Modeling and analysis of a 5 kWe HT-PEMFC system for residential heat and power generation

We present a high-temperature proton exchange membrane fuel cell (HT-PEMFC) system model that accounts for fuel reforming, HT-PEMFC stack, and heat-recovery modules along with heat exchangers and balance of plant (BOP) components. In the model developed for analysis, the reaction kinetics for the fuel reforming processes are considered to accurately capture exhaust gas compositions and reactor temperatures under various operating conditions. The HT-PEMFC stack model is simplified from the three-dimensional HT-PEMFC CFD models developed in our previous studies. In addition, the parasitic power consumption and waste heat release from the various BOP components are calculated based on their heat-capacity curves. An experimental fuel reforming reactor for a 5.0 kW_e HT-PEMFC system was tested to experimentally validate the fuel reforming sub model. The model predictions were found to be in good agreement with the experimental data in terms of exhaust gas compositions and bed temperatures. Additionally, the simulation revealed the impacts of the burner air-fuel ratio (AFR) and the steam reforming reactor steam-carbon ratio on the system performance and efficiency. In particular, the combined heat and power efficiency of the system increased up to 78% when the burner AFR was properly adjusted. This study clearly illustrates that an HT-PEMFC system requires a high degree of thermal integration and optimization of the system configuration and operating conditions.     

Utilizing coke oven gases as a fuel for a cogeneration system based on high temperature proton exchange membrane fuel cell; energy,exergy and economic assessment

Based on a high temperature proton exchange membrane fuel cell (HT-PEMFC), a cogeneration system is proposed to produce heat and power. The system includes a coke oven gas steam reformer, a water gas shift reactor, and an afterburner. The system is analyzed in detail considering the energy, exergy and economic viewpoints. The analyses reveal the importance of HT-PEMFC in the system and according to the results, 9.03 kW power is generated with energy and exergy efficiencies of 88.2% and 26.2%, respectively and the total product unit cost is calculated as 91.8 $/GJ. Through a parametric study the effects on system performance are studied of such variables as the current density, fuel cell and reformer operating temperatures, and cathode stoichiometric ratio. It is found that an increase in the fuel cell temperature and/or a decrease in the reformer temperature enhance the exergy efficiency. The exergy efficiency is also maximized at the cathode stoichiometric ratio of 2.4. By performing a two-objective optimization using genetic algorithm, the best operating point is determined at which the exergy efficiency is (32.86%) and the total product unit cost is (78.68 $/GJ).     

Effect of cooling surface temperature difference on the performance of high-temperature PEMFCs

Internal temperature distribution of the high-temperature proton exchange membrane fuel cell (HT-PEMFC) is affected by the cooling temperature, heat generation and reactant gas flow. Reasonable temperature control is helpful to improve the fuel cell performance and durability. In this work, a three-dimensional model that couples the reactant flows, species transport, heat transfer, charge transfer, and electrochemical reaction, was developed to simulate the HT-PEMFC operation. A solid mechanics model was established to analyze the stress distribution of the fuel cell. The polarization curves, distributions of temperature, membrane proton conductivity, current density and stress are investigated for different cooling surface temperature. Furthermore, the effect of assembly temperature on the stress of phosphoric acid-doped polybenzimidazole (PBI) membrane is discussed. Results reveals that the peak power density and uniformity of current density decrease with the increase of cooling surface temperature difference. The peak power density decreases by 9.14% when the temperature difference increases from 0 K to 40 K. The cooling surface temperature difference of less than 10 K and the voltage range in 0.5–0.7 V can achieve better current density uniformity and smaller current density change rate. In addition, the membrane in fuel cell has the highest stress, and increasing the assembly temperature is helpful to reduce the membrane stress. When the assembly temperature increases from 293.15 K to 343.15 K, the max and min compressive stresses in membrane in-plane decrease from 39.436 MPa to 31.416 MPa–24.934 MPa and 17.369 MPa at the temperature difference of 30 K, which decreases by 36.77% and 44.7%, respectively.     

Numerical modeling and investigation of gas crossover effects in high temperature proton exchange membrane (PEM) fuel cells

A gas crossover model is developed for a high temperature proton exchange membrane fuel cell (HT-PEMFC) with a phosphoric acid-doped polybenzimidazole membrane. The model considers dissolution of reactants into electrolyte phase in the catalyst layers and subsequent crossover of reactant gases through the membrane. Furthermore, the model accounts for a mixed potential on the cathode side resulting from hydrogen crossover and hydrogen/oxygen catalytic combustion on the anode side due to oxygen crossover, which were overlooked in the HT-PEMFC modeling works in the literature. Numerical simulations are carried out to investigate the effects of gas crossover on HT-PEMFC performance by varying three critical parameters, i.e. operating current density, operating temperature and gas crossover diffusivity to approximate the membrane degradation. The numerical results indicate that the effect of gas crossover on HT-PEMFC performance is insignificant in a fresh membrane. However, as the membrane is degraded and hence gas crossover diffusivities are raised, the model predicts non-uniform reactant and current density distributions as well as lower cell performance. In addition, the thermal analysis demonstrates that the amount of heat generated due to hydrogen/oxygen catalytic combustion is not appreciable compared to total waste heat released during HT-PEMFC operations.     

Effect of channel and rib width on transport phenomena within the cathode of a proton exchange membrane fuel cell

A two-phase, half cell, non-isothermal model of a proton exchange membrane fuel cell has been developed. The model geometry includes a gas diffusion layer, a micro-porous layer and a catalyst layer along with the interfaces for channel, land and membrane. The effect of channel and rib width on transport phenomena has been examined. The model was run with saturated gas feed at different operating current densities and cell temperatures. The results show that increasing the channel to rib width ratio does not have any effect on the total amount of liquid saturation, however, its distribution is significantly affected under the channel and rib region within the porous layers. The degree of supersaturation and undersaturation extends, but, the supersaturation region shrinks and the undersaturation region extends with increase in channel to rib width ratio. It is concluded that the transport mechanism within the cathode is a highly coupled phenomena which interlinks local distribution of temperature, liquid saturation and the relative humidity.     

Numerical studies of liquid water behaviors in PEM fuel cell cathode considering transport across different porous layers

In this paper, a two-phase two-dimensional PEM fuel cell model, which is capable of handling liquid water transport across different porous materials, is employed for parametric studies of liquid water transport and distribution in the cathode of a PEM fuel cell. Attention is paid particularly to the coupled effects of two-phase flow and heat transfer phenomena. The effects of key operation parameters, including the outside cell boundary temperature, the cathode gas humidification condition, and the cell operation current, on the liquid water behaviors and cell performance have been examined in detail. Numerical results elucidate that increasing the fuel cell temperature would not only enhance liquid water evaporation and thus decrease the liquid saturation inside the PEM fuel cell cathode, but also change the location where liquid water is condensed or evaporated. At a cell boundary temperature of 80 °C, liquid water inside the catalyst layer and gas diffusion media under the current-collecting land would flow laterally towards the gas channel and become evaporated along an interface separating the land and channel. As the cell boundary temperature increases, the maximum current density inside the membrane would shift laterally towards the current-collecting land, a phenomenon dictated by membrane hydration. Increasing the gas humidification condition in the cathode gas channel and/or increasing the operating current of the fuel cell could offset the temperature effect on liquid water transport and distribution.     

Design and analysis of SOFC stack with different types of external manifolds

In this study, a four-cell stack of anode-supported planar solid oxide fuel cells (SOFCs) was designed and simulated to investigate the flow/heat transport phenomena and the performance of the SOFC stack. This SOFC stack was designed based on the external manifold types with one side open toward the cathode inlet and components such as base station, manifold, end plate, press jig, and housing. To investigate the performance of the SOFC stack, a step-by-step heat and flow analysis was conducted. First, the separator, functioning as a current collector and a gas channel, was designed to have repeated convex shapes. As the boundary of the flow passage was periodic in both streamwise and transverse directions, only a small part of the flow channel was simulated. In the case of simple homogeneous porous media, the computational results for flow resistance could be expressed by following Darcy's Law. Subsequently, these calculation results from the separator flow analysis were used in the housing and stack analysis. Second, the flow of the cathode region in the housing of SOFC stack was analyzed to verify the flow uniformity in the cathode channel of the separators. Finally, a stack analysis was executed using the electrochemical reaction model to investigate the performance and transport phenomena of the stack. Owing to the uniformity in flow and temperature, each SOFC cell exhibited similar contours of reactant gases, temperature, and current density. In the case of two different fuel utilizations with different flow rates, the low fuel utilization performed slightly better than the high fuel utilization.     

Combined effect of channel to rib width ratio and gas diffusion layer deformation on high temperature – Polymer electrolyte membrane fuel cell performance

The present study investigates the combined influence of Channel to Rib Width (CRW) ratio and clamping pressure on the structure and performance of High Temperature-Polymer Electrolyte Membrane Fuel Cell (HT-PEMFC) using a three-dimensional numerical model developed previously. It also considers the impact of interfacial contact resistance between the Gas Diffusion Layer (GDL) and Bipolar Plate (BPP). The structural analysis of the single straight channel HT-PEMFC geometry shows that the von-Mises stress greatly increases in the GDL under the ribs as the CRW ratio increases resulting in considerably high deformation. The cell performance analysis depicts the significance of ohmic resistance and concentration polarization for different CRW ratios, particularly at higher operating current densities. However, in low to medium current density regions, the CRW ratio has little influence on cell performance. A substantial impact on the species, overpotential, and current distributions is observed. The findings also reveal that the CRW ratio significantly affects the temperature distribution in the cell.     

Micro-cogeneration application of a high-temperature PEM fuel cell stack operated with polybenzimidazole based membranes

High temperature Proton Exchange Membrane Fuel Cells (HT-PEMFC) have attracted the attention of researchers in recent years due to their advantages such as working with reformed gases, easy heat management and compatibility with micro-cogeneration systems. In this study, it is aimed to designed, manufactured and tested of the HT-PEMFC stack based on Polybenzimidazole/Graphene Oxide (PBI/GO) composite membranes. The micro-cogeneration application of the PBI/GO composite membrane based stack was investigated using a reformat gas mixture containing Hydrogen/Carbon Dioxide/Carbon Monoxide (H₂/CO₂/CO). The prepared HT-PEMFC stack comprises 12 cells with 150 cm² active cell area. Thermo-oil based liquid cooling was used in the HT-PEMFC stack and cooling plates were used to prevent coolant leakage between the cells. As a result of HT-PEMFC performance studies, maximum 546 W and 468 W power were obtained from PBI/GO and PBI membranes based HT-PEMFC stacks respectively. The results demonstrate that introducing GO into the PBI membranes enhances the performance of HT-PEMFC technology and demonstrated the potential of the HT-PEMFC stack for use in micro-cogeneration applications. It is also underlined that the developed PBI/GO composite membranes have the potential as an alternative to commercially available PBI membranes in the future.     

Electric power generation by super-adiabatic combustion in thermoelectric porous element

A new system for converting combustion heat into electric power was proposed on the basis of reciprocating-flow super-adiabatic combustion in a catalytic and thermoelectric porous element. Self-sustaining combustion of an extremely low-calorific gas was successfully achieved in the element; because a reciprocating flow in the porous element recirculated energy, effectively regenerating combustion gas enthalpy into an enthalpy increase in the low-calorific gas. In the combustion system, a trapezoidal temperature distribution was established along the flow direction, resulting in a steep temperature gradient in the thermoelectric porous element. Numerical simulation showed that 94% of the combustion heat was transferred through the thermoelectric element by conduction. As a result, the total thermal efficiency, which was defined as the ratio of the electric power generated to the combustion heat, attained a value close to the conversion efficiency of the thermoelectric device itself.     

3D non-isothermal dynamic simulation of high temperature proton exchange membrane fuel cell in the start-up process

High temperature proton exchange membrane fuel cell (HT-PEMFC) with phosphoric acid doped polybenzimidazole (PBI) electrolyte shows multiple advantages over conventional PEMFC working at below 373 K, such as faster electrochemical kinetics, simpler water management, higher carbon monoxide tolerance. However, starting HT-PEMFC from room temperature to the optimal operating temperature range (433.15 K–453.15 K) is still a serious challenge. In present work, the start-up strategy is proposed and evaluated and a three-dimensional non-isothermal dynamic model is developed to investigate start-up time and temperature distribution during the start-up process. The HT-PEMFC is preheated by gas to 393.15 K, followed by discharging a current from the cell for electrochemical heat generation. Firstly, different current loads are applied when the average temperature of membrane reaches 393.15 K. Then, the start-up time and temperature distribution of co-flow and counter-flow are compared at different current loads. Finally, the effect of inlet velocity and temperature on the start-up process are explored in the case of counter-flow. Numerical results clearly show that applied current load is necessary to reduce start-up time and just 0.1 A/cm² current load can reduce startup time by 45%. It is also found that co-flow takes 18.8% less time than counter-flow to heat membrane temperature to 393.15 K, but the maximum temperature difference of membrane is 39% higher than the counter-flow. Increasing the inlet gas flow velocity and temperature can shorten the start-up time but increases the temperature difference of the membrane.     

Measurement of the temperature distribution in a large solid oxide fuel cell short stack

During the operation of solid oxide fuel cells (SOFCs), nonhomogeneous electrochemical reactions in both electrodes and boundary conditions may lead to a temperature gradient in the cell which may result in the development of thermal stresses causing the failure of the cell. Thus, in this study, effects of operating parameters (current density, flow configuration and cell size) on the temperature gradient of planar SOFCs are experimentally investigated. Two short stacks are fabricated using a small (16 cm² active area) and a large size (81 cm² active area) scandia alumina stabilized zirconia (ScAlSZ) based electrolyte supported cells fabricated via tape casting and screen printing routes and an experimental set up is devised to measure both the performance and the temperature distribution in short stacks. The temperature distribution is found to be uniform in the small short stack; however, a significant temperature gradient is measured in the large short stack. Temperature measurements in the large short stack show that the temperature close to inlet section is relatively higher than those of other locations for all cases due to the high concentrated fuel resulted in higher electrochemical reactions hence the generated heat. The operation current is found to significantly affect the temperature distribution in the anode gas channel. SEM analyses show the presence of small deformations on the anode surface of the large cell near to the inlet after high current operations.     

Influence of rib height on the local heat transfer distribution and pressure drop in a square channel with 90° continuous and 60° V-broken ribs

Internal channel cooling is employed in advanced gas turbines blade to allow high inlet temperatures so as to achieve high thrust/weight ratios and low specific fuel consumption. The objective of the present work is to study the effect of rib height to the hydraulic diameter ratio on the local heat transfer distributions in a double wall ribbed square channel with 90° continuous attached and 60° V-broken ribs. The effect of detachment of the rib in case of broken ribs on the heat transfer characteristics is also presented. Reynolds number based on duct hydraulic diameter is ranging from 10,000 to 30,000. A thin stainless steel foil of 0.05 mm thickness is used as heater and infrared thermography technique is used to obtain the local temperature distribution on the surface. The images are captured in the periodically fully developed region of the channel. It is observed that the heat transfer augmentations in the channel with 90° continuous attached ribs increase with increase in the rib height to hydraulic diameter ratio (e/D) but only at the cost of the pressure drop across the test section. The enhancements caused by 60° V-broken ribs are higher than those of 90° continuous attached ribs and also result in lower pressure drops. But, with an increase in the rib height, the enhancements are found to decrease in channel with broken ribs. The effect of detachment incase of broken ribs is not distinctly observed. The heat transfer characteristics degraded with increase in the rib height in both attached and detached broken ribbed cases.     

Performance analysis of HT-PEFC stacks

The performance analysis of a five-cell HT-PEFC stack is presented. The stack was operated either with pure hydrogen or synthetic reformate on the anode side and air on the cathode side. The overall electric performance and the heat management were analyzed. The local performance was assessed by current density and temperature distribution measurements. For this purpose, a tailor-made measuring board was integrated into the stack assembly. It is shown how the choice of fuel gas composition, reactant stoichiometry, flow direction and cooling affect the current density and temperature distribution.     

Experimental investigation of CO tolerance in high temperature PEM fuel cells

In the present work, the effect of operating a high temperature proton exchange membrane fuel cell (HT-PEMFC) with different reactant gases has been investigated throughout performance tests. Also, the effects of temperature on the performance of a HT-PEMFC were analyzed at varying temperatures, ranging from 140 °C to 200 °C. Increasing the operating temperature of the cell increases the performance of the HT-PEMFC. The optimum operating temperature was determined to be 160 °C due to the deformations occurring in the cell components at high working temperatures. To investigate the effects of CO on the performance of HT-PEMFC, the CO concentration ranged from 1 to 5 vol %. The current density at 0.6 V decreases from 0.33 A/cm² for H₂ to 0.31 A/cm² for H₂ containing 1 vol % CO, to 0.29 A/cm² for 3 vol % CO, and 0.25 A/cm² for 5 vol % CO, respectively. The experimental results show that the presence of 25 vol % CO₂ or N₂ has only a dilution effect and therefore, there is a minor impact on the HT-PEMFC performance. However, the addition of CO to H₂/N₂ or H₂/CO₂ mixtures increased the performance loss. After long-term performance test for 500 h, the observed voltage drop at constant current density was obtained as ～14.8% for H₂/CO₂/CO (75/22/3) mixture. The overall results suggest that the anode side gas mixture with up to 5 vol % CO can be supplied to the HT-PEMFC stack directly from the reformer.     

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.     

基于容阻建模技术的SOFC电堆数值模拟

借助MATLAB软件，通过建模对固体氧化物燃料电池堆的运行进行仿真。为了计算的方便，模型采用了容阻建模技术。采用由质量守恒、能量守恒、动量守恒和电化学过程组成的模型，对SOFC电堆通道中的气体组成、温度分布、电压分布和电流密度分布进行了计算分析。这些分析结果可对电堆的设计提供参考。     

There is a mistake in Chen's model published in volume 128

The performance of PEM fuel cells depends significantly on an appropriate flow field design. In this work, an investigation on the influence of channel geometries on stack performance has been carried out. A parallel flow field design was developed to widely eliminate the influence of other flow field design parameters, e.g. degree of parallelization and gas crossover effects at the turning points of the gas channels. Starting from a basic design, channel as well as rib dimensions were varied and their influence on stack performance were studied. An optimum between 0.7 and 1 mm was found for either channel or for rib widths. For wider dimensions, the influence on mass transport (ribs) or lateral conductivity (channels) has been found to become significant. For very small dimensions, the manufacturing effort becomes unacceptably high (ribs) and the probability of channel clogging by formation of water droplets increases. In general, narrow channel dimensions are preferred for high current densities, whereas wider dimensions are better at low current densities.     

On the current distribution at the channel – rib scale in polymer-electrolyte fuel cells

Experimental results based on in-situ measurements at the interface between the catalyst layer and the gas diffusion layer (GDL) on the cathode side at the channel – rib scale show an interesting variation of the current density distribution as the mean current density is increased. It is found that the local current density below the rib median axis corresponds to a maximum at low to intermediate mean current densities and to a minimum when the mean current density is sufficiently high. Also, the higher is the current density, the more marked the minimum. From numerical simulations, it is shown that the current density distribution inversion phenomenon is strongly correlated to the liquid water zone development within the GDL.