Temperature and Humidity Control of a Micro PEM Fuel Cell Stack

This paper deals with the control of a miniaturised fuel cell system. A single air blower is used to control both heat and water management of the fuel cell. As the number of manipulated variables is smaller than the number of control variables, classical control algorithms are not applicable. To find a suitable controller, a system model is developed that shows the qualitatively same behaviour as the experimental setup. The dynamic behaviour of the model and the influence of the blower are studied by phase portraits. A control algorithm is then conceived by qualitative analysis of the phase portraits and tested in simulations.     

Development of a Direct Alcohol Alkaline Fuel Cell Stack

Direct alcohol alkaline fuel cells (DAAFC) are one of the potential fuel cell types in the category of low temperature fuel cells, which could become an energy source for portable electronic equipment in future. In the present study, a simple DAAFC stack has been developed and studied to evaluate the maximum performance for a given fuel (methanol or ethanol) and electrolyte (KOH) at various concentrations and temperatures. The open circuit voltage of the stack of four cells was nearly 4.0 V. A particular combination, 2 M fuel (methanol or ethanol) and 3 M KOH, results in maximum power density of the stack. The maximum power density obtained from the DAAFC stack (25 °C) was 50 mW cm^–2 at 20 mA cm^–2 for methanol and 17 mA cm^–2 for ethanol. The stack power density corroborated with that obtained from a single cell, indicating there was no further loss in the stack.     

Analysis of Leakages in a Solid Oxide Fuel Cell Stack in a System Environment

A solid oxide fuel cell (SOFC) stack can exhibit both anodic and cathodic leakages, i.e. a fuel leak from the anode side and an air leak from the cathode side of the stack, respectively. This study describes the results of an in‐situ leakage analysis conducted for a planar SOFC stack during 2000 hours of operation in an actual system environment. The leakages are quantified experimentally at nominal system operating conditions by conducting composition analysis and flow metering of gases for both fuel and air subsystems. Based on the calculated atomic hydrogen‐to‐carbon ratio of the fuel and air gases, it is found that the fuel leakages are mostly selective by nature: the leaking fuel gas does not have the same composition as the fuel system gas. A simple diffusive leakage model, based on the leakage being driven by concentration differences weighted by diffusion coefficients, is applied to quantify the amount of leakages. The leakage model provides a good correspondence with the experimental results of the gas analysis.     

Thermo‐Mechanical Response of Fuel Cell Electrodes: Constitutive Model and Application in Studying the Structural Response of Polymer Electrolyte Fuel Cell

The characterization of the mechanical properties of fuel cell electrodes through the experimental techniques is a complex task due to the low thickness, constituents' heterogeneous composition, and fragile nature of the film. We present a preliminary investigation on the thermomechanical response of fuel cell catalyst layer (CL) obtained through the numerical experiment. Since the Nafion ionomer is one of the constituents' of the CL, a modified micromechanically motivated viscoplastic model is adopted to characterize the Nafion ionomer in terms of reduced density factor to account for the void content. The catalyst agglomerates are taken as inclusions in the ionomer matrix to form a composite unit which is used to plot the true stress–true strain response. Practicality of this work is tested by implementing the electrode layer as a separate component in the single fuel cell unit cell model. A remarkable difference in the magnitude of stress levels in the membrane is observed under thermal and hydrated conditions with the presence and absence of electrode layer in the simulation domain. The present work will assist in improved understanding of the localized stress distribution in the membrane, which is essential to understand its mechanical endurance.     

Modelling of a Pressure Element for Solid Oxide Fuel Cell Stack

The paper deals with the structural analysis of a corrugated pressure element for the SOFC stack. The aim of the element application is assurance of the fuel cell tightness and sufficient electrical contact between components by means of its uniform pressure. In this paper, a method of the pressure element modelling is proposed and results of its behaviour analysis for different assembly conditions (free ends, fixed ends and ends subjected to friction force) are presented. Vertical and horizontal stiffness of the corrugated element as a function of its geometrical parameters has been determined which facilitates computer simulation and engineering design of the element.     

Life Cycle Assessment of a Molten Carbonate Fuel Cell Stack

Fuel cells are considered to be ultra‐clean energy conversion systems because the pollutants emitted during their operating time are very low in concentration. For this reason, fuel cells can represent the answer to several environmental issues. What is important, however, is to carefully evaluate if the problem is really being solved, or just shifted. In other words, it is vital to consider not only the operating life, but also the pollution related to the construction, the disposal, the conversion of primary fuel into hydrogen and all the other related operations and systems. The present study is the first step for evaluating the environmental impact related to the whole life cycle of a fuel cell system. In this paper, in fact, the authors, making use of the Life Cycle Assessment (LCA) approach, analyze the impact associated with a Molten Carbonate Fuel Cell Stack. Since the MCFC construction can be considered a novel practice, this study can provide a significant indication for pollution reduction in the stack construction phase.     

Detailed Dynamic Modeling of a Molten Carbonate Fuel Cell Stack with Indirect Internal Reformers

A detailed dynamic model of a molten carbonate fuel cell (MCFC) stack that can predict the distribution of process variables over the cell surface and along the stacking direction is presented. The model describes a repeating unit of the stack, which is composed of eight cells and one indirect internal reformer (IIR). The set of partial differential equations for the MCFC model is reduced to a set of ordinary differential equations through discretization using the cubic spline collocation method and the finite difference method. The steady‐state and dynamic behaviors of the eight‐cell stack model are compared with those of the one‐cell model, which has been commonly used in previous studies on MCFC dynamics, and three‐cell aggregate model. The distribution of the process variables along the stacking direction cannot be disregarded, and thus more detailed model should be used for the accurate reproduction of MCFC dynamics.     

Evaluation of a Non‐sealed Solid Oxide Fuel Cell Stack with Cells Embedded in Plane Configuration

A non‐sealed solid oxide fuel cell stack with cells embedded in plane configuration was fabricated and operated successfully in a box‐like stainless‐steel chamber. For a two‐cell stack, it demonstrated an open circuit voltage (OCV) of 2.13 V and a maximum power output of 569 mW at the flow rate of 67 sccm CH₄ and 33 sccm O₂. A fuel utilization of 4.16% was obtained. The cell performance was dominated by two different mechanisms, the polarization of the cathode at low current and the concentration polarization of the anode at high current. Finally, a scaled‐up stack with six cells in series generated an OCV of 6.4 V and a maximum power output of 8.18 W.     

Stack Modelling of a Molten Carbonate Fuel Cell (MCFC)

A model of a molten carbonate fuel cell (MCFC) stack including internal steam reforming is presented. It comprises a symmetric section of the stack, consisting of one half indirect internal reforming unit (IIR) and four fuel cells. The model describes the gas phase compositions, the gas and solid temperatures and the current density distribution within the highly integrated system. The model assumptions, the differential equations and boundary conditions as well as the coupling equations used in the model are shown. The strategy to solve the system of partial differential equations is outlined. The simulation results show that the fuel cells within the stack operate at different temperatures. This is expected to have an impact on the voltages as well as the degradation rates within the individual fuel cells.     

Performance Analysis of a Stationary Fuel Cell Thermoelectric Cogeneration System

The main purpose of our study was to use an experimental method and system dynamic simulation technology to examine a proton exchange membrane fuel cell thermoelectric cogeneration system that provides both high‐quality electric power and heated water. In the second part of our study, we experimentally verified the development of key components of the fuel cell and conducted a comprehensive analysis of the subsystems, including the fuel cell module, hydrogen supply subsystem, air supply subsystem, humidifier subsystem, and heat recovery subsystem. Finally, we integrated all of the subsystems into a PEM fuel cell thermoelectric cogeneration system and performed efficiency tests and analysis of power generation, heat recovery, and thermoelectric cogeneration. After comparing this system's efficiency results using simulation and experimentation, we determined that the accuracy of the simulation values when compared to the experimental values was >95%, showing that this system's simulation nearly approached the efficiency of the actual experiment, including more than 53% for power generation efficiency, more than 39% for heat recovery efficiency, and more than 93% for thermoelectric cogeneration combined efficiency.     

Analysis of a Fuel Cell System by a Step Response

A simple method for analysing a fuel cell system using the voltage response to a step change in load resistance is presented in this work. With the modified Randles model, where two resistors and two capacitors are implemented for the Warburg impedance, the response curve is fitted with three exponential curves. Six independent equations corresponding to six parameters of the model can be obtained from the fitted values, and they are solved under two specific assumptions for the initial state. The parameters are the charge double layer capacitance, the charge transfer resistance, a resistor–capacitor pair related to gas diffusion and another pair related to water diffusion. A down curve of the voltage for the step down change in load and an up curve for the step up change are fitted and parameters for both cases are evaluated separately. Analysis is performed by comparing the parameters at different current levels. The impedance is also simulated using the estimated parameters in order to show which parameter is the rate‐determining factor of the fuel cell reaction with increasing current levels.     

Analysis of the Ripple Current in a 5 kW Polymer Electrolyte Membrane Fuel Cell Stack

Polymer electrolyte fuel cell systems are increasingly being used in applications requiring an inverter to convert the direct current (DC) output of the stack to an alternating current (AC). These inverters, and other time‐varying inputs to the stack such as the anode feed pressure, cause deviations from the average stack current, or ripple currents, which are undesirable for reasons of performance and durability. A dynamic fuel cell model has been developed and validated against experimental data for a 5 kW fuel cell stack, examining in detail the ripple current behaviour. It was shown that the ripple currents exceed the 2% maximum recommended value, and may lead to long‐term degradation of the fuel cell stack.     

PEM Fuel Cell Stack Cold Start Thermal Model

For passenger fuel cell vehicles (FCVs), customers will expect to start the vehicle and drive almost immediately, implying a very short system warmup to full power. While hybridization strategies may fulfill this expectation, the extent of hybridization will be dictated by the time required for the fuel cell system to reach normal operating temperatures. Quick‐starting fuel cell systems are impeded by two problems: (i) the freezing of residual water or water generated by starting the stack at below freezing temperatures and (ii) temperature‐dependent fuel cell performance, improving as the temperature reaches the normal range. Cold start models exist in the literature; however, there does not appear to be a model that fully captures the thermal characteristics of the stack during sub‐freezing startup conditions. Existing models lack the following features: (i) modeling of stack internal heating methods (other than stack reactions) and their impact on the stack temperature distribution and (ii) modeling of endplate thermal mass effect on end cells and its impact on the stack temperature distribution. Unlike a lumped model, which may use a single temperature as an indicator of the stack's thermal condition, a model considering individual cell layers can reveal the effect of the endplate thermal mass on the end cells, and accommodate the evaluation of internal heating methods that may mitigate this effect. This paper presents and discusses results from simulations performed with a new, layered model.     

Fuel Cell Vehicle Simulation – Part 1: Benchmarking Available Fuel Cell Vehicle Simulation Tools

Fuel cell vehicle simulation is one method for systematic and fast investigation of the different vehicle options (fuel choice, hybridization, reformer technologies). However, a sufficient modeling program, capable of modeling the different design options, is not available today. Modern simulation programs should be capable of serving as tools for analysis as well as development. Shortfalls of the existing programs, initially developed for internal combustion engine hybrid vehicles, are: (i)Insufficient modeling of transient characteristics; (ii) Insufficient modeling of the fuel cells system; (iii) Insufficient modeling of advanced hybrid systems; (iv) Employment of a non‐causal (backwards looking) structure; (v) Significant shortcomings in the area of controls. In the area of analysis, a modeling tool for fuel cell vehicles needs to address the transient dynamic interaction between the electric drive train and the fuel cell system. Especially for vehicles with slow responding on‐board fuel processor, this interaction is very different from the interaction between a battery (as power source) and an electric drive train in an electric vehicle design. Non‐transient modeling leads to inaccurate predictions of vehicle performance and fuel consumption. When applied in the area of development, the existing programs do not support the employment of newer techniques, such as rapid prototyping. This is because the program structure merges control algorithms and component models, or different control algorithms (from different components) are lumped together in one single control block and not assigned to individual components as they are in real vehicles. In both cases, the transfer of control algorithms from the model into existing hardware is not possible. This paper is the first part of a three part series and benchmarks the “state of the art” of existing programs. The second paper introduces a new simulation program, which tries to overcome existing barriers. Specifically it explicitly recognizes the dynamic interaction between fuel cell system, drive train and optional additional energy storage.     

Reliable Fuel Cell Simulation Using an Experimentally Driven Numerical Model

The necessity of a simulation tool for integrated power plant with MCFCs has led to the development of a numerical model by which it is possible to evaluate the performances of the cell and the plant the cell is embedded in. Many mathematic models can be found in the literature that evaluate electrochemical fuel cell performance [1–9]; however the model must be developed according to a specific target. In other words, if the main purpose of the study is, for example, shape optimization, the model must be very accurate concerning internal conditions, such as current density, temperature distribution, etc. The models found in the literature [1–3, 5, 9] seem to produce excellent results. However, the aim here is to create a mathematical model for fuel cell stacks, which can be integrated into a simulation modular code for the Balance of the Plant (BoP) optimization. Plant modelling can be performed using commercially available software. Often, since fuel cells are innovative devices, numerical codes that simulate their behaviour must be implemented by using a proprietary code. Previous papers can be found in the literature [4, 10, 11], where fuel cell stacks are inserted in an entire power plant, but very often the fuel cell block is zero-dimensional and it is not affected by the variation in input parameters. The authors, for this purpose, have developed a module to be integrated into commercial software for plants simulation, Aspen Plus. The model adopted is simple, but at the same time, it calculates with good accuracy fuel cell performance under any conditions. Another important element taken into account during the code development is the validation through experimental data. In the present work, the authors have adopted an analytic model for evaluating the electrochemical performance [6] of a fuel cell and results obtained are compared with those found out during the tests on a Molten Carbonate Fuel Cell. Some phenomena not described in the original model are considered and an enhanced model for the cell performance is developed. The model obtained is therefore used to analyse fuel cell behaviour under several operating conditions. These results seem to fit very well with experimental analyses conducted in the test rig of the University of Perugia.     

Characterization of a Ballard MK5‐E Proton Exchange Membrane Fuel Cell Stack

We present the results of an experimental investigation of the energy balance of a Ballard MK5‐E proton exchange membrane fuel cell (PEMFC) stack. We have investigated the transient phenomena that occur during PEMFC stack warm‐up, under load switching, and when the PEMFC stack is connected to a DC/AC inverter. A simple and convenient model describing the polarization curve as a function of the temperature is presented and validated by our experimental data. We also present experimental results on the increase PEMFC stack performance as a function of the current density for different oxygen concentrations of the oxidant gas.     

Impact of Ionomer Content on Proton Exchange Membrane Fuel Cell Performance

The effect of Nafion ionomer content on performance of a proton exchange membrane (PEM) fuel cell operated with home‐made anodic and cathodic electrodes fabricated from a novel metal organic framework (MOF) derived Pt‐based electrocatalyst was investigated via numerical simulation and experimental measurement. First, the parameter sensitivity analysis was performed to identify the most influential parameters of the model. Then, these parameters were calibrated for different fuel cell designs investigated in the current study by employing the corresponding experimental data. Afterwards, the calibrated model was used to examine the impact of Nafion content in the catalyst layer of home‐made electrodes. Finally, the qualitative trend predicted by this model was experimentally surveyed by varying the Nafion content between 10–50 wt.% in the catalyst layer of home‐made electrodes. At the anode side, the performance of home‐made electrode in a PEM fuel cell demonstrated small dependency on Nafion content. For the cathodic home‐made electrode, Nafion content was found to affect the PEM fuel cell performance more strongly. Although the model could correctly capture the impact of Nafion content on calculated polarization curves, the model predicted optimum values significantly deviate from the experimental results. This was related to the several simplifications made during model development.     

Experimental Investigation into a Novel Modular PEMFC Fuel Cell Stack

The polymer electrolyte membrane fuel cell (PEMFC), despite being regarded as an ideal replacement to the internal combustion engine, is still not an economically attractive prime‐mover due to a number of key challenges that have yet to be fully resolved; such as degradation to cell components resulting in inadequate lifetimes, specialized manufacturing processes, and poor gravimetric/volumetric energy densities. This paper presents a stack concept which replaces the conventional bipolar plate (BPP), a component that is responsible for a large proportion of stack cost and volume in traditional fuel cell stack designs. The stack architecture compromises of active and passive components which are suited to mass manufacture and maintain functionality that the BPP fulfilled. Furthermore, the design allows the implementation of a fault tolerant system (FTS) which can bypass faulty cells while still ensuring electrical output. The stack architecture is presented and characterized over a number of operating scenarios. The experimental studies suggest that the performance of the new design is similar to that of traditional stacks over a number of operating conditions despite the removal of the BPP and the FTS continued to operate at a desired operating criterion despite the loss of a cell within the stack     

Effect of Carbon Dioxide and Ammonia on Polymer Electrolyte Membrane Fuel Cell Stack Performance

Proton exchange membrane fuel cell (PEMFC) performance degrades when impurities are present in the anode fuel gas, referred to as catalyst poisoning. This paper investigates the effect of carbon dioxide and ammonia as impurities in the anode gas of the PEMFC, and found that the presence of CO₂ decreases the performance of the fuel cell by up to 10%. The performance loss depends on the CO₂ concentration and the exposure time. The voltage loss is recoverable on passing pure hydrogen gas, indicating that a permanent poisoning of the catalyst layer has not taken place. Exposure of the fuel cell to ammonia beyond 20 ppm, even for a short duration, causes permanent PEMFC failure, probably due to the deterioration of the membrane.     

Modelling and Analysis of Electro‐chemical,Thermal, and Reactant Flow Dynamics for a PEM Fuel Cell System

In this paper an approach for the dynamic modelling of polymer electrolyte membrane fuel cells is presented. A mathematical formulation based on empirical equations is discussed and several features, exhibiting dynamic phenomena, are investigated. A generalized steady state fuel cell model is extended for the development of a method for dynamic electrochemical analysis. Energy balance and reactant flow dynamics are also explained through physical and empirical relationships. A well‐researched system (Ballard MK5‐E stack based PGS‐105B system) is considered in order to understand the operation of a practical fuel cell unit. Matlab‐SIMULINK^TM has been used in simulating the models. The proposed method appears to be relatively simple and consequently requires less computation time. Simulation results are compared with available experimental findings and a good match has been observed.