Experimental study on the endplate effect on the cold-start performance of an open-cathode air-cooled proton exchange membrane fuel cell stack

The temperature gradient inside an open-cathode air-cooled fuel cell is large because it uses air as its reaction and cooling media; moreover, the temperature of single cells near the endplates is low because of the high heat capacity of the endplate compared to single cells. Therefore, the cold start of open-cathode air-cooled fuel cells is difficult. In this work, the cold-start performance of an open-cathode air-cooled fuel cell stack, including the stack voltage, single-cell voltage and temperature distribution, are tested in a climatic chamber. The results show that the endplate effect has a significant adverse influence on the cold-start performance. Due to the existence of the endplate effect, the voltages of the single cells near the endplate decrease significantly. The stack can be successfully started at −5 °C without any external heating; however, when the temperature decreases below −10 °C, it cannot be started. At this time, if a certain power of endplate heating is adopted, successful cold-start can be achieved. However, if the temperature continues to decrease, the stack cannot be successfully started only through endplate heating because both the endplates and cold air affect the cold-start performance. Combining endplate and air heating may be a feasible cold-start method.     

Influence of hydrogen fuel cell temperature safety on bus driving characteristics and stack heating mode

Hydrogen fuel cells have developed rapidly in bus because of their cleanliness. The driving parameters of hydrogen fuel cell bus and the internal temperature distribution of hydrogen fuel stack are investigated in this study. The 1-D model of hydrogen fuel cell bus and the 2-D model of hydrogen fuel cell stack are established based on AMESim. In the cooling process, for the existing typical thermal management system, the influence of ambient temperature on the fuel cell bus driving parameters is analyzed by the 1-D model. In the heating process, the internal temperature distribution of the stack is analyzed under three heating modes by the 2-D model. As the ambient temperature increases, to ensure the safe operating temperature of the fuel cells, the bus top speed should be reduced, and the acceleration time should be extended. When the stack needs to be heated, the gas-liquid heating mode can supply a more uniform temperature distribution inside the stack.     

An adiabatic cell as a model for stack in PEFC cold start study

Inadequate cold start capability impedes the diffusion of fuel cell vehicles in cold regions. While the isothermal cell commonly used in literature is valuable in examining failure mechanism, it is not suitable for exploring startup strategy and the associated degradation of polymer electrolyte fuel cells (PEFCs). A model cell capable of simulating the adiabatic thermal boundary conditions of central cells inside the stack is needed. Herein, we design an adiabatic cell as a model for the central cells in PEFC stack. The adiabatic thermal boundary condition is realized by utilizing thermal adjustment boards, thermocouples, and heating plates. The power of heating plates is adjusted by a feedback proportion-integration-differentiation controller in real time. The system is shown to be capable of providing an adiabatic thermal boundary condition with quick response rate and having no risk of overheating in successful starts with two kinds of strategies. This cell can work as a model system for reproducible and well-controlled study of cold start of PEFC stack.     

Low temperature operation and influence parameters on the cold start ability of portable PEMFCs

The start up behaviour of PEM fuel cells below 0 °C is one of the most challenging tasks to be solved before commercialisation. The automotive industry started to develop solutions to reduce the start up time of fuel cell systems in the middle of the nineties. The strategies varied from catalytic combustion of hydrogen on the electrode catalyst to fuel starvation or external stack heating via cooling loops to increase the stack temperature.Beside the automotive sector the cold start ability is as well important for portable PEMFC applications for outdoor use. But here the cold start issue is even more complicated, as the fuel cell system should be operated as passive as possible.Below 0 °C freezing of water inside the PEMFC could form ice layers in the electrode and in the gas diffusion layer. Therefore the cell reaction is limited or even inhibited. Product water during the start up builds additional barriers and leads to a strong decay of the output power at isothermal operating conditions.In order to find out which operational and hardware parameters affect this decay, potentiostatic experiments on single cells were performed at isothermal conditions. These experiments comprise investigations of the influence of membrane thickness and different GDL types as well as the effect of gas flow rates and humidification levels of the membrane. As pre stage to physical based models, empirical based prediction models are used to gain a better understanding of the main influence parameters during cold start. The results are analysed using the statistical software Cornerstone 4.0.The experience of single cell investigations are compared to start up behaviour of portable fuel cell stacks which are operated in a climate chamber at different ambient temperatures below 0 °C. Additional flow sharing problems in the fuel cell stack could be seen during cold start up experiments.     

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.     

Unsteady 2D PEM fuel cell modeling for a stack emphasizing thermal effects

Models currently used for analyses of thermal and water behavior of a PEM fuel cell are based 3D computational fluid dynamics (CFD). However, the analyses are limited to a single cell with static behavior. Thus, these models cannot be used for analyses of dynamic behavior of a stack that continuously varies according to operating conditions. The model proposed describes dynamic behavior of a stack with two adjoining cells and endplate assembly, and work as a current controlled voltage source that can be used for optimization of BOPs and the associated controls. Simulations have been conducted to analyze start-up behaviors and the performance of the stack. Our analyses deliver following results: (1) dynamic temperature distribution in both the through-plane direction and the along channel direction of the fuel cell stack, (2) effects influencing the source terms of current density, and (3) dynamic oxygen concentration distribution. The temperature profile and its variation propensity are comparable to the previous results [Y. Shan, S.Y. Choe, J. Power Sources, 145 (1) (2005) 30–39; Y. Shan, S.Y. Choe, J. Power Sources, in press].     

Thermal stress analysis of a planar SOFC stack

The aim of this study is, by using finite element analysis (FEA), to characterize the thermal stress distribution in a planar solid oxide fuel cell (SOFC) stack during various stages. The temperature profiles generated by an integrated thermo-electrochemical model were applied to calculate the thermal stress distributions in a multiple-cell SOFC stack by using a three-dimensional (3D) FEA model. The constructed 3D FEA model consists of the complete components used in a practical SOFC stack, including positive electrode–electrolyte–negative electrode (PEN) assembly, interconnect, nickel mesh, and gas-tight glass-ceramic seals. Incorporation of the glass-ceramic sealant, which was never considered in previous studies, into the 3D FEA model would produce more realistic results in thermal stress analysis and enhance the reliability of predicting potential failure locations in an SOFC stack. The effects of stack support condition, viscous behavior of the glass-ceramic sealant, temperature gradient, and thermal expansion mismatch between components were characterized. Modeling results indicated that a change in the support condition at the bottom frame of the SOFC stack would not cause significant changes in thermal stress distribution. Thermal stress distribution did not differ significantly in each unit cell of the multiple-cell stack due to a comparable in-plane temperature profile. By considering the viscous characteristics of the glass-ceramic sealant at temperatures above the glass-transition temperature, relaxation of thermal stresses in the PEN was predicted. The thermal expansion behavior of the metallic interconnect/frame had a greater influence on the thermal stress distribution in the PEN than did that of the glass-ceramic sealant due to the domination of interconnect/frame in the volume of a planar SOFC assembly.     

A spatially resolved physical model for transient system analysis of high temperature fuel cells

This work builds upon previous high temperature fuel cell (HT-FC) modeling studies, capturing both steady state performance and transient behavior of HT-FC stacks by merging simplified dimensional aspects of a planar fuel cell stack with first principles physical modeling. Dynamic simulations are developed that spatially resolve fluctuations in temperature, pressure and concentration distributions during transient operation. A significant portion of the heat transfer occurs prior to and after the air passes over the electrochemically active portions of the cell, justifying additional heat transfer pathways from the stack to the air in order to accurately characterize the thermal transients and temperature distributions in the HT-FC stack. The highly configurable MatLab-Simulink® model developed can simulate both solid oxide and molten carbonate fuel cells utilizing either direct or indirect internal reforming. The perturbation response characteristics of the dynamic model to load, fuel flow, air flow and composition perturbations are discussed, and control strategies are introduced that minimize temperature fluctuations. Analysis indicates air flow and inlet temperature controls are sufficient to control average temperature and average internal temperature gradients. Internal heat transfer dynamics substantially change the spatial temperature distribution and local temperature gradients during typical operating conditions and perturbations.     

Control-orientated thermal model for proton-exchange membrane fuel cell systems

A lumped parameter dynamic model is developed for predicting the stack temperature, temperatures of the exit reactant gases and coolant water outlet in a proton-exchange membrane fuel cell (PEMFC) system. A dynamic model for a water pump is also developed and can be used along with the thermal model to control the stack temperature. The thermal and water pump models are integrated with the air flow compressor and PEMFC stack current–voltage models developed by Pukrushpan et al. to study the fuel cell system under open and closed-loop conditions. The results obtained for the aforementioned variables from open-loop simulation studies are found to be similar to the experimental values reported in the literature. Closed-loop simulations using the model are carried out to study the effect of stack temperature on settling times of other variables such as stack voltage, air flow rate, oxygen excess ratio and net power of the stack. Further, interaction studies are performed for selecting appropriate input–output pairs for control purpose. Finally, the developed thermal model can assist the designer in choosing the required number of cooling plates to minimize the difference between the cooling water outlet temperature and stack temperature.     

Modeling and simulation of a PEM fuel cell stack considering temperature effects

Management of the water and heat ejected as byproducts in an operating PEM fuel cell stack are crucial factors in their optimal design and safe operations. Models currently available for a PEM fuel cell are based on either empirical or 3-D computational fluid dynamics (CFD). Both models do not fully meet the need to represent physical behavior of a stack because of either their simplicity or complexity. We propose a highly dynamic PEM fuel cell stack model, taking into account the most influential property of temperature affecting performance and dynamics. Simulations have been conducted to analyze start-up behaviors and the performance of the stack in conjunction with the cells. Our analyses demonstrate static and dynamic behaviors of a stack. Major results presented are as follows: (1) operating dependent temperature gradient across through-plane direction of the fuel cell stack, (2) endplate effects on the temperature profile during start-up process, (3) temperature profile influences on the output voltage of individual cells and the stack, (4) temperature influence on the water content in membranes of different cells, and (5) cathode inlet relative humidity influence on the temperature profile of the stack.     

A study of operation strategy of cooling module with dynamic fuel cell system model for transportation application

A fuel cell system model with detailed cooling module model was developed to evaluate the control algorithms of cooling module which is used for the thermal management of a proton exchange membrane fuel cell (PEMFC) system. The system model is composed of a dynamic fuel cell stack model and a detailed dynamic cooling module model. To extend modeling flexibility, the fuel cell stack model utilizes analytic approach to capture the transient behavior of the stack temperature corresponding to the change of the coolant temperature and the flow rate during load follow-up. The cooling module model integrated model of fan, water pump, coolant passage, and electric motors so that the model is capable of investigation of operating strategy of pump and fan.The fuel cell system model is applied to the investigation of the control logics of the cooling module. Since, it is necessary for the control of cooling module to define the reference conditions such as coolant temperature and fuel cell stack temperature, this study presents such thermal management criteria. Finally, two control algorithms were compared, a conventional control algorithm and a feedback control algorithm. As a consequence, the feedback control algorithm was found to be more suitable for the cooling module of the PEMFC stack, as they consume less parasitic power while producing more stack power compared to a conventionally controlled cooling module.     

Cold start dynamics and temperature sliding observer design of an automotive SOFC APU

This paper presents a dynamic model for studying the cold start dynamics and observer design of an auxiliary power unit (APU) for automotive applications. The APU is embedded with a solid oxide fuel cell (SOFC) stack which is a quiet and pollutant-free electric generator; however, it suffers from slow start problem from ambient conditions. The SOFC APU system equips with an after-burner to accelerate the start-up transient in this research. The combustion chamber burns the residual fuel (and air) left from the SOFC to raise the exhaust temperature to preheat the SOFC stack through an energy recovery unit. Since thermal effect is the dominant factor that influences the SOFC transient and steady performance, a nonlinear real-time sliding observer for stack temperature was implemented into the system dynamics to monitor the temperature variation for future controller design. The simulation results show that a 100 W APU system in this research takes about 2 min (in theory) for start-up without considering the thermal limitation of the cell fracture.     

An integrated modeling approach for temperature driven water transport in a polymer electrolyte fuel cell stack after shutdown

The concept of using controlled temperature gradients to non-parasitically remove excess water from porous media during PEFC stack shutdown has been numerically investigated. An integrated modeling approach focusing both at stack and single cell level is presented. The stack thermal model is developed to obtain detailed temperature distribution across the PEFC stack. The two-phase unit fuel cell model is developed to investigate the detailed water and thermal transport in the PEFC components after shutdown, which for the first time includes thermo-osmotic flow in the membrane. The model accounts for capillary and phase-change induced flow in the porous media, and thermo-osmotic and diffusive flow in the polymer membrane. The single cell model is used to estimate the local water distribution with land or channel boundary condition, and the experimentally validated stack thermal model provided the transient temperature boundary conditions. Two different stack designs are compared to quantify the residual water in the stack. Model results indicate that a favorable temperature gradient can be formed in the stack to enhance the water drainage rate, esp. at anode end cell locations, where freeze/thaw damage has been observed to occur.     

Thermal characteristics of an air-cooled open-cathode proton exchange membrane fuel cell stack via numerical investigation

In light of stricter emissions regulations and depleting fossil fuel reserves, fuel cell vehicles (FCVs) are one of the leading alternatives for powering future vehicles. An open-cathode, air-cooled proton exchange membrane fuel cell (PEMFC) stack provides a relatively simple electric generation system for a vehicle in terms of system complexity and number of components. The temperature within a PEMFC stack is critical to its level of performance and the electrochemical efficiency. Previously created computational models to study and predict the stack temperature have been limited in their scale and the inaccurate assumption that temperature is uniform throughout. The present work details the creation of a numerical model to study the temperature distribution of an 80-cell Ballard 1020ACS stack by simulating the cooling airflow across the stack. Using computational fluid dynamics, a steady-state airflow simulation was performed using experimental data to form boundary conditions where possible. Additionally, a parametric study was performed to investigate the effect of the distance between the stack and cooling fan on stack performance. Model validation was performed against published results. The temperature distribution across the stack was identical for the central 70% of the cells, with eccentric temperatures observed at the stack extremities, while the difference between coolant and bipolar plate temperatures was approximately 10°C at the cooling channel outlets. The results of the parametric study showed that the fan-stack distance has a negligible effect on stack performance. The assumptions regarding stack temperature uniformity and measurement were challenged. Lastly, the hypothesis regarding the negligible effect of fan-stack distance on stack performance was confirmed.     

One-dimensional thermal model of cold-start in a polymer electrolyte fuel cell stack

A transient, one-dimensional thermal model for a generic polymer electrolyte fuel cell (PEFC) stack is developed to investigate the cold-start ability and the corresponding energy requirement over different operating and ambient conditions. The model is constructed by applying the conservation of energy on each stack component and connecting the component's relevant boundaries to form a continuous thermal model. The phase change of ice and re-circulation of coolant flow are included in the analytical framework and their contribution to the stack thermal mass and temperature distribution of the components is also explored. A parametric study was conducted to determine the governing parameters, relative impact of the thermal mass of each stack component and ice, and anticipated temperature distribution in the stack at start-up for various operating conditions. Results indicate that 20 cells were sufficient to accurately experimentally and computationally simulate the full size stack behavior. It was observed that an optimum range of operating current density exists for a chosen stack design, in which rapid start-up of the stack from sub-zero condition can be achieved. Thermal isolation of the stack at the end plates is recommended to reduce the start-up time. Additionally, an end plate thickness exceeding a threshold value has no added effect on the stack cold-start ability. Effect of various internal and external heating mechanisms on the stack start-up were also investigated, and flow of heated coolant above 0 °C was found to be the most effective way to achieve the rapid start-up.     

One-dimensional thermal model for direct methanol fuel cell stacks: Part II. Model based parametric analysis and predicted temperature profiles

Using the one-dimensional thermal model for the direct methanol fuel cell (DMFC) (presented in Part 1), based on the differential thermal energy conservation equation, which describes the thermal behaviour of a DMFC stack comprised of up to 25 large (272 cm²) cells, temperature profiles are predicted along the stack length. The model is used to assess the effect of operating parameters (temperature gradient, current density, flow rate and pressure) on the temperature profile along the stack. In addition, it enables investigation of the stack thermal management and the effect of altering a number of systematic parameters such as the number of cells in the stack, the active and exposed area and the interactions between the physical properties of the various components. The model aids the fuel cell system designer to gain an insight in the stack structure and select materials and geometric configurations that are optimal from a thermal management point of view.     

Effects of various operating and initial conditions on cold start performance of polymer electrolyte membrane fuel cells

Cold start is a challenging and important issue that hinders the commercialization of polymer electrolyte membrane fuel cell (PEMFC). In this study, a three-dimensional multiphase model has been developed to simulate the cold start processes in a PEMFC. Numerical simulations have been conducted for a single PEMFC starting at various operating and initial conditions, which are cell voltages, initial water contents and distributions, anode inlet relative humidity (RH), surrounding heat transfer coefficients, and cell temperatures. It is found that the heating-up time can be significantly reduced by decreasing the cell voltage and effective purge is critical for PEMFC cold start. The largest heating source at high cell voltages is the activational heat, and it becomes the ohmic heat at low cell voltages. The water freezing in the membrane is not observed when the cell is producing current due to the heat generation and the slow water diffusion into the membrane at subzero temperatures, and it is only observed after the cold start is failed, further confirming the importance of purge. Humidification of the supplied hydrogen has negligible effect on the cold start performance since only small amounts of water vapour can be taken by the gas streams at subzero temperatures. The surrounding heat transfer coefficients have significant influence on the heating-up time, indicating the importance of cell insulation or heating. The rate of cell heating up is reduced when the startup temperature is lowered due to the more sluggish electrochemical reaction kinetics.     

Computational model to predict thermal dynamics of planar solid oxide fuel cell stack during start-up process

Solid oxide fuel cell (SOFC) systems have been recognized as the most advanced power generation system with the highest thermal efficiency with a compatibility with wide variety of hydrocarbon fuels, synthetic gas from coal, hydrogen, etc. However, SOFC requires high temperature operation to achieve high ion conductivity of ceramic electrolyte, and thus SOFC should be heated up first before fuel is supplied into the stack. This paper presents computational model for thermal dynamics of planar SOFC stack during start-up process. SOFC stack should be heated up as quickly as possible from ambient temperature to above 700 °C, while minimizing net energy consumption and thermal gradient during the heat up process. Both cathode and anode channels divided by current-collecting ribs were modeled as one-dimensional flow channels with multiple control volumes and all the solid structures were discretized into finite volumes. Two methods for stack-heating were investigated; one is with hot air through cathode channels and the other with electric heating inside a furnace. For the simulation of stack-heating with hot air, transient continuity, flow momentum, and energy equation were applied for discretized control volumes along the flow channels, and energy equations were applied to all the solid structures with appropriate heat transfer model with surrounding solid structures and/or gas channels. All transient governing equations were solved using a time-marching technique to simulate temporal evolution of temperatures of membrane-electrode-assembly (MEA), ribs, interconnects, flow channels, and solid housing structure located inside the insulating chamber. For electrical heating, uniform heat flux was applied to the stack surface with appropriate numerical control algorithm to maintain the surface temperature to certain prescribed value. The developed computational model provides very effective simulation tool to optimize stack-heating process minimizing net heating energy and thermal gradient within the stack.     

Numerical study of cold start performance of proton exchange membrane fuel cell with coolant circulation

It has been well recognized that cold start is one of the key issues of proton exchange membrane fuel cell (PEMFC) used as the engine of vehicles. Coolant circulation is usually launched synchronously with the fuel cell during cold start to avoid sudden large temperature variation, which greatly increases the cell thermal mass, lowers the heating rate, and worsens the cell performance. Considering the flow and heat transfer of coolant circulation, a three-dimensional, transient, multi-disciplinary model for cold start is built up. The numerical results agree reasonably well with experimental data, indicating that the model can be used for the investigation of PEMFC cold start processes. The analysis of circulation parameter effects shows that increasing the coolant flow rate or coolant tank capacity has little influence on the cell voltage, but will increase the non-uniformity of temperature distribution along flow direction. At lower start-up temperature, this non-uniformity is more obvious. With higher coolant flow rate, although the distribution of current density becomes more evenly, the ice formation amount increases and its distribution and location are greatly affected.     

Numerical simulation and analysis of thermal stress distributions for a planar solid oxide fuel cell stack with external manifold structure

A three-dimensional numerical model based on the finite element method (FEM) is constructed to calculate the thermal stress distribution in a planar solid oxide fuel cell (SOFC) stack with external manifold structure. The stack is composed of 5 units which include cell, metallic interconnect, seal and anode/cathode current collectors. The temperature profile is described according to measured temperature points in the stack. It can be clearly seen that the maximum stress concentration area appears at the corner of the components when the stack is heated from room temperature (RT) to 780 °C. The effects of stack components on maximum stress concentration have been investigated under the operation temperature, as well as the thermal stress simulation results. It is obvious that the coefficient of thermal expansion (CTE) mismatch between the interconnect and the seal plays an important role in determining the thermal stress distribution in the stack. However, different compressive loads have almost no effect on stress distribution, and the influence of glass-based seal depends on the elastic modulus. The simulation results can be applied for optimizing the structural design of the stack and minimizing the high stress concentration in components.