure and temperature conditions

An open-air cathode proton exchange membrane fuel cell (PEMFC) was developed. This paper presents a study of the effect of several critical operating conditions on the performance of an 8-cell stack. The studied operating conditions such as cell temperature, air flow rate and hydrogen pressure and flow rate were varied in order to identify situations that could arise when the PEMFC stack is used in low-power portable PEMFC applications. The stack uses an air fan in the edge of the cathode manifolds, combining high stoichiometric oxidant supply and stack cooling purposes. In comparison with natural convection air-breathing stacks, the air dual-function approach brings higher stack performances, at the expense of having a lower use of the total stack power output. Although improving the electrochemical reactions kinetics and decreasing the polarization effects, the increase of the stack temperature lead to membrane excessive dehydration (loss of sorbed water), increasing the ohmic resistance of the stack (lower performance).     

Cooling strategy for effective automotive power trains: 3D thermal modeling and multi-faceted approach for integrating thermoelectric modules into proton exchange membrane fuel cell stack

The 3D Thermal modeling utilizes a Finite Differencing heat alteration method augmented with empirical boundary conditions is employed to develop 3D thermal model for the integration of thermoelectric modules with proton exchange membrane fuel cell stack. Hardware-in-Loop was designed under pre-defined drive cycle to obtain fuel cell performance parameters along with anode and cathode gas flow-rates and surface temperatures. The fuel cell model is used to conjugate the experimental boundary conditions with the Finite Differencing code, which implemented heat generation across the stack to depict the chemical composition process. The structural and temporal temperature contours obtained from this model are in compliance with the actual recordings obtained from the infrared detector and thermocouples. The model is harmonized with thermo-electric modules with a modeling strategy, which enables optimize better temporal profile across the stack. This study presents the improvement of a 3D thermal model for proton exchange membrane fuel cell stack along with the interfaced thermo-electric module. The model provided a virtual environment using a model-based design approach to assist the design engineers to manipulate the design correction earlier in the process and eliminate the need for costly and time consuming prototypes.     

An air-cooled proton exchange membrane fuel cell with combined oxidant and coolant flow

Combining the oxidant and coolant flow in an air-cooled proton exchange membrane fuel cell can significantly simplify the fuel cell design. In this paper, an air-cooled PEM fuel cell stack with an open cathode flow field, which supplied the oxidant and removed the heat produced in the fuel cell, was fabricated and tested. The influence of different operating parameters on cell voltage performance and the overall cell ohmic resistance, such as cell temperature and airflow rate, was investigated. The cell temperature and the temperature difference between the cell and the hydrogen humidifier were shown to serve important roles in reducing the fuel cell ohmic resistance. The test results also showed a noteworthy temperature gradient between each cell of a 5-cell stack. A hydrophilic treatment of the cathode flow field channels was demonstrated to be an effective way to mitigate water management issues caused at elevated operating temperatures.     

Experimental study on spatiotemporal distribution and variation characteristics of temperature in an open cathode proton exchange membrane fuel cell stack

This paper experimentally explores the spatiotemporal distribution and variation characteristics of temperature in an open cathode proton exchange membrane fuel cell stack based on thermal imager and thermocouples inserted in the cathode flow channels. The temperature distribution and evolution during the dynamic process are analyzed in detail. Besides, the effects of air flow rate and load current on the thermal characteristics of the stack are also investigated. The results show that during the start-up, the hot spot first sprouts in the central area and then spreads rapidly to the surrounding area. During the shutdown, the central and lower regions are first cooled, followed by the hydrogen inlet region, and finally the endplates. The temperature during the load stepwise increase is inconsistent with that during the load stepwise decrease, showing a temperature drift phenomenon. Moreover, there is a time lag in the response of temperature and voltage to changes in current.     

An application of indirect model reference adaptive control to a low-power proton exchange membrane fuel cell

Nonlinearity and the time-varying dynamics of fuel cell systems make it complex to design a controller for improving output performance. This paper introduces an application of a model reference adaptive control to a low-power proton exchange membrane (PEM) fuel cell system, which consists of three main components: a fuel cell stack, an air pump to supply air, and a solenoid valve to adjust hydrogen flow. From the system perspective, the dynamic model of the PEM fuel cell stack can be expressed as a multivariable configuration of two inputs, hydrogen and air-flow rates, and two outputs, cell voltage and current. The corresponding transfer functions can be identified off-line to describe the linearized dynamics with a finite order at a certain operating point, and are written in a discrete-time auto-regressive moving-average model for on-line estimation of parameters. This provides a strategy of regulating the voltage and current of the fuel cell by adaptively adjusting the flow rates of air and hydrogen. Experiments show that the proposed adaptive controller is robust to the variation of fuel cell system dynamics and power request. Additionally, it helps decrease fuel consumption and relieves the DC/DC converter in regulating the fluctuating cell voltage.     

Innovative cathode flow-field design for passive air-cooled polymer electrolyte membrane (PEM) fuel cell stacks

A novel cathode flow-field design suitable for a passive air-cooled polymer electrolyte membrane (PEM) fuel cell stack is proposed to enhance the water-retaining capability under excess dry air supply conditions. The innovative cathode flow-field is designed to supply more air to the cooling channels and further enables deceleration of the reactant air in the gas channels and acceleration of the coolant air in the cooling channels simultaneously along the air flow path. Therefore, the design facilitates the waste heat removal through the cooling channels while the water removal by the reactant air is minimized. The conceptual cathode flow-field design is validated using a three-dimensional PEM fuel cell model. The detailed simulation results clearly demonstrate that the new cathode flow-field design exhibits superior water-retaining capability compared with a conventional cathode flow-field design (parallel flow channel configuration) under typical air-cooled fuel cell operating conditions. This study provides a new strategy to design cathode flow-fields to alleviate notorious membrane dehydration and unstable performance issues in a passive air-cooled PEM fuel cell stack.     

Cell and stack‐level study of steady‐state and transient behaviour of temperature uniformity of open‐cathode proton exchange membrane fuel cells

The steady‐state temperature uniformity and thermal transients of open‐cathode proton exchange membrane fuel cell (PEMFC) at cell and stack level are researched experimentally in this study. The local temperatures are obtained by 30 thermocouples contacting the surfaces of cathode gas diffusion layers (GDL). The s temperature homogeneity under different load currents and air flow rates are investigated. The results reveal that the fluctuation of temperature distribution under different currents is small under the lowest air flow rate set in the experiments. Comparatively, the temperature is less uniform when the load current is higher under other air flow rates. The evaluation indicator, temperature uniformity index (TUI), varies nearly linearly with the current. And the maximum variation is 55.6% to 59.0%. This distinct behaviour is probably related to the existence of liquid water and its nonuniform distribution which can enlarge the temperature difference at high current. With respect to thermal transients, there is rapid deterioration in temperature uniformity when the load current is stepped up. It may arise from the uneven liquid water distribution which can lead to different temperature variation rates. Further, the research gives direction for optimization of cooling strategy and thermal management of open‐cathode PEMFC stack in application.     

Performance of a proton exchange membrane fuel cell stack with thermally conductive pyrolytic graphite sheets for thermal management

This work experimentally investigates the effects of the pyrolytic graphite sheets (PGS) on the performance and thermal management of a proton exchange membrane fuel cell (PEMFC) stack. These PGS with the features of light weight and high thermal conductivity serve as heat spreaders in the fuel cell stack for the first time to reduce the volume and weight of cooling systems, and homogenizes the temperature in the reaction areas. A PEMFC stack with an active area of 100 cm² and 10 cells in series is constructed and used in this research. Five PGS of thickness 0.1 mm are cut into the shape of flow channels and bound to the central five cathode gas channel plates. Four thermocouples are embedded on the cathode gas channel plates to estimate the temperature variation in the stack. It is shown that the maximum power of the stack increase more than 15% with PGS attached. PGS improve the stack performance and alleviate the flooding problem at low cathode flow rates significantly. Results of this study demonstrate the feasibility of application of PGS to the thermal management of a small-to-medium-sized fuel cell stack.     

质子交换膜燃料电池堆的热力学分析

建立了质子交换膜燃料电池(PEMFC)堆的热力学分析模型,研究了运行温度、气体分压和阳极流量等工作参数对燃料电池堆能量效率和火用效率的影响。结果表明:对气体加压,能提高热力学能效率和火用效率;温度升高时,系统性能无明显变化;阳极流量增加时,系统的热力学能效率和火用效率有所降低。     

Optimization of fuel cell system operating conditions for fuel cell vehicles

Proton exchange membrane fuel cell (PEMFC) technology for use in fuel cell vehicles and other applications has been intensively developed in recent decades. Besides the fuel cell stack, air and fuel control and thermal and water management are major challenges in the development of the fuel cell for vehicle applications. The air supply system can have a major impact on overall system efficiency. In this paper a fuel cell system model for optimizing system operating conditions was developed which includes the transient dynamics of the air system with varying back pressure. Compared to the conventional fixed back pressure operation, the optimal operation discussed in this paper can achieve higher system efficiency over the full load range. Finally, the model is applied as part of a dynamic forward-looking vehicle model of a load-following direct hydrogen fuel cell vehicle to explore the energy economy optimization potential of fuel cell vehicles.     

Effect of operational parameters on the performance of PEMFC assembled with Au-coated Ni-foam

An innovative proton exchange membrane fuel cell was assembled using Au-coated nickel foam instead of the conventional flow field (carbon plate). The effect of operational parameters on the performance of this cell was investigated by DC polarization and electrochemical impedance spectroscopy techniques. Parameters such as cell operating temperature, cathode humidification temperature, and cathode-gas stoichiometry were of concern.     

Performance evaluation of an air breathing polymer electrolyte membrane (PEM) fuel cell in harsh environments – A case study under Saudi Arabia's ambient condition

A key parameter that determines the efficiency of proton exchange membrane fuel cells is their operating conditions. Optimization of various components in these fuel cells is pivotal in improving cell performance, as their performance is directly related to the operational conditions the cells are subjected to.This investigation examined the viability of an air breathing fuel cell subjected to ambient conditions in Riyadh in Saudi Arabia. A validated three-dimensional air breathing 5-cell stack, modelled in ANSYS was utilised to generate the results. Furthermore, the work also considered the feasibility of deploying a humidifier unit for the hydrogen inlet, so as to ascertain the physical behaviour of the PEMFC stack. It was observed that the performance of the stack reaches its peak during the summer time (June–August), and hydrogen humidification improves output performance by 40%.     

Performance investigation of a transportation PEM fuel cell system

In this study, a comprehensive performance analysis of a transportation system powered by a PEM fuel cell engine system is conducted thermodynamically both through energy and exergy approaches. This system includes system components such as a compressor, humidifiers, pressure regulator, cooling system and the fuel cell stack. The polarization curves are studied in the modeling and compared with the actual data taken from the literature works before proceeding to the performance modeling. The system performance is investigated through parametric studies on energy, exergy and work output values by changing operating temperature, operating pressure, membrane thickness, anode stoichiometry, cathode stoichiometry, humidity, reference temperature and reference pressure. The results show that the exergy efficiency increases with increase of temperature from 323 to 353 K by about 8%, pressure from 2.5 to 4 atm by about 5%, humidity from 97% to 80% by about 10%, and reference state temperature from 253 to 323 K by about 3%, respectively. In addition, the exergy efficiency increases with decrease of membrane thickness from 0.02 to 0.005 mm by about 9%, anode stoichiometry from 3 to 1.1 by about 1%, and cathode stoichiometry from 3 to 1.1 by about 35% respectively.