Enhanced water removal in a fuel cell stack by droplet atomization using structural and acoustic excitation

This work examines new methods for enhancing product water removal in fuel cell stacks. Vibration and acoustic based methods are proposed to atomize condensed water droplets in the channels of a bipolar plate or on a membrane electrode assembly (MEA). The vibration levels required to atomize water droplets of different sizes are first examined using two different approaches: (1) exciting the droplet at the same energy level required to form that droplet; and (2) by using a method called ‘vibration induced droplet atomization’, or VIDA. It is shown analytically that a 2 mm radius droplet resting on a bipolar-like plate can be atomized by inducing acceleration levels as low as 250 g at a certain frequency. By modeling the direct structural excitation of a simplified bipolar plate using a realistic source, the response levels that can be achieved are then compared with those required levels. Furthermore, a two-cell fuel cell finite element model and a boundary element model of the MEA were developed to demonstrate that the acceleration levels required for droplet atomization may be achieved in both the bipolar plate as well as the MEA through proper choice of excitation frequency and source strength.     

Performance of PEMFC stack using expanded graphite bipolar plates

The bipolar plates are in weight and volume the major part of PEM fuel cell stack, and also a significant effect to the stack cost. To develop the low-cost and low-weight bipolar plate for PEM fuel cell, we have developed a kind of cheap expanded graphite plate material and a production process for fuel cell bipolar plates. The plates have a high electric conductivity and low density, and can be stamped directly forming fuel cell bipolar plates. Then, 1 and 10 kW stacks using expanded graphite bipolar plates are successfully assembled. The contact resistance of the bipolar plate is investigated and the electrochemical performances of the fuel cell stacks are tested. Good fuel cell performance is obtained and the voltage distribution among every single cell in the stacks is very uniform.     

Contact mechanics approach to determine contact surface area between bipolar plates and current collector in proton exchange membrane fuel cells

A significant portion of the power loss in a fuel cell stack can be attributed to the contact resistance between the gas diffusion layer/bipolar plate and the current collector/bipolar plate interfaces, especially when an oxide layer is formed on a stainless steel bipolar plate. Researchers have already studied methods to decrease the contact resistance between fuel cell components, but never has a theoretical contact mechanics model been applied to contact resistance problems in fuel cells. Therefore, the purpose of this research is to utilize a theoretical contact mechanics model in order to study real contact area in fuel cell components as a function of surface roughness, material properties, and clamping force. Specifically, the effects of bipolar plate surface roughness, coating thickness of the gold plating on the current collector, and the clamping force on real contact area have been studied. It was found that smoother materials, thicker gold coating, and higher clamping force resulted in a higher real percentage contact area.     

Modelling of polymer electrolyte membrane fuel cell stacks based on a hydraulic network approach

Polymer electrolyte membrane (PEM) fuel cells convert the chemical energy of hydrogen and oxygen directly into electrical energy. Waste heat and water are the reaction by‐products, making PEM fuel cells a promising zero‐emission power source for transportation and stationary co‐generation applications. In this study, a mathematical model of a PEM fuel cell stack is formulated. The distributions of the pressure and mass flow rate for the fuel and oxidant streams in the stack are determined with a hydraulic network analysis. Using these distributions as operating conditions, the performance of each cell in the stack is determined with a mathematical, single cell model that has been developed previously. The stack model has been applied to PEM fuel cell stacks with two common stack configurations: the U and Z stack design. The former is designed such that the reactant streams enter and exit the stack on the same end, while the latter has reactant streams entering and exiting on opposite ends. The stack analysed consists of 50 individual active cells with fully humidified H₂ or reformate as fuel and humidified O₂ or air as the oxidant. It is found that the average voltage of the cells in the stack is lower than the voltage of the cell operating individually, and this difference in the cell performance is significantly larger for reformate/air reactants when compared to the H₂/O₂ reactants. It is observed that the performance degradation for cells operating within a stack results from the unequal distribution of reactant mass flow among the cells in the stack. It is shown that strategies for performance improvement rely on obtaining a uniform reactant distribution within the stack, and include increasing stack manifold size, decreasing the number of gas flow channels per bipolar plate, and judicially varying the resistance to mass flow in the gas flow channels from cell to cell. Copyright © 2004 John Wiley & Sons, Ltd.     

Integration of Heat Pipe into Fuel Cell Technology

This paper describes recent applications of heat pipe technology in fuel cell systems, which include new stack designs with heat pipes to improve heat transfer as well as work on fuel cell system level design and engineering with adopting the heat pipe concept. In one design, micro-heat pipes are inserted and bonded in bipolar plates for thermal control in the fuel cell stack. In another design, flat heat pipes are integrated with a carbon bipolar plate for improving thermal control in the fuel cell stack. Finally, based on the heat pipe concept, we specifically developed a series of direct methanol fuel cell (DMFC) systems characterized as passive technology for methanol fuel delivery, water recirculation, and air and thermal management. Long-term durability and stability of the passive DMFC systems have been proved experimentally.     

Measurement of deuterium isotope separation by polymer electrolyte fuel cell stack

Processes for separating hydrogen isotopes are important for future energy applications. Several separation methods are based on electrolytic process; however, electrolysis consumes large amounts of electric energy. In this study, we demonstrate deuterium isotope separation from a mixture of H₂ and D₂ gases using a polymer electrolyte fuel cell stack. To identify the most efficient process, we investigated two flow patterns for the fuel gas, namely, parallel and serial flow. The electrical power of the stacks depended on the flow pattern when a high current was generated. We attribute this dependence on membrane dehydration and water droplet formation in the serial flow, which passed through the single cells in a straight path. However, the stack with the serial path showed a high separation factor (α = 6.6) indicating enrichment of deuterium water during the operation. The long reaction path of the fuel gas contributed to effective separation. The fuel utilization in individual cells suggested the potential for even more effective separation processes by a serial flow path.     

Modelling and evaluation of heating strategies for high temperature polymer electrolyte membrane fuel cell stacks

Experiments were conducted on two different cathode air cooled high temperature PEM (HTPEM) fuel cell stacks; a 30 cell 400 W prototype stack using two bipolar plates per cell, and a 65 cell 1 kW commercial stack using one bipolar plate per cell. The work seeks to examine the use of different heating strategies and find a strategy suited for fast start-up of the HTPEM fuel cell stacks. Fast start-up of these high temperature systems enables use in a wide range of applications, such as automotive and auxiliary power units, where immediate system response is needed. The development of a dynamic model to simulate the temperature development of a fuel cell stack during heating can be used for assistance in system and control design. The heating strategies analyzed and tested reduced the start-up time of one of the fuel cell stacks from 1 h to about 6 min.     

Design analysis of PEMFC bipolar plates considering stack manufacturing and environment impact

Stack and vehicle performance, design for manufacturing, and design for environment principles are used to develop bipolar plate design requirements and analyze design concepts for PEM fuel cells. Specifically, a list of 18 requirements identified in the literature is extended to 51 requirements and design rules. Given these design requirements, engineering characteristics or metrics used to indicate how well different bipolar plate designs meet each requirement and related targets and benchmarks are identified. Next, a subset of the engineering characteristics are used to evaluate six example bipolar plate designs made from graphite, stainless steel, and carbon composite in solid and integrated cooling configurations for a specific hybrid vehicle. For the case study of bipolar plates, correlations are interpreted for the considering relationships to compressive strength, the mass of the bipolar and cooling plates, the size of the stack required to move the ‘generic vehicle’, stack volume, disassembly efficiency, and select manufacturability metrics. Also, advantages and disadvantages specific to materials and design configurations are presented and discussed. Finally, power density and specific volume without consideration for vehicle performance was found not to be enough to assess the case study plates and, because of their common use in assessing fuel cell system design, is an important conclusion of this research.     

Performance evaluation of an energy recovery system for fuel reforming of PEM fuel cell power plants

This paper describes an energy recovery system that recovers waste thermal energy from a fuel cell stack and uses it for fuel reforming purposes. The energy recovery system includes a throttling valve, a heat exchanger, and a compressor, and is coupled with a coolant loop of the fuel cell stack. The feed stock of a fuel reformer, which is primarily a mixture of water and fuel, is vaporized in the heat exchanger and is compressed to a sufficiently high pressure before it is ducted into the fuel reformer. The performance of a fuel cell power plant equipped with the energy recovery system is evaluated. The results indicate that the power plant efficiency can be increased by more than 40% compared to that of a fuel cell power plant without the energy recovery system. Additionally, up to 90% of the waste heat generated in the fuel cell stack is recovered. As a result, the required heat dissipation capacity of the radiator that is used for cooling the fuel cell stack can be drastically reduced.     

Investigation of water droplet kinetics and optimization of channel geometry for PEM fuel cell cathodes

The kinetics and transport mechanisms of water droplets in model flow field channels of a low temperature polymer electrolyte fuel cell were investigated. The pressure drop at different air flows was measured for different channel geometries in a graphite plate as employed for fuel cell bipolar plates. The minimum air flow required for the movement of a water droplet in the flow channel was identified. From the experimental findings, recommendations for the development of flow fields with high condensate removal capabilities combined with low pressure differences were drawn to allow for an efficient operation of PEM fuel cells.     

Design and simulation of a novel bipolar plate based on lung‐shaped bio‐inspired flow pattern for PEM fuel cell

Finding the optimal flow pattern in bipolar plates of a proton exchange membrane is a crucial step for enhancing the performance of the device. This design plays a critical role in fluid mass transport through microporous layers, charge transfer through conductive media, management of the liquid water produced in microchannels, and microporous layers and heat management in fuel cells. This article investigates different types of common flow patterns in bipolar plates while considering a uniform pressure and velocity distribution as well as a uniform distribution of reactants through all the surfaces of the catalyst layer as the design criteria so that there would be a consistent electron production by the catalyst layer. Then, by identifying the important parameters in achieving the best performance of a fuel cell, a microfluidic flow pattern is inspired from the lungs in the human body, and an innovative bipolar plate is suggested, which was not proposed before. Afterwards, numerical simulations were carried out using computational fluid dynamics methods, and the mentioned bipolar plate called lung‐shaped bipolar plate was modeled. Simulations in this research showed that the lung‐shaped microfluidic flow pattern is an appropriate flow pattern to gain maximum power and energy density. In other words, the best polarization curve and power density curve are obtained by using the lung‐shaped bipolar plate in a proton exchange membrane fuel cell compared with previously suggested patterns. Copyright © 2017 John Wiley & Sons, Ltd.     

Factors influencing the performance and durability of polymer electrolyte membrane water electrolyzer: A review

Hydrogen is the best energy vector for renewable and intermittent power sources. Electrolysis coupled with renewable energy resources is the most promising for the production of green hydrogen among the current hydrogen production methods. The polymer electrolyte membrane water electrolyzer (PEMWE) is the frontrunner of electrolyzer technology because of its ability to operate at high current densities and compact design, thereby enabling high-pressure operation. This review summarizes the static parameters (stack assembly and design aspects) and dynamic parameters (operating parameters and gas bubble removal) affecting PEMWE's performance, as well as static parameters (stack design) and dynamic parameters (operating parameters) affecting durability. For PEMWE, stack design plays an important role in the electrolyzer performance and durability, such as the fabrication of the bipolar plate and the material selection of the stack components. Recently, novel stack designs have shown promising performance and durability enhancements by lowering the ohmic and mass losses, enabling constant clamping pressure inside the stack, and eliminating the degradation of the BPP plates by using 3D-printed plastic plates, which also greatly lower the cost of the stack. Operating parameters, including temperature, pressure, and water flow rate, can be regulated during operation. However, temperature and pressure have a more significant impact on PEMWE's performance and durability than water flow rate. Research on magnetic fields, ultrasonic power, pulsed power, and pressure swings has shown promising results in increasing gas removal rate to enhance PEMWE's performance by addressing intrinsic mass transport limitations. However, their application to large PEMWE systems has not been extensively tested. Further studies are needed to elucidate their mechanism and potential in PEMWE applications.     

千瓦级质子交换膜燃料电池电堆的实验研究

空泡率是汽液两相流动的基本参数之一,而已有过冷沸腾空泡率计算方法研究以高质量流速为主。且大量文献报道现有空泡率模型难以适用于低流速过冷沸腾工况。该文基于低流速过冷沸腾净蒸汽产生点(NVG)理论模型,进一步建立了计算过冷沸腾空泡率的分布拟合模型。在较宽广的压力、质量流速、热流密度和流道尺寸范围内将模型计算结果与现有空泡率实验数据进行了比较,低流速工况下该模型与实验数据符合良好,表明该模型可适用于低流速过冷沸腾工况。     

Operational characteristics of the direct methanol fuel cell stack on fuel and energy efficiency with performance and stability

This paper is presented to investigate operational characteristics of a direct methanol fuel cell (DMFC) stack with regard to fuel and energy efficiency, including its performance and stability under various operating conditions. Fuel efficiency of the DMFC stack is strongly dependent on fuel concentration, working temperature, current density, and anode channel configuration in the bipolar plates and noticeably increases due to the reduced methanol crossover through the membrane, as the current density increases and the methanol concentration, anode channel depth, and temperature decreases. It is, however, revealed that the energy efficiency of the DMFC stack is not always improved with increased fuel efficiency, since the reduced methanol crossover does not always indicate an increase in the power of the DMFC stack. Further, a lower methanol concentration and temperature sacrifice the power and operational stability of the stack with the large difference of cell voltages, even though the stack shows more than 90% of fuel efficiency in this operating condition. The energy efficiency is therefore a more important characteristic to find optimal operating conditions in the DMFC stack than fuel efficiency based on the methanol utilization and crossover, since it considers both fuel efficiency and cell electrical power. These efforts may contribute to commercialization of the highly efficient DMFC system, through reduction of the loss of energy and fuel.     

New electroplated aluminum bipolar plate for PEM fuel cell

Further improvement in the performance of the polymer electrolyte membrane fuel cells as a power source for automotive applications may be achieved by the use of a new material in the manufacture of the bipolar plate. Several nickel alloys were applied on the aluminum substrate, the use of aluminum as a bipolar plate instead of graphite is to reduce the bipolar plate cost and weight and the ease of machining. The electroplated nickel alloys on aluminum substrate produced a new metallic bipolar plate for PEM fuel cell with a higher efficiency and longer lifetime than the graphite bipolar plate due to its higher electrical conductivity and its lower corrosion rate. Different pretreatment methods were tested; the optimum method for pretreatment consists of dipping the specimen in a 12.5% NaOH for 3 min followed by electroless zinc plating for 2 min, then the specimen is dipped quickly in the electroplating bath after rinsing with distilled water. The produced electroplate was tested with different measurement techniques, chosen based on the requirement for a PEM fuel cell bipolar plate, including X-ray diffraction, EDAX, SEM, corrosion resistance, thickness measurement, microhardness, and electrical conductivity.     

Mass production cost of PEM fuel cell by learning curve

A learning curve model has been developed to analyze the mass production cost structure of proton exchange membrane fuel cells for automobiles. The fuel cell stack cost is aggregated by the cost of membranes, platinum, electrodes, bipolar plates, peripherals and assembly process. The mass production effects on these components are estimated. Nine scenarios with different progress ratios and future power densities are calculated by the learning curve for cumulative production of 50 000 and 5 million vehicles. The results showed that the fuel cell stack cost could be reduced to the same level as that of an internal combustion engine today, and that the key factors are power density improvement and mass production process of bipolar plates and electrodes for reducing total cost of fuel cell stack.     

Bipolar plate made of carbon fiber epoxy composite for polymer electrolyte membrane fuel cells

Polymer electrolyte membrane fuel cells (PEMFCs) are able to efficiently generate high power densities, making the technology potentially attractive for certain mobile and portable applications. Since the bipolar plate is a major part of the PEMFC stack both in weight and volume, the bipolar plate should be developed with its weight and thickness in mind.     

Recent development in design a state-of-art proton exchange membrane fuel cell from stack to system: Theory,integration and prospective

As an efficient energy converter, the proton exchange membrane fuel cell (PEMFC) is developed to couple various applications, including portable applications, transportation, stationary power generation, unmanned underwater vehicles, and air independent propulsion. PEMFC is a complex system consisting of different components that can be influenced by many factors, such as material properties, geometric designs operating conditions, and control strategies. The interaction between components and subsystems could affect the performance, durability, and lifespan of PEMFC system. To design a high performance, long lifespan, high durability PEMFC, it's essential to comprehensively understand the coupling effect of different factors on the overall performance and durability of PEMFCs. This review will present existing research on basis of four aspects, involving fuel cell stack design, subsystems design and management, mass transfer enhancement, and system integration. Firstly, the multi-physics intergradation and component design of PEMFC are reviewed with the designing mechanisms and recent progress. Besides, mass transfer enhancement methods are discussed by bipolar plate design and membrane electrode assembly optimization. Then, water management, thermal management, and fuel management are summarized to provide design guidance for PEMFC. The specifications design and system management for various engineering applications are briefly presented.     

Performance evaluation of different configurations of biogas-fuelled SOFC micro-CHP systems for residential applications

Three configurations of solid oxide fuel cell (SOFC) micro-combined heat and power (micro-CHP) systems are studied with a particular emphasis on the application for single-family detached dwellings. Biogas is considered to be the primary fuel for the systems studied. In each system, a different method is used for processing the biogas fuel to prevent carbon deposition over the anode of the cells used in the SOFC stack. The anode exit gas recirculation, steam reforming, and partial oxidation are the methods employed in systems I–III, respectively. The results predicted through computer simulation of these systems confirm that the net AC electrical efficiency of around 42.4%, 41.7% and 33.9% are attainable for systems I–III, respectively. Depending on the size, location and building type and design, all the systems studied are suitable to provide the domestic hot water and electric power demands for residential dwellings. The effect of the cell operating voltage at different fuel utilization ratios on the number of cells required for the SOFC stack to generate around 1 kW net AC electric power, the thermal-to-electric ratio (TER), the net AC electrical and CHP efficiencies, the biogas fuel consumption, and the excess air required for controlling the SOFC stack temperature is also studied through a detailed sensitivity analysis. The results point out that the cell design voltage is higher than the cell voltage at which the minimum number of cells is obtained for the SOFC stack.     

Non-active area water mitigation in PEM fuel cells via bipolar plate surface energy modification

The management of liquid water from either internal chemical reactions or externally humidified reactants is an important design consideration for proton exchange membrane (PEM) fuel cells because of the effects on both cell performance and durability. To achieve proper water management, significant effort has been devoted to developing new fuel cell materials, hardware designs, and appropriate stack operating conditions. However, water management in the region of the channel-to-manifold interfaces has received limited attention. This region covers the ends of the bipolar plate from where liquid water exits the active area to the entrance of the stack exhaust manifolds where excess reactant flows from individual cells are combined and leave the stack. For practical applications, there is a small driving force to expel liquid water in this region, especially in the anode flow field. Under severe operating conditions such as freezing temperatures, the buildup of water may cause a channel-scale blockage. In this study, hydrophilic and hydrophobic bipolar plate treatments were investigated to identify the effectiveness of water mitigation through ex-situ experiments performed using a dedicated freeze test rig. Water mitigation behavior with various locations of hydrophilic/hydrophobic coatings was characterized using measurements of differential pressure and gas flow rate. It was found that the hydrophilic coatings provide better performance, as water accumulation can be readily mitigated with less potential to cause full channel-scale blockages.