Study of the characteristics of temperature rise and coolant flow rate control during malfunction of PEM fuel cells

In actual PEM fuel cell systems, the coolant flow rate is generally controlled to maintain a preset temperature at the coolant outlet. This implies that a change in coolant supply flow rate is a good early indicator of a malfunctioning PEM fuel cell stack and system components. In this study, various fuel cell malfunctions are simulated based on the practical coolant flow control strategy by using a three-dimensional, two-phase, multiscale PEM fuel cell model developed in our previous studies. The focus is on analysis of the characteristics of coolant flow rate change along with voltage degradation in various fuel cell malfunction cases. The model predictions show that in general, the coolant flow rate tends to increase proportionally with the degree of voltage degradation, but the increase in temperature inside the membrane electrode assembly (MEA) is not always related to the voltage drop and is influenced more directly by local current density distribution. Although the present numerical comparison between the normal and malfunctioning cases is conducted at the low current density of 0.3 A cm^?2, the general cell behavior will not be altered at higher current densities due to inverse relationship between cell performance and waste heat generation. The present work elucidates the complex interplay among increase in coolant flow rate, increase in MEA temperature, voltage drop, and change in local current density distribution when a PEM fuel cell malfunctions.     

Non-isothermal dynamic modelling and optimization of a direct methanol fuel cell

A non-isothermal dynamic optimization model of direct methanol fuel cells (DMFCs) is developed to predict their performance with an effective optimum-operating strategy. After investigating the sensitivities of the transient behaviour (the outlet temperature, crossovers of methanol and water, and cell voltage) to operating conditions (the inlet flow rates into anode and cathode compartments, and feed concentration) through dynamic simulations, we find that anode feed concentration has a significantly larger impact on methanol crossover, temperature, and cell voltage than the anode and cathode flow rates. Also, optimum transient conditions to satisfy the desired fuel efficiency are obtained by dynamic optimization. In the developed model, the significant influence of temperature on DMFC behaviour is described in detail with successful estimation of its model parameters.     

Temperature and hydrogen flow rate controls of diesel autothermal reformer for 3.6 kW PEM fuel cell system with autoignition delay time analysis

In this paper catalyst temperature and hydrogen flow rate controls are an area of interest for autothermal reforming (ATR) of diesel fuel to provide continuous and necessary hydrogen flow to the on-board fuel cell vehicle system. ATR control system design is important to ensure proper and stable performance of fuel processor and fuel cell stack. Fast system response is required for varying load changes in the on-board fuel cell system. To cope with control objectives, a combination of PI and PID controllers are proposed to keep the controlled variables on their setpoints. ATR catalyst temperature is controlled with feedback PID controller through variable OCR (oxygen to carbon ratio) manipulation and kept to the setpoint value of 900 °C. Additionally diesel auto-ignition delay time is implemented through fuel flow rate delay to avoid complete oxidation of fuel. Hydrogen flow rate to the fuel cell stack is kept to setpoint of required hydrogen flow rate according to fuel cell load current using PI controller. An integrated dynamic model of fuel processor and fuel cell stack is also developed to check the fuel cell voltage. Product gas composition of 35, 18 and 4% is achieved for hydrogen, nitrogen, and carbon dioxide, respectively. The results show fast response capabilities of fuel processor following the fuel cell load change and successfully fulfills the control objectives.     

A novel strategy based on salp swarm algorithm for extracting the maximum power of proton exchange membrane fuel cell

Cell temperature and water content of the membrane have a significant effect on the performance of fuel cells. The current-power curve of the fuel cell has a maximum power point (MPP) that is needed to be tracked. This study presents a novel strategy based on a salp swarm algorithm (SSA) for extracting the maximum power of proton-exchange membrane fuel cell (PEMFC). At first, a new formula is derived to estimate the optimal voltage of PEMFC corresponding to MPP. Then the error between the estimated voltage at MPP and the actual terminal voltage of the fuel cell is fed to a proportional-integral-derivative controller (PID). The output of the PID controller tunes the duty cycle of a boost converter to maximize the harvested power from the PEMFC. SSA determines the optimal gains of PID. Sensitivity analysis is performed with the operating fuel cell at different cell temperature and water content of the membrane. The obtained results through the proposed strategy are compared with other programmed approaches of incremental resistance method, Fuzzy-Logic, grey antlion optimizer, wolf optimizer, and mine-blast algorithm. The obtained results demonstrated high reliability and efficiency of the proposed strategy in extracting the maximum power of the PEMFC.     

Experimental study and mitigation of abnormal behavior of cell voltage in a proton exchange membrane fuel cell stack

The aim of this study is to investigate the abnormal behavior of cell voltage in a proton exchange membrane fuel cell stack and a mitigation strategy. The proposed strategy is simple and requires only a three‐way solenoid valve to replace the direct way solenoid valve of the original system. It is applied to a proton exchange membrane fuel cell stack with a dead‐ended anode to verify its validity. The behavior of the cell voltages in the stack is discussed in detail, especially the cell reversal process. The results show that the proposed strategy can significantly reduce the severity of hydrogen starvation. And the maximum power of the stack is increased by 10.67%. It is a sudden increase related to cell reversal mitigation. Uneven hydrogen distribution is the cause of low cell voltage and cell reversal. This strategy increases the cell voltage by increasing the hydrogen content in the anode flow channel downstream. It also significantly reduces the fluctuations in cell voltage and improves the uniformity of the cell voltage. This experimental study contributes to mitigate hydrogen starvation in cells of proton exchange membrane fuel cell stacks in application.     

Characterization of a novel, highly integrated tubular solid oxide fuel cell system using high-fidelity simulation tools

A novel, highly integrated tubular SOFC system intended for small-scale power is characterized through a series of sensitivity analyses and parametric studies using a previously developed high-fidelity simulation tool. The high-fidelity tubular SOFC system modeling tool is utilized to simulate system-wide performance and capture the thermofluidic coupling between system components. Stack performance prediction is based on 66 anode-supported tubular cells individually evaluated with a 1-D electrochemical cell model coupled to a 3-D computational fluid dynamics model of the cell surroundings. Radiation is the dominate stack cooling mechanism accounting for 66-92% of total heat loss at the outer surface of all cells at baseline conditions. An average temperature difference of nearly 125 °C provides a large driving force for radiation heat transfer from the stack to the cylindrical enclosure surrounding the tube bundle. Consequently, cell power and voltage disparities within the stack are largely a function of the radiation view factor from an individual tube to the surrounding stack can wall. The cells which are connected in electrical series, vary in power from 7.6 to 10.8 W (with a standard deviation, σ = 1.2 W) and cell voltage varies from 0.52 to 0.73 V (with σ = 81 mV) at the simulation baseline conditions. It is observed that high cell voltage and power outputs directly correspond to tubular cells with the smallest radiation view factor to the enclosure wall, and vice versa for tubes exhibiting low performance. Results also reveal effective control variables and operating strategies along with an improved understanding of the effect that design modifications have on system performance. By decreasing the air flowrate into the system by 10%, the stack can wall temperature increases by about 6% which increases the minimum cell voltage to 0.62 V and reduces deviations in cell power and voltage by 31%. A low baseline fuel utilization is increased by decreasing the fuel flowrate and by increasing the stack current demand. Simulation results reveal fuel flow as a poor control variable because excessive tail-gas combustor temperatures limit fuel flow to below 110% of the baseline flowrate. Additionally, system efficiency becomes inversely proportional to fuel utilization over the practical fuel flow range. Stack current is found to be an effective control variable in this type of system because system efficiency becomes directly proportional to fuel utilization. Further, the integrated system acts to dampen temperature spikes when fuel utilization is altered by varying current demand. Radiation remains the dominate heat transfer mechanism within the stack even if stack surfaces are polished lowering emissivities to 0.2. Furthermore, the sensitivity studies point to an optimal system insulation thickness that balances the overall system volume and total conductive heat loss.     

An innovating application of PEM fuel cell: Current source controlled by hydrogen supply

Fuel cells are complex systems which can be considered as low voltage electrical source. Preliminary investigations led with a single proton exchange membrane fuel cell, either short-circuited or hybridized by discharged supercapacitors, could evidence the behavior as a current source, in which the current is directly controlled by the hydrogen flow rate. This operation mode imposes the fuel cell voltage to be far below the threshold recommended by the fuel cell manufacturer. The paper deals with this unusual application of fuel cell and its benefits such as the high quality current, free of oscillations that might be upgraded for superconducting coil supply. To investigate this operation mode an appropriate single fuel cell model is established and then validated by means of a test bench equipped with a proton exchange membrane single fuel cell.     

Transient analysis of a solid oxide fuel cell unit with reforming and water-shift reaction and the building of neural network model for rapid prediction in electrical and thermal performance

This study investigates the effect of reforming reaction, water-shift reaction, and operating parameters on the transient performance of a solid oxide fuel cell unit, because the transient analysis is necessary and helpful for the applications of a SOFC with cross-flow configuration. The primary results show that all properties approach the steady state at similar time except the cell temperature. The reforming and water-shift reaction obviously promote the average current density by 5%, and lower the maximum cell temperature by 20 K. The molar flow rate variation deduces about 15 K difference of maximum cell temperature. The effect of inlet temperature and operating voltage on the average current density and maximum cell temperature is more obvious than the molar flow rate effect. Moreover, this study builds a neural network model to predict the steady average current density and maximum cell temperature rapidly and correctly, which is helpful for the control of a SOFC.     

Study of gas pressure and flow rate influences on a 500 W PEM fuel cell,thanks to the experimental design methodology

The behaviour of a 500 W PEM fuel cell stack, fed by pure hydrogen and humidified compressed air, is currently investigated on the fuel cell test platform of Belfort.In this paper, the influences on fuel cell performance of gas pressure and flow rate parameters are studied. The fuel cell is operated in the pressure regulation mode: the gas flow rates are regulated thanks to mass flow controllers placed upstream of the stack and the gas pressures at stack inlets are controlled by regulation valves located downstream of the stack. The choice of the various tests to perform is made thanks to experimental design methodology, which is a suitable technique to characterise, analyse and to improve a complex system such as a fuel cell generator. In this study, the four physical factors considered are both hydrogen/air pressures and anode/cathode flow rates. Each factor has two levels, leading to a full factorial design requiring 16 experiments (16 current–voltage curves). The test bench developed at the laboratory allows setting the other factors (for instance: stack temperature, relative humidity and dew point temperature of the air at stack inlet) at fixed values. The test responses are the maximal output power and the efficiency computed for this power. Statistical sensitivity analyses (ANOVA analyses) are used to compute the effects and the contributions of the various factors to the fuel cell maximal power. The use of fractional designs shows also how it is possible to reduce the number of experiments. Some graphic representations are employed in order to display the results of the statistical analyses made for different current values.     

Performance prediction of PEM fuel cell with wavy serpentine flow channel by using artificial neural network

Effects of serpentine flow channel having sinusoidal wave at the rib surface on performance of PEMFC having 25 cm² active area are investigated at different flow rates, three different amplitudes changing from 0.25 mm to 0.75 mm and three different cell operation temperatures. A proton exchange membrane fuel cell (PEMFC) is modeled for the prediction of the output current by using artificial neural network (ANN) that is utilized the aforementioned experimental parameters. Effect of hydrogen and air flow rate, the fuel cell temperature, amplitude of channel is tested. The results indicated that model C1 having lowest amplitude is enhanced maximum power output up to 20.15% as compared to indicated conventional serpentine channel (model C4) for 0.7 SLPM H₂ and 1.5 SLPM air and also model C1 has better performance than C2, C3 and C4 models. The maximum power output is augmented with increasing the cell temperature due to raising the fuel and oxidant diffusion ratio. Cell temperature, amplitude, H₂ and air flow rate and input voltage is used as input variables in train and test of the developing ANN model. MAPE of training and testing is determined as 2.89 and 2.059, respectively. Prediction results of developed ANN model including two hidden layer shows similar trend with experimental results. Developed ANN model can be used to both decrease the number of required experiments and find the optimum operation condition within the range of input parameters.     

Adaptive thermal control for PEMFC systems with guaranteed performance

Proton exchange membrane fuel cell (PEMFC) s are faced with dynamical load scenario in practical applications, and the resulting temperature variation will decrease the performance and consequently shorten the fuel cell lifetime. To address this problem, a control strategy for regulating the stack temperature is proposed in this paper. Firstly, a thermal management-oriented dynamic model of a water-cooled PEMFC system is built to facilitate the control design. Secondly, considering that the stack temperature should be maintained in a certain range regardless of the dynamical changing current demand, a Barrier Lyapunov function is employed to construct a feedback error of the stack temperature. Thirdly, a set of adaptation laws is designed to estimate the unknown parameters related to the gas flow rates in the flow fields. Particularly, a dynamic inversion tracking methodology is applied to design the non-affine input. A Lyapunov method based analysis demonstrates the stability and convergence of the closed-loop properties. Simulation results are provided to show that the proposed control strategy can satisfy all the control objectives and enhance the control performance compared to the proportional-integral controlled case.     

Experimental investigation on voltage response to operation parameters of a unitized regenerative fuel cell during mode switching from fuel cell to electrolysis cell

In this work, a proton exchange membrane unitized regenerative fuel cell with a 25 cm² active area and a transparent window was designed to study the influence of mode switching from the fuel cell mode to the electrolysis cell mode on the cell voltage and the gas‐liquid two‐phase flow behaviors in the oxygen flow channels. Results indicate that: the growth rate of electrolysis cell voltage before the water pumped to the oxygen flow channels decreases with the increase of the fuel cell current density; while the growth rate of electrolysis cell voltage before the water pumped to the oxygen flow channels increases with the cell temperature; the voltage of electrolysis cell mode before the water pumped to the oxygen flow channels decreases with the increase of water flow rate; the different voltage reduction speeds are attributed to the different water flow rates. The water temperature has an obscure influence on the cell voltage of electrolysis cell mode before the water pumped to the oxygen flow channels.     

Simulation of the water and heat management in proton exchange membrane fuel cells

A non-isothermal and three-dimensional numerical model of a PEM fuel cell was developed to compute the water and heat management. Transport of water in the polymer membrane, phase change of water in the cathode porous medium and capillary flow to the gas channels were determined. Influences of these phenomena on fuel cells and conditions that may affect their performance have been numerically evaluated. Output variables are velocity, temperature, mass fraction, current density, voltage loss, water content of the polymer membrane, saturation and liquid flow fields. Cell voltage and total current density of PEM fuel cell were computed as well. Results show that there may be severe mass transfer limitations depending either on the design or on the water management of the cell. For the chosen conditions, the polymer membrane can keep and even increase its water content, as long as inlet flows are injected at 100% relative humidity. In case the fuel cell is operated under dehydrating conditions, the decrease of the water content of the polymer electrolyte may affect the performance. The variations of temperature were small. However, temperature plays an important role in the cathode reaction rate of the cell and in the dehydration of the polymer membrane. Numerical results and experimental data were found to be in good agreement.     

Performance simulation for an anode-supported SOFC using Star-CD code

Experimental activities and computational fluid dynamics (CFD) simulation are presented in this paper for investigating the performance of an anode-supported solid oxide fuel cell (SOFC). The goal of this work is to assess a commercial CFD code, Star-CD with es-sofc module, to simulate the current–voltage (I–V) characteristics with respect to the experimental data. Compiled with the geometry of cell test housing, a 3D numerical model and test conditions were established to analyze the anode-supported cell (ASC) performance including current density and temperature distributions, fuel concentration, and fuel utilization. After adjusting parameters in the electrochemical model, the simulation results showed good agreements with the experimental data. The results also revealed that the power density increased while the fuel utilization decreased as the fuel flow rate increased.     

Parallel serpentine-baffle flow field design for water management in a proton exchange membrane fuel cell

In a proton exchange membrane fuel cell (PEMFC) water management is one of the critical issues to be addressed. Although the membrane requires humidification for high proton conductivity, water in excess decreases the cell performance by flooding. In this paper an improved strategy for water management in a fuel cell operating with low water content is proposed using a parallel serpentine-baffle flow field plate (PSBFFP) design compared to the parallel serpentine flow field plate (PSFFP). The water management in a fuel cell is closely connected to the temperature control in the fuel cell and gases humidifier. The PSBFFP and the PSFFP were evaluated comparatively under three different humidity conditions and their influence on the PEMFC prototype performance was monitored by determining the current density–voltage and current density–power curves. Under low humidification conditions the PEMFC prototype presented better performance when fitted with the PSBFFP since it retains water in the flow field channels.     

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.     

Three dimensional CFD modeling and experimental validation of an electrolyte supported solid oxide fuel cell fed with methane-free biogas

In the present study a comprehensive numerical model of a planar cross-flow electrolyte-supported solid oxide fuel cell (SOFC) is reported. This model is solved in a 3D environment using COMSOL Multiphysics software. To verify the simulation results, an experimental set-up of a six-cell stack was built. Cell temperature and current–voltage measurements are used for validation of the simulation results. Good agreement between the simulation results and the experimental measurements is achieved. Temperature validation in addition to the popular current/voltage validation ensures that the model performs well in predicting local processes like chemical reactions. In this study methane-free biogas (CO₂ + H₂) is fed to the SOFC, and the performance of the system is investigated and explained. It is concluded that the methane-free biogas reduces the cooling air flow due to endothermic reverse water gas shift reaction and gives better current density distribution over the cell compared to hydrogen.     

Analytical modelling of boiling phase change phenomenon in high-temperature proton exchange membrane fuel cells during warm-up process

This paper investigates the thermal and water balance as well as the electro-kinetics during the warm-up process of a Hydrogen/Oxygen high-temperature proton exchange membrane fuel cell (HT-PEMFC) from room temperature up to the desired temperature of 180 °C. The heating strategy involves the extraction of constant current from the fuel cell, while an external heating source with a constant heat input rate is applied at the end plates of the cell simultaneously. A simple analytical unsteady model is derived addressing the boiling phase changing phenomenon in the cathode catalyst layer (CCL) and cathode gas diffusion layer (CGDL) of the cathode that occurs when the temperature of the fuel cell reaches the boiling temperature of water. Parameters such as the heat input rate, extracted current, cathode pressure and cathode stoichiometric flow ratio are varied and their effects on the temperature, liquid water fraction and most importantly, the voltage profiles with respect to time, are explored. A comparison between other existing heating strategies using the model suggests that there is insignificant improvement in warm-up time when current is extracted from room temperature considering a single cell. However, considering the solution for a typical 1-kW stack suggests that reductions in warm-up time and energy consumption can be expected. In addition, the results show that boiling phase change is found to be a key factor that affects the level of water saturation in the porous media such as the CCL and CGDL during the warm-up process, when current is extracted from the start of the process i.e. room temperature. However, the energy consumption due to boiling phase change is found to be negligible as compared to external heating input rate. The parametric studies show that the variation of heat input rate, extracted current and cathode pressure have significant effect on the cell voltage that is strongly dominated by the liquid water fraction in the porous media. On the other hand, the variation of cathode stoichiometric flow ratio is found to have minimal effect on the output cell voltage. The parametric studies also indicate that boiling phase change is present for a significant period of time under typical operating conditions.     

Two-dimensional dynamic simulation of a direct internal reforming solid oxide fuel cell

This study presents a two-dimensional mathematical model of a direct internal reforming solid oxide fuel cell (DIR-SOFC) stack which is based on the reforming reaction kinetics, electrochemical model and principles of mass and heat transfer. To stimulate the model and investigate the steady and dynamic performances of the DIR-SOFC stack, we employ a computational approach and several cases are used including standard conditions, and step changes in fuel flow rate, air flow rate and stack voltage. The temperature distribution, current density distribution, gas species molar fraction distributions and dynamic simulation for a cross-flow DIR-SOFC are presented and discussed. The results show that the dynamic responses are different at each point in the stack. The temperature gradients as well as the current density gradients are large in the stack, which should be considered when designing a stack. Further, a moderate increase in the fuel flow rate improves the performances of the stack. A decrease in the air flow rate can raise the stack temperature and increase fuel and oxygen utilizations. An increased output voltage reduces the current density and gas utilizations, resulting in a decrease in the temperature.     

Three‐dimensional nonisothermal modeling of solid oxide fuel cell coupling electrochemical kinetics and species transport

A three‐dimensional (3D) nonisothermal model is developed and applied for anode‐supported planar solid oxide fuel cell (SOFC). The mass and momentum, species, ion, electric, and heat transport equations are solved simultaneously by implementing the electrochemical kinetics and electrochemical reaction as volumetric source terms. The interconnect land limits the O₂ transport under the land and lowers the local current density under the land. The effects of interconnect land width and cathode substrate thickness on SOFC cell performance are quantified in this study. Cathode stoichiometry is found to have a large effect on the SOFC cell temperature distribution. Under low‐cathode stoichiometry, significant temperature gradients are seen in the SOFC cell. Higher‐cathode stoichiometry is beneficial for lower temperature and more uniform current density distribution in SOFC cell. Co‐flow and counter‐flow arrangements are investigated and discussed with the model. Counter‐flow arrangement is found to induce a high temperature and high current density region near the H₂ inlet. On the other hand, co‐flow arrangement leads high temperature and high current density to occur relatively downstream, a slightly lower maximum temperature on cell and considerably more uniform current density distribution. A 67.2‐cm² SOFC cell is simulated considering the side cooling effect. The side cooling effectively lowers the cell temperature, at the same time, causes temperature, current density, and fuel utilization nonuniformity in the across multichannel direction. Because of the strong coupling of the in‐plane current density distribution and temperature distribution, limiting the locally high temperature and temperature gradient is critical for achieving a more uniform current density distribution in anode‐supported planar SOFC.