Numerical study for diagnosing various malfunctioning modes in PEM fuel cell systems

The proton exchange membrane fuel cell (PEMFC) is promising technology for efficient power generation and has wide applications. In PEMFC development, it is important to diagnose malfunctions in a system with defective components and a PEMFC stack can act as an effective sensor to detect the various malfunctioning modes. Hence, the focus of this study is to analyze the response of a PEMFC under various malfunction conditions including humidifier, air blower, and coolant pump, catalyst layer degradation, and membrane aging based on 3D PEMFC simulations. Except for the coolant supply malfunction, other malfunctions exhibit similar behavior in terms of voltage drop and temperature rise, requiring more detailed measurement techniques such as Electrochemical Impedance Spectroscopy to identify the cause of malfunctions. In addition, measuring the relative humidity of the outlet gas may not provide sufficient information to distinguish the malfunction of the anode or cathode humidifier. The results of the study suggest fault detection and isolation methods under these malfunction conditions to prevent more severe failure of the PEMFC stack and system. An extensive multi-dimensional contour comprising temperature, relative humidity, liquid saturation, water content, and current density is also provided for the better analyzation of system malfunctioning behaviors.     

Effect of flow field design on performances of high temperature PEM fuel cells: Experimental analysis

Among all types of fuel cells, attention is being drawn lately on high temperature Polybenzimidazole (PBI) PEM because their operative temperature range (120-180 °C) increases the tolerance to carbon monoxide. This feature allows working with low quality hydrogen produced by hydrocarbon reformation. Most of the literature on PBI PEM deals with membrane and MEA related issues, however, cell efficiency and specially, commercial feasibility are conditioned by other fuel cell components as bipolar plates. In the present study the focus is on the effect of the flow field geometry of high temperature PBI PEM composite bipolar plates on the overall performance of the cell. For this purpose, three different channel geometries are studied: two serpentine flow fields and parallel channels flow field. Results show that serpentine geometry yields higher performance though it introduces higher pressure drop along the cell as well.     

Flow rate and humidification effects on a PEM fuel cell performance and operation

A new algorithm is presented to integrate component balances along polymer electrolyte membrane fuel cell (PEMFC) channels to obtain three-dimensional results from a detailed two-dimensional finite element model. The analysis studies the cell performance at various hydrogen flow rates, air flow rates and humidification levels. This analysis shows that hydrogen and air flow rates and their relative humidity are critical to current density, membrane dry-out, and electrode flooding. Uniform current densities along the channels are known to be critical for thermal management and fuel cell life. This approach, of integrating a detailed two-dimensional across-the-channel model, is a promising method for fuel cell design due to its low computational cost compared to three-dimensional computational fluid dynamics models, its applicability to a wide range of fuel cell designs, and its ease of extending to fuel cell stack models.     

Computational model of a PEM fuel cell with serpentine gas flow channels

A three-dimensional computational fluid dynamics model of a PEM fuel cell with serpentine flow field channels is presented in this paper. This comprehensive model accounts for the major transport phenomena in a PEM fuel cell: convective and diffusive heat and mass transfer, electrode kinetics, and potential fields. A unique feature of the model is the implementation of a voltage-to-current (VTC) algorithm that solves for the potential fields and allows for the computation of the local activation overpotential. The coupling of the local activation overpotential distribution and reactant concentration makes it possible to predict the local current density distribution more accurately. The simulation results reveal current distribution patterns that are significantly different from those obtained in studies assuming constant surface overpotential. Whereas the predicted distributions at high load show current density maxima under the gas channel area, low load simulations exhibit local current maxima under the collector plate land areas.     

Fast start-up of a diesel fuel processor for PEM fuel cells

Fuel cell systems based on liquid fuels are particularly suitable for auxiliary power generation due to the high energy density of the fuel and its easy storage. Together with industrial partners, Oel-Waerme-Institut is developing a 3 kW_el PEM fuel cell system based on diesel steam reforming to be applied as an APU for caravans and yachts. The start-up time of a fuel cell APU is of crucial importance since a buffer battery has to supply electric power until the system is ready to take over. Therefore, the start-up time directly affects the battery capacity and consequently the system size, weight, and cost.     

Operating line analysis of fuel processors for PEM fuel cell systems

At least three different definitions of fuel processor efficiency are in widespread use in the fuel cell industry. In some instances the different definitions are qualitatively the same and differ only in their quantitative values. However, in certain limiting cases, the different efficiency definitions exhibit qualitatively different trends as system parameters are varied. In one limiting case that will be presented, the use of the wrong efficiency definition can lead a process engineer to believe that a theoretical maximum in fuel processor efficiency exists at a particular operating condition, when in fact no such efficiency optimum exists. For these reasons, the objectives of this paper are to: (1) quantitatively compare and contrast these different definitions, (2) highlight the advantages and disadvantages of each definition and (3) recommend the correct definition of fuel processor efficiency.     

Three-dimensional computational fluid dynamics model of a tubular-shaped PEM fuel cell

A full three-dimensional, non-isothermal computational fluid dynamics model of a tubular-shaped proton exchange membrane (PEM) fuel cell has been developed. This comprehensive model accounts for the major transport phenomena in a PEM fuel cell: convective and diffusive heat and mass transfer, electrode kinetics, and potential fields. In addition to the tubular-shaped geometry, the model feature an algorithm that allows for more realistic representation of the local activation overpotentials which leads to improved prediction of the local current density distribution. Three-dimensional results of the species profiles, temperature distribution, potential distribution, and local current density distribution are presented. The model is shown to be able to understand the many interacting, complex electrochemical, and transport phenomena that cannot be studied experimentally.     

A Study of dynamic characteristics of PEM fuel cells by measuring local currents

Dynamic characteristics are important information in analyzing and understanding PEM fuel cell performances and detailed local dynamic information is extremely important in the understanding of fundamental mechanisms. In this study, the input parameters studied include humidification temperature of the reactant gases, operating temperature, and operating backpressure and the local dynamic responses are the local current densities. The fuel cell used is a PEM fuel cell with a single channel serpentine flow field and the local current densities are directly measured using the current density distribution measurement gasket and a multi-channel potentiostat. The experimental results show that very different local dynamic responses exist even though the response of the average current shows very little dynamics. Especially when the humidification temperature increases from a low value to moderate value, the responses of the local current density in the upstream are very different from those in the downstream.     

Air mass flow and pressure optimisation of a PEM fuel cell range extender system

In order to eliminate the local CO₂ emissions from vehicles and to combat the associated climate change, the classic internal combustion engine can be replaced by an electric motor. The two most advantageous variants for the necessary electrical energy storage in the vehicle are currently the purely electrochemical storage in batteries and the chemical storage in hydrogen with subsequent conversion into electrical energy by means of a fuel cell stack. The two variants can also be combined in a battery electric vehicle with a fuel cell range extender, so that the vehicle can be refuelled either purely electrically or using hydrogen. The air compressor, a key component of a PEM fuel cell system, can be operated at different air excess and pressure ratios, which influence the stack as well as the system efficiency. To asses the steady state behaviour of a PEM fuel cell range extender system, a system test bench utilising a commercially available 30 kW stack (96 cells, 409 cm² cell area) was developed. The influences of the operating parameters (air excess ratio 1.3 to 1.7, stack temperature 20 °C–60 °C, air compressor pressure ratio up to 1.67, load point 122 mA/cm² to 978 mA/cm²) on the fuel cell stack voltage level (constant ambient relative humidity of 45%) and the corresponding system efficiency were measured by utilising current, voltage, mass flow, temperature and pressure sensors. A fuel cell stack model was presented, which correlates closely with the experimental data (0.861% relative error). The air supply components were modelled utilising a surface fit. Subsequently, the system efficiency of the validated model was optimised by varying the air mass flow and air pressure. It is shown that higher air pressures and lower air excess ratios increase the system efficiency at high loads. The maximum achieved system efficiency is 55.21% at the lowest continuous load point and 43.74% at the highest continuous load point. Future work can utilise the test bench or the validated model for component design studies to further improve the system efficiency.     

A 5 kWt catalytic burner for PEM fuel cells: Effect of fuel type,fuel content and fuel loads on the capacity of the catalytic burner

For proton exchange membrane fuel cell systems (PEMFC) integrated with fuel processors, the calorific value of reformate gases produced during the start-up phase must be recovered. An appropriate exhaust after treatment system has crucial importance for PEMFC systems. Catalytic combustion is a promising alternative regarding its total oxidation capability of low calorific value gases at low temperatures, thereby reducing environmentally hazardous emissions. The aim of the study is to develop an after treatment system using a catalytic burner with a nominal capacity of 5 kW_t, which is also adaptive to partial loads of PEM fuel cell capacity. Fuel type, fuel composition and fuel loads are important parameters determining the operating window of the catalytic burner. Precious metal based catalysts, as proved to be the most active catalysts for the oxidation of hydrocarbons, can withstand temperatures of about 1073 K without exhibiting a rapid deactivation. This is the main barrier dictating the operating window and thereby determining the capacity of the burner. In this work, 1.5% natural gas (NG) alone was found to be the upper limit to control the catalyst bed temperature below 1073 K. In the case of catalytic combustion of hydrogen–NG mixture, 7% of hydrogen with NG up to 0.6% could be totally oxidized below 1073 K. Within the experimented ranges of fuel loads, between 2.5 kW_t and 5.5 kW_t, the temperature of the catalyst bed was seen to increase with increasing the fuel load at constant fuel percentages. It has been observed that fuel type was another parameter affecting the exhaust gas temperature.     

Numerical investigation of the performance of symmetric flow distributors as flow channels for PEM fuel cells

This work reports on the performance of a single PEM fuel cell using symmetric flow patterns as gas delivery channels. Three flow patterns, two symmetric and one serpentine, are taken from the literature on cooling of electronics and they are implemented in a computational model as gas flow channels in the anode and cathode side of a PEMFC. A commercial CFD code was used to solve the physics involved in a fuel cell namely: the flow field, the mass conservation, the energy conservation, the species transport, and the electric/ionic fields under the assumptions of steady state and single phase. An important feature of the current modeling efforts is the analysis of the main irreversibilities at different current densities showing the main energy dissipation phenomena in each cell design. Also, the hydraulic performance of the flow patterns was studied by evaluating the pressure drop and pumping power. The first part of this work reveals the advantages of using a serpentine pattern over the base symmetric distributors. The second part is an optimization of the symmetric patterns using the entropy minimization criteria. Such an optimization led to the creation of a flow structure that promotes an improved performance from the point of view of power generation, uniformity of current density, and low pumping power.     

Thermal analysis of air-cooled PEM fuel cells

Air-cooled proton exchange membrane fuel cells (PEMFCs), having combined air cooling and oxidant supply channels, offer significantly reduced bill of materials and system complexity compared to conventional, water-cooled fuel cells. Thermal management of air-cooled fuel cells is however a major challenge. In the present study, a 3D numerical thermal model is presented to analyze the heat transfer and predict the temperature distribution in air-cooled PEMFCs. Conservation equations of mass, momentum, species, and energy are solved in the oxidant channel, while energy equation is solved in the entire domain, including the membrane electrode assembly (MEA) and bipolar plates. The model is validated with experiments and can reasonably predict the maximum temperature and main temperature gradients in the stack. Large temperature variations are found between the cool incoming air flow and the hot bipolar plates and MEA, and in contrast to water-cooled fuel cells, significant temperature gradients are detected in the flow direction. Furthermore, the air velocity and in-plane thermal conductivity of the plate are found to play an important role in the thermal performance of the stack.     

Numerical investigation of flow field configuration and contact resistance for PEM fuel cell performance

A steady-state three-dimensional non-isothermal computational fluid dynamics (CFD) model of a proton exchange membrane fuel cell is presented. Conservation of mass, momentum, species, energy, and charge, as well as electrochemical kinetics are considered. In this model, the effect of interfacial contact resistance is also included. The numerical solution is based on a finite-volume method. In this study the effects of flow channel dimensions on the cell performance are investigated. Simulation results indicate that increasing the channel width will improve the limiting current density. However, it is observed that an optimum shoulder size of the flow channels exists for which the cell performance is the highest. Polarization curves are obtained for different operating conditions which, in general, compare favorably with the corresponding experimental data. Such a CFD model can be used as a tool in the development and optimization of PEM fuel cells.     

Effects of humidification temperatures on local current characteristics in a PEM fuel cell

It is well known that water plays a very important role in the performance of proton exchange membrane (PEM) fuel cells. Non-uniform water content in the membrane leads to non-uniform ionic resistance, and non-uniform liquid water fraction in the porous electrode causes varied mass transfer resistances. The objective of this work is to study the effects of different anode and cathode humidification temperatures on local current densities of a PEM fuel cell with a co-flow serpentine flow field. The method used is the current distribution measurement gasket technique [H. Sun, G.S. Zhang, L.J. Guo, H. Liu, J. Power Sources 158 (2006) 326–332]. The experimental results show that both air and the hydrogen need to be humidified to ensure optimal cell performance, and too high or too low humidification temperature can cause severe non-uniform distribution of local current density. From the experimental results of local current density distributions, the local membrane hydration, the optimal humidification temperature, and if flooding occurs can be obtained. Such detailed local measurement results could be very valuable in fuel cell design and operation optimizations.     

Methanol fuel processor and PEM fuel cell modeling for mobile application

The use of hydrocarbon fed fuel cell systems including a fuel processor can be an entry market for this emerging technology avoiding the problem of hydrogen infrastructure. This article presents a 1 kW low temperature PEM fuel cell system with fuel processor, the system is fueled by a mixture of methanol and water that is converted into hydrogen rich gas using a steam reformer. A complete system model including a fluidic fuel processor model containing evaporation, steam reformer, hydrogen filter, combustion, as well as a multi-domain fuel cell model is introduced. Experiments are performed with an IDATECH FCS1200™ fuel cell system. The results of modeling and experimentation show good results, namely with regard to fuel cell current and voltage as well as hydrogen production and pressure. The system is auto sufficient and shows an efficiency of 25.12%. The presented work is a step towards a complete system model, needed to develop a well adapted system control assuring optimized system efficiency.     

Optimization of hydrogen feeding procedure in PEM fuel cell systems for transportation

The hydrogen feeding sub-system is one of balance of plant (BOP) components necessary for the correct operation of a fuel cell system (FCS). In this paper the performance of a 6 kW PEM (Proton Exchange Membrane) FCS, able to work with two fuel feeding procedures (dead-end or flow-through), was experimentally evaluated with the aim to highlight the effect of the anode operation mode on stack efficiency and durability. The FCS operated at low reactant pressure (<50 kPa) and temperature (<330 K), without external humidification. The experiments were performed in both steady state and dynamic conditions. The performance of some cells in dead-end mode worsened during transient phases, while a more stable working was observed with fuel recirculation. This behavior evidenced the positive role of the flow-through procedure in controlling flooding phenomena, with the additional advantage to simplify the management issues related to hydrogen purge and air stoichiometric ratio. The flow-through modality resulted a useful way to optimize the stack efficiency and to reduce the risks of fast degradation due to reactant starvation during transient operative phases.     

Pressure control in a PEM fuel cell via second order sliding mode

Pressure difference inside the Polymer Electrolyte Membrane Fuel Cells (PEMFC) arises due to load variations, during which the pressure difference between anode and cathode rises. Practically, this problem can be avoided by equalizing anode and cathode pressures, to protect the fuel cell from permanent damage. This paper focuses on pressure regulation in the anode and cathode sides of the PEMFC. The control objective is achieved using second order sliding mode multi-input multi-output (MIMO) controller based on “Twisting algorithm”. Parametric uncertainty is formally presented and included in a nonlinear dynamic fuel cell model. The resultant nonlinear controller is robust and is proved to guarantee performance around any equilibrium point and under parametric uncertainty. Simulation results show that the proposed controller has a good transient response under load variations.     

Three‐dimensional multiphase flow model to study channel flow dynamics of PEM fuel cells

A three‐dimensional multiphase flow model has been developed to study dynamics of a water droplet on the surface of the channel of proton exchange membrane fuel cell. Results are presented based on solving full Navier–Stokes equations for Newtonian liquids. The volume‐of‐fluid method is used to numerically determine the deformation of free surfaces. Water droplet and channel fluid properties determine whether the droplet deforms and remain stationary or disintegrate. We have shown the dependency of the water flooding to the flow rate and pressure drop in the channel has been introduced as a tool to determine water flooding in the channel. Copyright © 2010 John Wiley & Sons, Ltd.     

A simple model of a high temperature PEM fuel cell

We develop a simple analytical model of a high temperature hydrogen fuel cell with proton exchange membrane. The model is validated against experimental results obtained in our group. The model is pseudo two dimensional, steady-state and isothermal, it accounts for the crossover of reactant gases through the membrane and it can be solved analytically. The role of the crossover is considered in detail.