Two-phase Non-isothermal Model Research Articles

Investigation of Water and Heat Transfer Mechanism in PEMFCs Based on a Two-Phase Non-Isothermal Model

In a proton exchange membrane fuel cell (PEMFC) system, proper management of water and heat transport is essential to improve its overall performance and durability. To comprehensively investigate the internal processes of PEMFCs, an improved two-phase non-isothermal model based on heat and water transfer mechanisms inside the fuel cell is developed. The results show that the model proposed in this work can predict the fuel cell’s performance accurately and is capable of exploring water and heat transfer phenomena inside fuel cells. Additionally, the water and heat transfer of cathodes and anodes under different relative humidity and temperatures are studied. It can be concluded that when the PEMFC operates under a constant voltage, the anode water content gradually increases, while the cathode water content gradually decreases. The maximum water content occurs at the interface between cathode catalyst layer and cathode gas diffusion layer, while the minimum value is attained at the interface between anode catalyst layer and anode gas diffusion layer. When the fuel cell operates at 0.75 V, although the water content of CCL is the highest, no back-diffusion of dissolved water occurs.

Improving round-trip energy efficiency of a unitized regenerative fuel cell by adopting staircase flow channel and counter flow configuration

The flow field design is an effective measure to improve hydrothermal management within a fuel cell. This paper proposes a staircase flow channel used in the oxygen-side flow channel of a unitized regenerative fuel cell. Based on a two-dimensional two-phase non-isothermal model, the hydrothermal distribution variation induced by staircase channels with different step numbers is investigated in detail. Results show that oxygen transport is boosted and liquid water accumulation is reduced in the fuel cell mode with the use of staircase structures, thus local oxygen starvation under high current density is alleviated. In addition, the temperature uniformity of electrodes is effectively improved, and the fuel cell net power is enhanced by up to 14.24% at 0.2 V with a five-step flow channel. In electrolytic cell mode, however, the advantage of downward staircase structures is not prominent. Therefore, a counter-flow method is suggested during the mode switching stage, which can improve average current density by up to 10.83%. Besides, round-trip energy efficiency is introduced to evaluate the comprehensive performance. The staircase flow channel coupled with counter-flow method improves the efficiency of unitized regenerative fuel cell by up to 49.63% under high current density, compared with conventional flow channels with co-flow configuration.

Improvement of thermal and water management of air-cooled polymer electrolyte membrane fuel cells by adding porous media into the cathode gas channel

In air-cooled polymer electrolyte membrane fuel cells (PEMFCs), the contradiction between cooling and water retention renders the improvement of cell performance really challenging. In the present study, cathode flow field is designed by inserting porous media into the gas channel to properly distribute flow rate between gas channel and cooling channel. A three dimensional two-phase non-isothermal model of PEMFCs is adopted to investigate the reactive transport processes in air-cooled PEMFCs. The results show that by inserting porous media into the outlet of the gas channel, the current density can be improved by 18.2% compared with the conventional parallel flow field. Then, effects of permeability of the porous media and the cathode outlet set-point temperature on the performance of PEMFC are examined, which both show a nonmonotonic influence on fuel cell performance. The reason can be attributed to the complex trade-off between cooling, water retention and reactant supply. Finally, effects of the altitude are explored, and it is found that the proposed novel design also shows advantage in alleviating the performance reduction due to the increase of altitude. The proposed design is proved to be a promising scheme to alleviate the contradiction between cooling and water retention in air-cooled PEMFCs.

Hydration and dehydration cycles in polymer electrolyte fuel cells operated with wet anode and dry cathode feed: A neutron imaging and modeling study

Abstract Proper water management plays an essential role in the performance and durability of Polymer Electrolyte Fuel Cells (PEFCs), but it is challenged by the variety of water transport phenomena that take place in these devices. Previous experimental work has shown the existence of fluctuations between low and high current density levels in PEFCs operated with wet hydrogen and dry air feed. The alternation between both performance states is accompanied by strong changes in the high frequency resistance, suggesting a cyclic hydration and dehydration of the membrane. This peculiar scenario is examined here considering liquid water distributions from neutron imaging and predictions from a 3D two-phase non-isothermal model. The results show that the hydration-dehydration cycles are triggered by the periodic condensation and shedding of liquid water at the anode inlet. The input of liquid water humidifies the anode channel and offsets the membrane dry-out induced by the dry air stream, thus leading to the high-performance state. When liquid water is flushed out of the anode channel, the dehydration process takes over, and the cell comes back to the low-performance state. The predicted amplitude of the current oscillations grows with decreasing hydrogen and increasing air flow rates, in agreement with previous experimental data.

Transient Phase Change in the Cathode Side of a PEM Fuel Cell

We numerically investigated the temporal variation and spatial distribution of interfacial mass-transfer phase-change rate in the cathode side gas diffusion layer of a proton exchange membrane (PEM) fuel cell during startup. For this purpose, a three-dimensional transient two-phase nonisothermal model was used. A nonequilibrium evaporation–condensation interfacial mass-transfer rate is incorporated in the model, which enables us to take supersaturation and subsaturation into consideration. This helps us to investigate the most significant contribution of the phase-change rate to the transient response and thermal behavior of the cell. The effects of the operating temperature and channel inlet humidity on the phase-change rate are also investigated. It is observed that when condensation is dominant, the transient time decreases. It is also observed that the maximum temperature decreases with time due to vapor-phase diffusion and phase change.

Phase change in the cathode side of a proton exchange membrane fuel cell

A three-dimensional steady state two-phase non-isothermal model which highly couples the water and thermal management has been developed to numerically investigate the spatial distribution of the interfacial mass transfer phase-change rate in the cathode side of a proton exchange membrane fuel cell (PEMFC). A non-equilibrium evaporation–condensation phase change rate was incorporated in the model which allowed supersaturation and undersaturation take place. The most significant effects of phase-change rate on liquid saturation and temperature distributions are highlighted. A parametric study was also carried out to investigate the effects of operating conditions; namely as the channel inlet humidity, cell operating temperature, and inlet mass flow rate on the phase-change rate. It was also found that liquid phase assumption for produced water in the cathode catalyst layer (CL) changed the local distribution of phase-change rate. The maximum evaporation rate zone (above the channel near the CL) coincided with the maximum temperature zone and resulted in lowering the liquid saturation level. Furthermore, reduction of the channel inlet humidity and an increase of the operation temperature and inlet mass flow rate increased the evaporation rate and allowed for dehydration process of the gas diffusion layer (GDL) to take place faster.

Non-isothermal modeling of direct methanol fuel cell

In this work, a two-dimensional, two-phase non-isothermal model is developed for DMFC. The natural convection heat transfer at the out surface of the current collector is considered as the thermal boundary conditions to obtain a more realistic simulation of the DMFC working conditions. The heat and mass transfer, along with the electrochemical reactions occurring in the DMFC are modeled and numerically solved by a self-developed simulation code. The numerical results show that cell performance is enhanced with the increase in the inlet temperature. The distribution of temperature in the DMFC mainly depends on the inlet temperature of the dilute methanol aqueous in the anode side. The mean temperature of MEA and temperature difference in MEA increase with the increase in current density and the profiles show the same trend. With the decrease in MEA thermal conductivity and the increase in the inlet temperature the temperature difference in MEA becomes larger.

C214 Dynamic model for a GDL of a polymer electrolyte fuel cell water and thermal management

Temporal variation and spatial distribution of interfacial mass-transfer rate in the cathode side gas diffusion layer (GDL) of a proton exchange membrane fuel cell during start-up were numerically investigated. For this purpose, a 3D transient two-phase nonisothermal model was used. A nonequilibrium evaporation-condensation interfacial mass-transfer rate is incorporated in the model, which enables us to take supersaturation and subsaturation into consideration. This helps us to investigate the most significant contribution of the phase-change rate to the transient response and thermal behavior of the cell.

Analyses of heat and water transport interactions in a proton exchange membrane fuel cell

A two-phase non-isothermal model is developed to explore the interaction between heat and water transport phenomena in a PEM fuel cell. The numerical model is a two-dimensional simulation of the two-phase flow using multiphase mixture formulation in a single-domain approach. For this purpose, a comparison between non-isothermal and isothermal fuel cell models for inlet oxidant streams at different humidity levels is made. Numerical results reveal that the temperature distribution would affect the water transport through liquid saturation amount generated and its location, where at the voltage of 0.55 V, the maximum temperature difference is 3.7 °C. At low relative humidity of cathode, the average liquid saturation is higher and the liquid free space is smaller for the isothermal compared with the non-isothermal model. When the inlet cathode is fully humidified, the phase change will appear at the full face of cathode GDL layer, whereas the maximum liquid saturation is higher for the isothermal model. Also, heat release due to condensation of water vapor and vapor-phase diffusion which provide a mechanism for heat removal from the cell, affect the temperature distribution. Instead in the two-phase zone, water transport via vapor-phase diffusion due to the temperature gradient. The results are in good agreement with the previous theoretical works done, and validated by the available experimental data.

Analysis of a Two-Phase Non-Isothermal Model for a PEFC

A non-isothermal, two-phase model for a polymer electrolyte fuel cell (PEFC) is presented, analyzed, and solved numerically under three different thermal, and two hydrodynamic, modeling assumptions; the consequences of these are then discussed in terms of thermal and water management and cell performance. The study is motivated by recent experimental results that suggest the presence of previously unreported, and thus unmodeled, thermal contact resistances between the components of PEFCs and the discrepancy in the value for the capillary pressure that is used by different authors when modeling the two-phase flow in PEFCs. For the three different thermal assumptions (assuming effective heat conductivities, isothermal flow, and interfacial and bulk conductivites), liquid saturations of around 10% are obtained at the cathode active layer for and a cell voltage of 0.6 V. When lowering the capillary pressure (hydrodynamic assumption), liquid saturations of almost 30% and locally up to 100% are observed at the active layer of the cathode. At this current density and voltage, temperature differences across the cell of around 9°C are predicted. In addition, the effect of varying clamping pressure within the framework of the model is touched upon. The benefits of the scaling analysis conducted here, to predict correctly, prior to numerical computations, important characteristic cell performance quantities such as current density and temperature drop are also highlighted.