A numerical study of dynamic behaviors of a unitized regenerative fuel cell during gas purging

The gas purging states affect electricity output and energy storage capacity of unitized regenerative fuel cells. In this study, a model of unitized regenerative fuel cell is established. Cell voltages and operating temperatures influences on the dynamic distribution of thermal fluid during purging process and the discharge of residual liquid water in electrolytic cell mode are investigated. The motivation of the present study is better understanding the gas purging characteristics and its effect on reaction behaviors of unitized regenerative fuel cells. Simulation results reveal a significant influence of purging gas temperature on the water flooding and a great effect of operating voltage on the water diffusion. The operating temperature of electrolytic cell model almost has little effect on purging results at different cell temperature and the same purging gas temperature. When the purging gas temperature is changed, higher temperatures of cell and purging gas facilitate liquid water discharging out from the cell regions. In cell water flooding situation, when having large liquid content, the purging gas has little effects on the water expelling process.     

Experimental study on rapid cold start-up performance of PEMFC system

The cold start-up of a proton exchange membrane fuel cell is considered one of the main factors affecting the commercialization of fuel cell vehicles. In this study, an automotive fuel cell system was designed and tested for cold start-up at low temperatures. In the absence of PTC (Positive Temperature Coefficient) heating device, the stack was directly loaded to generate heat, which provided the cold start-up characteristics of system at low temperatures. Cold start-up process and purging control strategies were analyzed at −20 °C and −30 °C. It was found that the fuel cell system could produce 50% power in 25 s at −20 °C, the coolant temperature's heating rate was 0.78 °C/s, the coolant outlet temperature could reach 20 °C within 40 s and no apparent low voltage of single cell occurred. While, the cell close to the end plate had low cell voltage and reverse polar phenomena throughout the −30 °C cold start-up process. The heating rate of the coolant temperature was 0.44 °C/s, and the temperature of coolant outlet reached 20 °C within 90 s. The purging time ranged from 180 to 260 s according to the voltage drop value of stack and the ohmic resistance of stack was 360–470 mΩ after the high-volume air purging at different tests. After 30 cold start-up tests, the rated point performance of the stack declined by about 1%, and the consistency of cell voltages did not change significantly. Future work will focus on optimizing cold start-up strategy and speeding up purging time to minimize the performance impact of the cold start-up.     

Voltage instability in a simulated fuel cell stack correlated to cathode water accumulation

Single fuel cells running independently are often used for fundamental studies of water transport. It is also necessary to assess the dynamic behavior of fuel cell stacks comprised of multiple cells arranged in series, thus providing many paths for flow of reactant hydrogen on the anode and air (or pure oxygen) on the cathode. In the current work, the flow behavior of a fuel cell stack is simulated by using a single-cell test fixture coupled with a bypass flow loop for the cathode flow. This bypass simulates the presence of additional cells in a stack and provides an alternate path for airflow, thus avoiding forced convective purging of cathode flow channels. Liquid water accumulation in the cathode is shown to occur in two modes; initially nearly all the product water is retained in the gas diffusion layer until a critical saturation fraction is reached and then water accumulation in the flow channels begins. Flow redistribution and fuel cell performance loss result from channel slug formation. The application of in-situ neutron radiography affords a transient correlation of performance loss to liquid water accumulation. The current results identify a mechanism whereby depleted cathode flow on a single cell leads to performance loss, which can ultimately cause an operating proton exchange membrane fuel cell stack to fail.     

PEMFC purging at low operating temperatures: An experimental approach

In this paper, the effect of operating temperature on optimal purge interval for maximum energy efficiency in a proton exchange membrane fuel cell (PEMFC) with dead‐ended anode (DEA) was experimentally investigated. The study was conducted with a focus on challenges associated with operation at temperatures lower than the recommended designed temperature. With DEA, gradual voltage drop happens due to the accumulation of water and impurities such as nitrogen. Hence, periodic purging of the anode side is required to remove excess water and impurities that are accumulated at the anode side over time. Short purge intervals increase hydrogen loss that translates into low fuel utilisation, whereas long purge intervals result in voltage drop due to high water and impurity accumulations. Therefore, an optimal purge strategy should be implemented to maximise the stack energy efficiency. Depending on the operating conditions and loads, there are instances that a fuel cell stack operates at temperatures lower than its recommended designed temperature. Considering the temperature effect on the cell water management, operating temperature is an important factor to consider for optimising the purge strategy in PEMFCs. At lower operating temperatures (ie, below 50°C), more water is formed in liquid form, which makes the optimisation of purge strategy more challenging. For a stack temperature of 40°C, it was observed that with an increase in stack current from 0.25 to 0.45 A cm^?2, the optimal purge interval decreases from 90 seconds to around 45 seconds, respectively. Increasing the stack temperature from 40°C to 50°C resulted in an increase in the optimal purge interval to 120 seconds and 90 seconds for stack currents of 0.25 (ie, low current density) and 0.45 A cm^?2, respectively. At lower operating temperatures, more frequent purging schedules are needed accordingly to remove the liquid water from the cell. These results indicated that at lower operating temperatures, water accumulation at the anode side becomes more dominant compared with higher operating temperatures.     

Analysis and control of a fuel delivery system considering a two-phase anode model of the polymer electrolyte membrane fuel cell stack

The fuel delivery system using both an ejector and a blower for a PEM fuel cell stack is introduced as a fuel efficiency configuration because of the possibility of hydrogen recirculation dependent upon load states.A high pressure difference between the cathode and anode could potentially damage the thin polymer electrolyte membrane. Therefore, the hydrogen pressure imposed to the stack should follow any change of the cathode pressure. In addition, stoichiometric ratio of the hydrogen should be maintained at a constant to prevent a fuel starvation at abrupt load changes.Furthermore, liquid water in the anode gas flow channels should be purged out in time to prevent flooding in the channels and other layers. The purging control also reduces the impurities concentration in cells to improve the cell performance.We developed a set of control oriented dynamic models that include a anode model considering the two-phase phenomenon and system components The model is used to design and optimize a state feedback controller along with an observer that controls the fuel pressure and stoichiometric ratio, whereby purging processes are also considered. Finally, included is static and dynamic analysis with respect to tracking and rejection performance of the proposed control.     

Control-oriented modeling of gas purging process on the cathode of polymer electrolyte membrane fuel cell during shutting down

Gas purging process of cathode side during the shut-down procedure of a polymer electrolyte membrane fuel cell (PEMFC) system is of great importance for a successful cold start. This paper proposes a study on the modeling and control of the cathodic gas purging process, whose main purpose is to remove liquid water in the gas diffusion layer (GDL) and the membrane. The water removal process can be divided into three steps, which are called (a) the through-plane drying of the GDL, (b) the in-plane drying of the GDL, and (c) the vapor-transport from the membrane. A nonlinear model is firstly developed to describe the water removal process in the GDL and the membrane. It includes a one-dimensional three-step purging sub-model and an energy consumption sub-model considering the properties of the air compressor. Experiments are carried out to validate the water-remove model by using the membrane HFR. An optimal constant purging control strategy that minimizes energy consumption during the cathodic purging process is designed based on the model and verified in simulation.     

Computationally efficient modeling of the dynamic behavior of a portable PEM fuel cell stack

A numerically efficient mathematical model of a proton exchange membrane fuel cell (PEMFC) stack is presented. The aim of this model is to study the dynamic response of a PEMFC stack subjected to load changes under the restriction of short computing time. This restriction was imposed in order for the model to be applicable for nonlinear model predictive control (NMPC). The dynamic, non-isothermal model is based on mass and energy balance equations, which are reduced to ordinary differential equations in time. The reduced equations are solved for a single cell and the results are upscaled to describe the fuel cell stack. This approach makes our calculations computationally efficient. We study the feasibility of capturing water balance effects with such a reduced model. Mass balance equations for water vapor and liquid water including the phase change as well as a steady-state membrane model accounting for the electro-osmotic drag and diffusion of water through the membrane are included. Based on this approach the model is successfully used to predict critical operating conditions by monitoring the amount of liquid water in the stack and the stack impedance. The model and the overall calculation method are validated using two different load profiles on realistic time scales of up to 30 min. The simulation results are used to clarify the measured characteristics of the stack temperature and the stack voltage, which has rarely been done on such long time scales. In addition, a discussion of the influence of flooding and dry-out on the stack voltage is included. The modeling approach proves to be computationally efficient: an operating time of 0.5 h is simulated in less than 1 s, while still showing sufficient accuracy.     

Removal of excess product water in a PEM fuel cell stack by vibrational and acoustical methods

The research presented here investigates the use of vibro-acoustic methods to improve the performance of a PEM fuel cell by enhancing water removal from the active reaction sites within the fuel cell. Removing the water increases the available reaction sites and thus increases the available power for a given operating condition. To examine the new water removal methods, first, the production of water in fuel cells and current water removal methods are reviewed. Then, the new methods are proposed that are based on structural and acoustical excitation of the stack. Specifically, the use of flexural waves, acoustic waves and surface waves to remove water from a fuel cell stack are examined. Analytical formulations are given in order to calculate the excitation frequency and amplitude required to move a droplet resting on a vibrating bipolar plate. Depending on the droplet radius and other parameters, it is estimated that a water droplet resting on a bipolar plate can be moved by structural displacement levels as low as 1 μm. The different approaches to droplet removal are compared in terms of the minimum vibration energy required per droplet. Water production in a commercial fuel cell stack is then estimated and used as a test case to compare the power required to effect removal of a certain number of droplets with the amount of power produced by the stack. It is shown that a water droplet clogging a plate channel may be moved with parasitic power requirements as low as 21 mW. For each method, the energy required to effect droplet removal is quite small, although among the three, the use of surface acoustic waves may be the best option in terms of minimal vibration energy and implementation feasibility.     

Effective purge method with addition of hydrogen on the cathode side for cold start in PEM fuel cell

The purge process is essential for successful cold start of fuel cell vehicles during winter, and it plays an important role in the removal of the residual water inside the fuel cell in a short time. In this study, a new purge method is introduced by adding a small amount of hydrogen to the cathode gas flow in order to increase the purge performance. The experimental results demonstrate that the hydrogen addition purge method is very effective in removing the residual water near the catalyst layer. The water removal is verified by measuring the resistance of the fuel cell, dew point temperature of the outlet purge gas, and weights of the membrane electrolyte assembly (MEA) and gas diffusion layer (GDL). In addition, the image of the GDL after the purge process is captured to show the advantage of the hydrogen addition purge method. Cold start experiments are also conducted after the optimal purge process. It is also found that the degradation of the catalyst layer is not serious after the hydrogen addition purge process.     

Transient behavior of water generation in a proton exchange membrane fuel cell

The effect of water generation on the performance of proton exchange membrane fuel cell (PEMFC) was investigated by using a periodical linear sweep method. Three different kinds of I–V curves were obtained, which reflected different amount of water uptake in the fuel cell. The maximum water uptake that could avoid flooding in the fuel cell and the hysteresis of water diffusion were also discussed. Quantitative analysis of water uptake and water transport phenomena in this study were conducted both experimentally and theoretically. Results showed that the water uptake capacity for the fuel cell under no severe flooding was 27.837 mg cm⁻². The transient response of the internal resistance indicated that the high frequency resistance (HFR) lagged the current with a value of about 20 s. The effect of purging operation on the internal resistance of the fuel cell was also explored. Experimental data showed that the cell experienced a continuous 8-min purging process can maintain at a relatively steady and dry state.     

氢燃料电池氢气利用率提升策略研究

针对燃料电池堆再循环管线的再循环速率低的问题,提出用于燃料电池的氢气供应系统的循环控制方案,根据再循环管线中再循环的气体量精确估计由吹扫阀吹扫的氢体浓度,通过反馈每种气体的吹扫量,调节吹扫阀的开度,提升氢气利用率,并对该方案进行仿真分析。仿真结果表明,燃料电池阳极侧氢气利用率明显提升,最高可达92.733%,可提高燃料电池堆的耐久性。     

Investigation of mechanical vibration effect on proton exchange membrane fuel cell cold start

A three-axis vibration platform is first constructed and utilized in the investigation of the effects of mechanical vibration on the cold start performance of a proton exchange membrane (PEM) fuel cell. In addition, an intermittent pattern of purging is adopted to improve the purging efficiency. The applied vibrations are found to promote water dispersion, but ultimately do not enhance water removal. Under subzero conditions (−13 °C), the vibration of the fuel cell improves cold start performance via delayed freezing, especially when vibrating at the fuel cell natural frequency (10 Hz). With an increase in vibration amplitude, the freezing rate is found to be slow and eventually plateau. Finally, the vibration in the vertical axis is found to play a positive role in improving cold start performance; the effects of other orientations depend on the startup temperature. The result of cold start under vibration might indirectly prove the existence of super-cooled water.     

Thermal management for a hydrogen-fueled 1-kW PEMFC based on thermoeconomic analysis

Thermal management is a critical issue in optimizing the performance of proton exchange membrane fuels cells (PEMFC). The heat balance between the heat generation in fuel cell stack (FCS) and the heat removal by coolant liquid determines the operating temperature of the PEMFC and the dehydration or flooding condition in FCS. In this study, the amount of water condensed among all the water produced during the electrochemical reaction in FCS of a hydrogen-fueled 1-kW PEMFC at various conditions was determined using a thermoeconomic method called modified productive structure analysis (MOPSA) and the calculated results were compared with observed ones. The amount of the condensed water which should be removed through the cathode channel is dependent crucially on the cooling rate of FCS, which indicates that thermal management for FCS can be done by controlling of the cooling rate of FCS.     

氢燃料电池氢气管道吹扫系统冷启动策略研究

针对现有车载燃料电池管道在吹扫过程中氢气侧管道由于低温结冰易受阻,从而导致冷启动困难的问题,提出氢气管道两级吹扫方案,以实现对燃料电池堆氢管中剩余水、蒸汽含量的独立控制,降低冷启动时的加热能耗,避免冷启动失效。通过三维仿真对4种不同工况下阴极中段在不同时间的温度、阴极中段在不同时间的冰体积分数、电池纵向截面表面不同时间的温度进行分析,结果表明该方案可实现在-20 ℃温度下20 s内的平稳冷启动,既解决了低温冷启动困难问题,又保证了冷启动后的电堆输出性能。     

Development of the novel control algorithm for the small proton exchange membrane fuel cell stack without external humidification

Small PEM (proton exchange membrane) fuel cell systems do not require humidification and have great commercialization possibilities. However, methods for controlling small PEM fuel cell stacks have not been clearly established. In this paper, a control method for small PEM fuel cell systems using a dual closed loop with a static feed-forward structure is defined and realized using a microcontroller. The fundamental elements that need to be controlled in fuel cell systems include the supply of air and hydrogen, water management inside the stack, and heat management of the stack. For small PEM fuel cell stacks operated without a separate humidifier, fans are essential for air supply, heat management, and water management of the stack. A purge valve discharges surplus water from the stack. The proposed method controls the fan using a dual closed loop with a static feed-forward structure, thereby improving system efficiency and operation stability. The validity of the proposed method is confirmed by experiments using a 150-W PEM fuel cell stack. We expect the proposed algorithm to be widely used for controlling small PEM fuel cell stacks.     

Kinetic studies of cathode degradation on PEM fuel cell short stack level undergoing freeze startups with different states of residual water and current draws

One of the biggest challenges for a wide spread introduction of polymer electrolyte membrane fuel cells in automotive applications is the freeze start at subzero temperatures as this poses a severe threat to fuel cell performance and overall lifetime. Therefore, the impact of current draw during stack freeze startup at various rest water contents and current densities was investigated applying state of the art in-situ testing as quasi cyclic voltammetry.The results indicate a clear dependency between number of freeze startups and performance loss, whereas higher initial water content within the stack reduced its destructive impacts.In our earlier work we were able to show the dependency between residual water and degradation for non-freeze-startup-capable systems at the unit cell level [31]. With this work we confirm that the same physical relationship also applies to freeze-startup-capable systems on the short stack level.Furthermore, reversible performance losses that were encountered during this study can be assigned to oxidizable fuel contaminants, which are believed to be CO, and an easy cleansing procedure is being suggested.     

Novel closed anode pressure-swing system for proton exchange membrane fuel cells

To overcome the low system efficiency and fuel efficiency in conventional recirculation systems and purging systems, a new anode closed pressure-swing system for proton exchange membrane fuel cells (PEMFCs) is proposed in this paper. Only two pressure regulators set at different pressures and a buffer tank are applied to produce pressure-swing inside the anode through the process of hydrogen feed and reaction, thereby achieving the anode-dead-end (ADE) operation (no purging). This system was successfully tested on a 20-cell open-cathode stack. The results indicate that the performance of the stack with the developed system is stable and efficient over 13,000 s, while its performance in ADE mode without purging deteriorates rapidly because of active area reduction. It can be concluded that the pressure swing system provides the following advantages: close to 100% hydrogen utilization; improved stack performance of around 12.5% for the power compare to the ADE mode with intermittent purging at 12 V load; improved humidification of the anode and membrane; and ease of implementation as there are no extra pumps.     

Integration of Miniature Heat Pipes into a Proton Exchange Membrane Fuel Cell for Cooling Applications

In proton exchange membrane fuel cell (PEMFC) operations, the electrochemical reactions produce a rise in temperature. A fuel cell stack therefore requires an effective cooling system for optimum performance. In this study, miniature heat pipes were applied for cooling in PEMFC. Three alternatives were considered in tests: free convection, forced convection cooling with air, and also water. An analytical model was developed to show the possibility of evoking heat from inside a fuel cell stack with different numbers of miniature heat pipes. An experiment setup was designed and then used for further analysis. The proposed experiment setup consisted of a simulated fuel cell that produced heat and a number of thermosyphon miniature heat pipes to evoke heat from the simulated fuel cell. The experiment results reported in this paper present advantages and disadvantages of each tested cooling scenario. Results show that each cooling scenario, using a different number of heat pipes, provided different heat removal rates for PEMFC cooling.     

Effect of orthohydrogen–parahydrogen composition on performance of a proton exchange membrane fuel cell

The effect of orthohydrogen–parahydrogen concentration on the performance of a proton exchange membrane fuel cell is calculated and experimentally investigated. Gibbs free energy and reversible cell potential calculations predict that parahydrogen at room temperature produces a voltage 20 mV/cell higher than normal hydrogen and a 1.6% increase in efficiency over normal hydrogen. Experimental data based on a 1 kW proton exchange membrane fuel cell rapidly switched between normal and parahydrogen did not show a statistically significant difference in performance. Variations due to stack humidity and anode purging are found to dominate fuel cell output. The experimental results confirm that, as anticipated, parahydrogen concentration has a negligible impact on fuel cell performance for the majority of practical applications.     

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.