The maximization of an alkaline membrane fuel cell (AMFC) net power output

This study optimizes numerically the relative sizes, spacings (internal structure), and aspect ratios (external configuration) of a single alkaline membrane fuel cell for maximum net power. The alkaline membrane fuel cell (AMFC) cellulosic membrane brings new light to the possibility of having alkaline fuel cells that are nontoxic and asbestos free as compared with static electrolyte cells that use an asbestos separator and ammonium‐based alkaline anion‐exchange membranes. A dimensionless dynamic mathematical model is utilized in the process, and the results are presented in normalized charts for generality. Two degrees of freedom are considered as follows: (i) the relative thicknesses of two reaction and diffusion layers and the membrane space (internal structure); and (ii) the external aspect ratios of a square section plate that contains all single alkaline membrane fuel cell components (external configuration). The optimized internal and external configurations result from the optimal balance between electrical power output and pumping power to supply fuel and oxidant to the AMFC. A third level of optimization is found, that is, the KOH mass fraction in the electrolyte that leads to a 3‐way‐maximized net power output. A sixfold variation in AMFC net power output is observed as the internal and external configurations, and KOH mass fraction are changed. Such effect stresses the importance of pinpointing the optimal AMFC configuration in order to avoid poor performance. New algebraic correlations are derived to indicate in dimensionless form, the optimal configurations for the internal and external structure, and resulting maximum net power output, which are important for scaling up and down the AMFC design with ease, without having to perform new simulations. Copyright © 2016 John Wiley & Sons, Ltd.     

Experimental investigation on scaling and stacking up of proton exchange membrane fuel cells

The performance of a proton exchange membrane fuel cell (PEMFC) with various flow channel design (serpentine and interdigitated) with different landing to channel ratios (L:C = 1:1; 2:2) for an active area of 25 cm² and 70 cm², for single cell and two cells stack is studied and compared. The effect of back pressure on the PEMFC performance is also investigated. This study establishes a strong relation between back pressure and power output from a PEMFC. It was concluded that the interdigitated flow channel gives better results than the serpentine flow channel configuration for various landing to channel ratios. It was also found that power outputs do not proportionally increase with active area of the membrane electrode assembly (MEA). Similarly, stacking up studies with single cell and two cell stack shows that the two cell stack has reduced power densities when compared to that of a single cell. The effect of cooling channels with natural and forced convection by using induced draught fan on the performance of a PEMFC stack is also studied. Fuel distribution and temperature management are found to be the significant factors which determine the performance of a PEMFC stack.     

Reliability computing of polymer-electrolyte-membrane fuel cell stacks through Petri nets

In this paper a model is introduced which computes reliability data of PEMFC (polymer-electrolyte-membrane fuel cell) stacks, especially the average lifetime of a single stack or the reliability of stacks of a whole fuel cell vehicle fleet within a given timing. The stack and its behaviour over time is modelled by a Petri net. The behaviour is divided into degradation, spontaneous and reversible events. Through the worsening over time the characteristics voltage, internal and external leakages, which are assigned to the components MEA (membrane electrolyte assembly) and BIP (bipolar plate), are changed. Thresholds for every characteristic monitor the operating ability of the whole 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.     

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.     

Collaborative simulation for dynamical PEMFC power systems

In this paper a collaborative simulation platform for proton exchange membrane fuel cell (PEMFC) power systems is presented, where the stack is simulated by a two-phase distributed parameter model and the auxiliary units by lumped parameter models. By exchanging the dynamic data between the external load/auxiliary units and PEMFC stack, dynamic simulation of PEMFC stack has been carried out during the load changes for various states associated with different characteristic variables. The internal states of the stack can be observed due to variation of external load/auxiliary units. Numerical experiments are provided for a special case with multiple cycles of load changes derived from an acceleration mode of a fuel cell vehicle. The numerical results demonstrate that the “undershoot” of output voltage is due to the response lag of the auxiliary units and liquid water accumulation in the fuel cell stack.     

Design of control systems for portable PEM fuel cells

The current evolution in the design of fuel cell systems, together with the considerable development of integrated control techniques in microprocessor systems allows the development of portable fuel cell applications in which optimized control of the fuel cells performance is possible. Control, in the strict sense, implies a thorough knowledge of both the static and dynamic behaviour of the system comprising the stack, manifold and the compressor that enables oxygen supply. The objective of this control, far from being simply to maintain the stack free from oxygen and hydrogen shortages, is to achieve the necessary values of these gases, minimizing compressor consumption, which is the cause of the greatest inefficiency of fuel cells. This objective is essential when fuel cell systems are involved in situations where the net power of the stack is reduced and any unnecessary consumption lowers the total power available to the user. The design of an efficient control system requires the following steps: (1) modeling of the stack, compressor and other pneumatic elements involved in the system. (2) Calculation of the control equations and simulation of the entire system (including control). (3) Emulation of the stack and other pneumatic elements and simulation utilizing the designed control system. (4) Physical realization of the control system and testing within the fuel cell system. The design of a control system for fuel cell systems is introduced to manage PEMFC stacks. The control system will guarantee the correct performance of the stack around its optimal operation point, in which the net power is maximized. This means that both, the air flow and the stack temperature are controlled to a correct value.     

The experimental validation of a simplified PEMFC simulation model for design and optimization purposes

Geometric design, including the internal structure and external shape, considerably affect the thermal, fluid and electrochemical characteristics of a polymer electrolyte membrane fuel cell (PEMFC), which determines the polarization curves as well as the thermal and net power responses. In order to predict the response of PEM fuel cells according to the variation of manufacturing materials physical properties, operating and design parameters, a reliable simulation model (and computationally fast) is necessary, which accounts for the power losses due to pressure drops in the gas channels. In this paper, a simplified and comprehensive PEMFC mathematical model introduced in previous studies is experimentally validated. Numerical results are obtained with the model for an existing set of ten commercial unit PEM fuel cells. The model accounts for pressure drops in the gas channels, and for temperature gradients with respect to space in the flow direction, and current increase that are investigated by direct infrared imaging, showing that even at low current operation such gradients are present in fuel cell operation, and therefore should be considered by a PEMFC model, since large coolant flow rates are limited due to induced high pressure drops in the cooling channels. The computed polarization and power curves are directly compared to the experimentally measured ones with good qualitative and quantitative agreement. The combination of accuracy and low computational time allow for the future utilization of the model as a reliable tool for PEMFC simulation, control, design and optimization purposes.     

Optimization of single SOFC structural design for maximum power

This paper improves previously published models by the authors for a single solid oxide fuel cell (SOFC), and introduces a procedure to optimize its external configuration and operating conditions, so that the net power is maximized. The previous models are hereby improved to include: i) a constant offset overpotential in total potential drop; ii) heat generation associated with all the potential losses; iii) temperature-dependent thermo-physical properties of fuel and air, and iv) pumping power to maximize fuel cell performance. The thermodynamic model is derived from physical laws (e.g., the first law of thermodynamics, Fick's law, Fourier's law) to obtain the temperature and pressure spatial distribution in the SOFC. The electrochemical model is validated by direct comparison with experimental data from the Pacific Northwest National Laboratory (PNNL), and allows for the computation of the SOFC voltage, current, and power output. Based on the simulation results, the structural design, the active three phase boundaries regions at the electrodes and the fuel utilization factor, and their impact on the SOFC performance are discussed. Subjected to fixed total volume, the optimal geometric and operating parameters are pursued so that the net power of the SOFC is maximized through a 4-way-optimization procedure. The method used is general and the numerically obtained maxima are sharp, taking into account that up to a 631% single SOFC performance variation was observed within the studied parameters' range. The fixed volume constraint was then relaxed, and the effect of total volume variation on performance was investigated, delivering the general optimal parameters for the 4-way maximized SOFC net power output within the studied total dimensionless fuel cell volume range. These findings show the potential to use the model as a tool for future SOFC design, simulation and optimization.     

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.     

Studies on the cathode humidification by exhaust gas recirculation for PEM fuel cell

For high efficiency and long durability of proton exchange membrane fuel cells (PEMFCs), polymer electrolyte membranes should be kept wet. Reactant gases should be humidified on this account. For the humidification, the PEMFC system uses an external or internal humidifier as a part of balance of plants (BOPs). However, external humidifiers have many disadvantages such as parasitic power loss, system complexity, high cost and bulky volume. As such, efforts have been made to remove the external humidifier or replace it with an advanced humidifier. In this work, to remove a humidifier, humidification by exhaust gas recirculation is investigated by theoretical analysis and experiments with 5-cell stack of an active area 250 cm². In the theoretical analysis, species conservation equations and energy conservation equation are solved to obtain the O₂ concentration, stoichiometric ratio, humidity ratio, temperature, amount of condensed water and so on. With the theoretical results, experiments with 5-cell, 250 cm² fuel cell stack were carried out in order to analyze the stack performance at the theoretical conditions of the cathode process stream of exhaust gas recirculation.     

Experimental analysis of the performance of the air supply system in a 120 kW polymer electrolyte membrane fuel cell system

For analyzing the performance of 120 kW polymer electrolyte membrane fuel cell (PEMFC) system and its air supply system, an air system test bench was built, then applied on a 120 kW PEMFC system test bench composed of air supply subsystem, hydrogen supply subsystem, stack, cooling subsystem and electronic control subsystem. The strategy composed of feedforward table and Piecewise proportional integral (PI) feedback control strategy is employed to regulate the flow rate and pressure of air supply system. Firstly, the air compressor map and the mapping relationship between the speed of air compressor, opening of back-pressure valve and stack current are obtained by carrying out experiments on the PEMFC air system bench. Then, the max output performance, steady-state performance, the startup performance, the dynamic response abilities of PEMFC system are tested, respectively. During the experiments, performances under different test conditions were analyzed by comparing parameters such as voltage inconsistency, average voltage, minimum voltage, voltage range, net power of the PEMFC system, and stack power. The test results show that the air supply system can provide qualified flow rate and pressure for the PEMFC stack. The peak power of the stack is 120 kW and net power of the system is 97 kW when the current is 538 A. The response time from rated net power to idle net power is 12 s and from idle net power to rated net power is 23 s. The overshoot of average voltage and minimum voltage in the process of increasing load is both 0.01 V, which are 0.015 V and 0.02 V lower than that when the load is decreased, respectively. The dynamic response speed and stability of the PEMFC system in the process of decreasing the load are better than those in the process of increasing the load.     

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.     

A high efficient assembly technique for large PEMFC stacks: Part I. Theory

A high efficient assembly technique for large proton exchange membrane fuel cell (PEMFC) stacks is proposed to obtain the optimal clamping load. The stack system is considered as a mechanical equivalent stiffness model consisting of numerous elastic elements (springs) in either series or parallel connections. We first propose an equivalent stiffness model for a single PEM fuel cell, and then develop an equivalent stiffness model for a large PEMFC stack. Based on the equivalent stiffness model, we discuss the effects of the structural parameters and temperature on the internal stress of the components and the contact resistance at the contact interfaces, and show how to determine the assembly parameters of a large fuel cell stack using the equivalent stiffness model. Finally, a three-dimensional finite element analysis (FEA) for a single PEMFC is compared with what the equivalent stiffness model predicts. It is found that the presented model gives very good prediction accuracy for the component stiffness and the clamping load.     

Determination of optimal operating conditions for a polymer electrolyte membrane fuel cell stack: optimal operating condition based on multiple criteria

A methodology for optimal control of the polymer electrolyte membrane fuel cell (PEMFC) with multiple criteria is presented here. In this regard, thermoelectric objectives and thermoeconomic objective are considered, simultaneously. The proposed fuel cell is a 1200 W Ballard PEMFC namely Nexa? power module. The net power density and exergetic efficiency of the PEMFC are maximized, and the unit cost of the generated power is minimized in a multi‐objective optimization procedure using the NSGA‐II (non‐dominated sorting genetic algorithm). Operating temperature and pressure, air stoichiometric coefficient at the cathode and the current density are considered as controlling parameters in order to acquire optimal performance of the PEMFC. A set of optimal solution namely the Pareto frontier is obtained, and a final optimal solution is selected from available solutions located on the Pareto frontier using the fuzzy decision‐making process based on the Bellman–Zadeh approach. Results are compared with corresponding results obtained previously in single objective optimization scenarios. It has been shown that the optimal operating condition obtained based on the multiple criteria approach has least deviation from the ideal features of the fuel cell in comparison to the corresponding optimal solution obtained in conventional single‐objective optimization approaches. Copyright © 2013 John Wiley & Sons, Ltd.     

A review of unitized regenerative fuel cell stack: Material,design and research achievements

The stack design of a unitized regenerative fuel cell (URFC) can modify the structure of cells that can be used as storage and energy regenerator aside from cells that use other sources such as solar or wind energy. A reversible unitized polymer electrolyte membrane fuel cell (PEMFC) contains a dual-functional single cell that is less expensive and has enhanced performance. The use of URFCs on hydrogen and oxygen is preferred because it is highly efficient, environmentally friendly, and uses power generators. The stack, then, must be made affordable or accessible. The expenses of URFC stack must be reduced by improving its design, materials, and performance. This study referred to recent studies on developing a method to cut the expenses of the URFC stack. The study also aims to determine its main constituents and to look further into its design by observing its performance and electrochemical behaviors. It also presents the issues that are currently encountered in this field.     

Comprehensive influences measurement and analysis of power converter low frequency current ripple on PEM fuel cell

To deeply understand the influences of power converter's low frequency current ripple (LFCR) and harmonics on a proton exchange membrane fuel cell (PEMFC) in its power conditioning system (PCS), a comprehensive measurement and analysis of the influences of LFCR and harmonics on PEMFC's performance and durability is investigated in this paper. Based on an equivalent circuit model of PEMFC stack and a mechanism model for evaluating the LFCR effects on the PEMFC, this paper studies primarily and systematically the comprehensive influences of LFCR and harmonics on PEMFC performances and durability, such as (1) degrading the PEMFC performance, (2) shortening the lifetime of PEMFC, (3) reducing the stack output power, (4) lowing its availability efficiency, (5) producing more heat and raising the PEMFC temperature, (6) consuming more fuel, and (7) decreasing the fuel utilization. Finally, a Horizon 300 W PEMFC stack is implemented and tested.     

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

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

Development of new test instruments and protocols for the diagnostic of fuel cell stacks

In the area of fuel cell research, most of the experimental techniques and equipments are still devoted to the analysis of single cells or very short stacks. However, the diagnosis of fuel cell stacks providing significant power levels is a critical aspect to be considered for the integration of fuel cell systems into real applications such as vehicles or stationary gensets. In this article, a new instrument developed in-lab is proposed in order to satisfy the requirements of electrochemical impedance studies to be led on large FC generators made of numerous individual cells. Moreover, new voltammetry protocols dedicated to PEMFC stack analysis are described. They enable for instance the study of membrane permeability and loss of platinum activity inside complete PEMFC assemblies.