Coolant controls of a PEM fuel cell system

When operating the polymer electrolyte membrane (PEM) fuel cell stack, temperatures in the stack continuously change as the load current varies. The temperature directly affects the rate of chemical reactions and transport of water and reactants. Elevated temperature increases the mobility of water vapor, which reduces the ohmic over-potential in the membrane and eases removal of water produced. Adversely, the high temperature might impose thermal stress on the membrane and cathode catalyst and cause degradation. Conversely, excessive supply of coolants lowers the temperature in the stack and reduces the rate of the chemical reactions and water activity. Corresponding parasitic power dissipated at the electrical coolant pump increases and overall efficiency of the power system drops.     

Unsteady 2D PEM fuel cell modeling for a stack emphasizing thermal effects

Models currently used for analyses of thermal and water behavior of a PEM fuel cell are based 3D computational fluid dynamics (CFD). However, the analyses are limited to a single cell with static behavior. Thus, these models cannot be used for analyses of dynamic behavior of a stack that continuously varies according to operating conditions. The model proposed describes dynamic behavior of a stack with two adjoining cells and endplate assembly, and work as a current controlled voltage source that can be used for optimization of BOPs and the associated controls. Simulations have been conducted to analyze start-up behaviors and the performance of the stack. Our analyses deliver following results: (1) dynamic temperature distribution in both the through-plane direction and the along channel direction of the fuel cell stack, (2) effects influencing the source terms of current density, and (3) dynamic oxygen concentration distribution. The temperature profile and its variation propensity are comparable to the previous results [Y. Shan, S.Y. Choe, J. Power Sources, 145 (1) (2005) 30–39; Y. Shan, S.Y. Choe, J. Power Sources, in press].     

Simultaneous thermal and visual imaging of liquid water of the PEM fuel cell flow channels

Water flooding and membrane dry-out are two major issues that could be very detrimental to the performance and/or durability of the proton exchange membrane (PEM) fuel cells. The above two phenomena are well-related to the distributions of and the interaction between the water saturation and temperature within the membrane electrode assembly (MEA). To obtain further insights into the relation between water saturation and temperature, the distributions of liquid water and temperature within a transparent PEM fuel cell have been imaged using high-resolution digital and thermal cameras. A parametric study, in which the air flow rate has been incrementally changed, has been conducted to explore the viability of the proposed experimental procedure to correlate the relation between the distribution of liquid water and temperature along the MEA of the fuel cell. The results have shown that, for the investigated fuel cell, more liquid water and more uniform temperature distribution along MEA at the cathode side are obtained as the air flow rate decreases. Further, the fuel cell performance was found to increase with decreasing air flow rate. All the above results have been discussed.     

Performance prediction of proton exchange membrane fuel cell engine thermal management system using 1D and 3D integrating numerical simulation

The cooling system of proton exchange membrane fuel cell (PEMFC) engine was simulated by 1D and 3D collaborative simulation method. Firstly, the resistance characteristics of the flow channel are obtained by simulating the airside flow model. A three-dimensional simulation model including dual fans and radiator is also established to simulate the airflow distribution. The one-dimensional simulation model of 30 kW PEMFC engine cooling system that are mainly composed of a thermostat, water pump, and fan and radiator model is established. Secondly, the heat dissipation performance of the cooling system is calculated by using the coupled simulation model. It is found that the simulation results of the amount of heat transferred are in good agreement with the experimental data by compromising, which proves that the model is reasonable. Finally, the thermal performance of the extreme operating conditions of the PEMFC system is simulated by means of a simulation model. By monitoring the flow of the pump and the fan speed, we can maintain the stack internal heat balances, so that the stack efficient and stable operation. The results demonstrate that the 3D simulation can get the distribution of fluid flow more accurately, while the simulation time of 1D thermal system is short and can guide the matching of heat transfer parts quickly.     

A method for evaluating the efficiency of PEM fuel cell engine

Efficiency is an important factor to reflect the performance of fuel cell engine (FCE). Evaluating efficiency should consider efficiency properties and common work conditions of FCE. In this paper, output power of FCE on real work conditions is analyzed according to driving cycles, and four efficiency evaluation points are obtained, as well as their weighted values. Then, a scoring function is used to convert the efficiency values of four evaluation points into scores. Multiplying scores by their weighted values and adding them together, we can get overall scores of efficiency properties. This method can evaluate the efficiency performance of FCE reasonably and objectively.     

Automotive fuel cell engine test cell design and its thermal flow analysis

A test cell for automotive PEM fuel cell engine is introduced and designed. Similarities and differences of facilities between PEM fuel cell engine and inner-combustion engine are illustrated. It turns out that, the air treatment, exhaust gas, cooling and electrical facilities are quite similar, the fuel treatment, power load type, ventilation and air-conditioning are quite different, while the vibration isolation and noise elimination facilities are completely simplified. Furthermore, a thermodynamic model is proposed to analysis the heat flow in fuel cell engine test cell. The Monte Carlo Simulation method is applied to get the proportion of outgoing thermal flows. A thermal flow rule of 4.5/4.5/0.6/0.4 is proposed as rule of thumb for cell designer.     

Development of a cylindrical PEM fuel cell

Proton exchange membrane fuel cells (PEMFCs) have strong potential as power conversion devices of the future, especially for man-portable and mobile applications. However, the manufacturing cost should be significantly reduced for making PEMFCs commercially attractive. An improvement of the power density with respect to the weight of the cell - termed as gravimetric power density in this study - can help in achieving lower manufacturing cost and reducing parasitic power losses, which is particularly important in man-portable applications. Furthermore, the power density of a PEMFC with respect to the overall volume of the cell - termed as volumetric power density in this study - must be improved for man-portable and automotive applications. The bipolar plates made out of graphite contribute significantly to the cost, weight, and volume of the cell. The state-of-the-art PEM fuel cells are of planar design. While several commercial planar prototypes have been demonstrated, cost and water management are still major issues. These problems arise partly as a result of the complicated bipolar plate design in planar PEMFC. Because the planar fuel cell concept has been so well-entrenched, alternate designs have not been seriously pursued. In this paper, we present some experimental studies on a novel cylindrical PEM fuel cell design that addresses the cost, gravimetric and volumetric power density issues. This study while highlighting the advantages of the tubular design also identifies areas of research that will have tremendous utility in further development of this technology.     

Performance study of a PEM fuel cell

This paper presents the installation, maintenance and the efficiency of a Polymer Electrolyte Membrane (PEM) fuel cell, Ballard Trade Mark that use pure hydrogen as fuel and air as an oxidant. A study of the overall efficiency, considering the co-generation of electrical and thermal energies, is performed. The system consists of the cell, a CC/CC converter, a battery, a DC/AC inverter and the load. The behavior of the system is experimentally analyzed for different load states (cases) by measuring and controlling all the parameters registered by the communication software of the cell. The software can adjust limit values for current intensity, hydrogen flow, pressure and the temperature.     

Study of thermal conductivity of PEM fuel cell catalyst layers

Thermal conductivities and compression of differently composed Polymer Electrolyte Membrane Fuel Cell (PEMFC) Catalyst Layers (CLs) were measured, both when dry and when containing liquid water. The results were compared using a 1-D thermal model.     

Experimental analysis of a degraded open-cathode PEM fuel cell stack

The well-known challenges to overcome in PEM fuel cell research are their relatively low durability and the high costs for the platinum catalysts. This work focuses on degradation mechanisms that are present in open-cathode PEM fuel cell systems and their links to the decaying fuel cell performance. Therefore a degraded, open-cathode, 20 cell, PEM fuel cell stack was analyzed by means of in-situ and ex-situ techniques. Voltage transients during external perturbations, such as changing temperature, humidity and stoichiometry show that degradation affects individual cells quite differently towards the end of life of the stack. Cells located close to the endplates of the stack show the biggest performance decay. Electrochemical impedance spectroscopy (EIS) data present non-reversible catalyst layer degradation but negligible membrane degradation of several cells. Post-mortem, ex-situ experiments, such as cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) show a significant active area loss of the first cells within the stack due to Pt dissolution, oxidation and agglomeration. Scanning electron microscope (SEM) images of the degraded cells in comparison with the normally working cells in the stack show severe carbon corrosion of the cathode catalyst layers.     

Consistent modeling of a single PEM fuel cell using Onsager's principle

In this paper a novel approach is proposed for a three-dimensional (3D) modeling of a High Temperature Exchange Membrane Fuel Cell (HTPEMFC). This new modeling is based on Onsager's principle of minimum energy dissipation that is applicable for near equilibrium and coupled irreversible systems. In particular, for low conductivity membranes, this leads to a one directional proton movement through the membrane. The resulting equations are numerically solved for a real single cell geometry, using a 3D finite volume discretization. Results are analyzed and validated against experimental data.     

Thermal modeling and temperature control of a PEM fuel cell system for forklift applications

Temperature changes in PEM fuel cell stacks are considerably higher during load variations and have a negative impact as they generate thermal stresses and stack degradation. Cell hydration is also of vital importance in fuel cells and it is strongly dependent on operating temperature. A combination of high temperature and reduced humidity increases the degradation rate. Stack thermal management and control are, thus, crucial issues in PEM fuel cell systems especially in automotive applications such as forklifts.     

Modeling and control of a PEM fuel cell based hybrid multilevel inverter

Multilevel inverter is an effective and practical solution for increasing power demand and reducing harmonics of AC waveforms. It is mainly employed in the distributed energy sources area because several batteries, fuel cell and solar cell can be connected through multilevel inverter to feed a load. This paper investigates the potentials of a single-phase Hybrid Cascaded Multilevel Inverter (HCMLI) fed from Proton Exchange Membrane Fuel Cell (PEMFC). A mathematical model of the PEMFC supplying HCMLI has been developed. This paper also presents the effect of a novel hybrid modulation on the device switching losses and harmonics of HCMLI. The proposed hybrid modulation technique combines the fundamental frequency switching scheme and Variable Frequency Inverted Sine Pulse Width Modulation (VFISPWM) technique. A comparison between the hybrid modulation strategy and the conventional Phase Disposition (PD) PWM method is also presented in terms of THD and switching losses. A suitable PID controller has been designed to control the output voltage of fuel cell based HCMLI, so that it can provide constant AC voltage with minimum THD up to the rated conditions. The inverter circuit topology and its control scheme are described in detail and their performance is verified based on simulation and experimental results.     

Experimental investigation of the thermal response of open-cathode proton exchange membrane fuel cell stack

The aim of this study is to investigate the thermal response characteristics of the proton exchange membrane fuel cell stack. In order to find out the regularities of temperature variation under rapidly increasing load change, a home-made 500 W open-cathode stack embedded with 30 thermocouples was made and tested. The result shows that the local temperature dominates the thermal response at the initial stage while the membrane hydration is the crucial impact factor at low power stage. Further, the anode flooding strongly affects the stability of the output performance and the change of temperature at the overloaded stage. The maximum temperature difference within one cell can reach a steady state faster than that of the temperature. At normal operation, there is little difference between the defined surfaces. The exergy analysis shows that the reaction air will have higher exergy if the temperature variation is more smooth. This experimental study contributes to the optimization of the cooling strategy and thermal management of the open-cathode stack in application.     

PEM fuel cell degradation effects on the performance of a stand-alone solar energy system

After comparing fresh and degraded performances of Polymer Electrolyte Membrane (PEM) based components of a hydrogen cycle with the help of computational fluid dynamics simulations, recently established stand-alone solar energy system producing hydrogen for energy storage is investigated focusing on the effects of degradation of fuel cells on the overall performance of the system. A complete model of the system has been developed using TRNSYS, and a degraded PEM Fuel Cell Subsystem has been incorporated into the model. Then, the effects of the PEM fuel cell degradation on the overall performance of the energy system are estimated. After reviewing the simulation results, the model shows that the PEM Fuel Cell degradation has a substantial impact on the overall system performance causing a system down time of approximately one month in a typical simulation year. Consequently, the stand-alone system is not capable of operating continuously for a complete year when the PEM fuel cells are degraded. Furthermore, an economic analysis is performed for a project lifetime of 25 years and the Levelized Cost of Electricity (LCE) value of the degraded system is found to be 0.08 $/kWh higher than the newly established system. Nevertheless, LCE calculations that are repeated for declining PV panel costs show that the considered hybrid system may be an economically competitive alternative to conventional diesel generators, even when the degradation of PEM based components and their regular maintenance are considered.     

Effect of cathode design on the performance of an air-breathing PEM fuel cell

Effect of cathode design on the performance of an air-breathing fuel cell is studied using a three dimensional, non-isothermal, steady state and single phase model developed using commercial CFD software FLUENT (version 6.3). Performances of ducted (channel) and ribbed (planar) cathode designs are compared and the cell characteristics such as current density, species, temperature distributions, velocity and net water transport coefficient are presented. Peak power density obtained for the cell with ducted cathode is 205 m W/cm², whereas with ribbed cathode it is 232 m W/cm². The limiting current density of the cell with ribbed cathode (690 m A/cm²) is much higher than that of the cell with ducted cathode (430 m A/cm²). The performance curves as well as the cell characteristics show that the ribbed cathode design is a better configuration compared to ducted design. Cell orientation has a significant effect on the cell performance. Best performance is obtained when the cell is oriented vertically for both ribbed and ducted cathode designs.     

Application of a thermally conductive pyrolytic graphite sheet to thermal management of a PEM fuel cell

This work experimentally investigates the thermal performance of a pyrolytic graphite sheet (PGS) in a single proton exchange membrane fuel cell (PEMFC). This PGS with high thermal conductivity serves as a heat spreader, reduces the volume and weight of cooling systems, and reduces and homogenizes the temperature in the reaction area of the fuel cells. A transparent PEMFC is constructed with PGS of thickness 0.1 mm cut into the shape of a flow channel and bound with the cathode gas channel plate. Eleven thermocouples are embedded at different positions on the cathode gas channel plate to measure the temperature distribution. The water and water flooding inside the cathode gas channels, with and without PGS, were successfully visualized. The locations of liquid water are correlated with the temperature measurement. PGS reduces the maximum cell temperature and improves cell performance at high cathode flow rates. The temperature distribution is also more uniform in the cell with PGS than in the one without PGS. Results of this study demonstrate the promising application of PGS to the thermal management of a fuel cell system.     

Neural network model for a commercial PEM fuel cell system

Performance prediction of a commercial proton exchange membrane (PEM) fuel cell system by using artificial neural networks (ANNs) is investigated. Two artificial neural networks including the back-propagation (BP) and radial basis function (RBF) networks are constructed, tested and compared. Experimental data as well as preprocess data are utilized to determine the accuracy and speed of several prediction algorithms. The performance of the BP network is investigated by varying error goals, number of neurons, number of layers and training algorithms. The prediction performance of RBF network is also presented. The simulation results have shown that both the BP and RBF networks can successfully predict the stack voltage and current of a commercial PEM fuel cell system. Speed and accuracy of the prediction algorithms are quite satisfactory for the real-time control of this particular application.     

Exergoeconomic analysis of a vehicular PEM fuel cell system

In this study, we deal with the exergoeconomic analysis of a proton exchange membrane (PEM) fuel cell power system for transportation applications. The PEM fuel cell performance model, that is the polarization curve, is previously developed by one of the authors by using the some derived and developed equations in literature. The exergoeconomic analysis includes the PEM fuel cell stack and system components as compressor, humidifiers, pressure regulator and the cooling system. A parametric study is also conducted to investigate the system performance and cost behaviour of the components, depending on the operating temperature, operating pressure, membrane thickness, anode stoichiometry and cathode stoichiometry. For the system performance, energy and exergy efficiencies and power output are investigated in detail. It is found that with an increase of temperature and pressure and a decrease of membrane thickness the system efficiency increases which leads to a decrease in the overall production cost. The minimization of the production costs is very crucial in commercialization of the fuel cells in transportation sector.