Dynamical modeling of water transport in polymer electrolyte membrane fuel cell (PEMFC) design

A two-dimensional finite element computational fluid dynamics (CFD) model, including coupled partial differential equations of mass, momentum and charge conservation inside a membrane electrode assembly of a polymer electrolyte membrane fuel cell (PEMFC) are developed. The CFD model is solved for PEMFCs with conventional and interdigitated gas flow fields. For the PEMFC with interdigitated flow fields both coflow and counterflow designs are studied. Furthermore a dynamic lumped model based on the formulation of Pukrushpan et al. (2003) is developed with the addition of membrane's transient water transport. Models are validated by comparing the polarization curves with the experimental data of Ticianelli et al. (1988) for MEAs with conventional gas distributors and He et al. (2000) for MEAs with counterflow interdigitated gas distributors. The results of the lumped model and the CFD model for conventional design are shown to be comparable and lumped model proves to be a good substitute of CFD model for control studies. For the interdigitated case, coflow is found to be superior to counterflow in the performance of the cell. Transient and steady-state responses of the fuel cell system to changes in cell voltage, air pressure and relative humidity of air are investigated for each design. The effect of transient water transport is emphasized and it is observed that it plays a critical role in the operation of a PEMFC for both designs

Multiphysics modeling of fuel cells

Fuel cells are expected to resist permanent changes in performance over time, to tolerate unexpected changes in the ambient conditions for a stable operation, and to sustain a structural integrity under different operating conditions. However, during the operation, both solid oxide fuel cells (SOFC) and polymer electrolyte fuel cells (PEFC) are prone to many hazards that may cause degradation of the performance even to the extent of complete failure of these devices. ^ In this study performance and degradation of SOFCs and PEFCs is studied. A computational modeling framework has been established to investigate the transport phenomena and the electrochemical performance as well as the mechanical behavior of SOFCs and PEFCs. ^ The electrochemical performance of the SOFC is investigated both in steady-state and transient operations while elucidating the transport phenomena related to the fuel cell operation. The proposed computational framework for the SOFC comprises two separate models for the test furnace and the single cell in order to more accurately model the actual test system while decreasing the computational cost. The fuel cell performance in transient operation is also studied. The performance of the SOFC is investigated in case of a failure in the fuel supply system. Mechanical behavior of the SOFC is also considered to help assessing the durability of the cells. ^ The same modeling framework is utilized for the PEFCs to investigate electrochemical and mechanical degradation during the fuel cell operation. To assess the performance degradation as a result of gas contamination, a cation transport model is presented. It is found that the effect of fuel side contamination of cationic species is much more significant than the air side contamination while there still is a significant performance degradation associated with the latter. ^ Further, the stresses induced during the PEFC operation due to the swelling and shrinkage of the membrane with hydration changes are investigated. The impact of the anisotropy in the gas diffusion layers on the mechanical stresses is investigated and found to have a significant effect on the stress distribution in the membrane.

Multiphysics modeling of fuel cells

Fuel cells are expected to resist permanent changes in performance over time, to tolerate unexpected changes in the ambient conditions for a stable operation, and to sustain a structural integrity under different operating conditions. However, during the operation, both solid oxide fuel cells (SOFC) and polymer electrolyte fuel cells (PEFC) are prone to many hazards that may cause degradation of the performance even to the extent of complete failure of these devices. ^ In this study performance and degradation of SOFCs and PEFCs is studied. A computational modeling framework has been established to investigate the transport phenomena and the electrochemical performance as well as the mechanical behavior of SOFCs and PEFCs. ^ The electrochemical performance of the SOFC is investigated both in steady-state and transient operations while elucidating the transport phenomena related to the fuel cell operation. The proposed computational framework for the SOFC comprises two separate models for the test furnace and the single cell in order to more accurately model the actual test system while decreasing the computational cost. The fuel cell performance in transient operation is also studied. The performance of the SOFC is investigated in case of a failure in the fuel supply system. Mechanical behavior of the SOFC is also considered to help assessing the durability of the cells. ^ The same modeling framework is utilized for the PEFCs to investigate electrochemical and mechanical degradation during the fuel cell operation. To assess the performance degradation as a result of gas contamination, a cation transport model is presented. It is found that the effect of fuel side contamination of cationic species is much more significant than the air side contamination while there still is a significant performance degradation associated with the latter. ^ Further, the stresses induced during the PEFC operation due to the swelling and shrinkage of the membrane with hydration changes are investigated. The impact of the anisotropy in the gas diffusion layers on the mechanical stresses is investigated and found to have a significant effect on the stress distribution in the membrane.

Local entropy generation and exergy analysis of the condenser in a direct methanol fuel cell system

This paper aims to identify the irreversibilities in the condenser of a direct methanol fuel cell (DMFC) system and present possible enhancements in its design through local entropy generation analysis (L-EGA). For this purpose, the local entropy generation terms originating from heat and mass calculated from results of a pseudo two-phase computational fluid dynamic (CFD) model of the condenser. Through this analysis, the total irreversibilities due to heat and mass transfer are calculated locally (e.g., film boundary layer, vapourgas boundary layer) under the variable operating conditions of a DMFC (undersaturated, saturated, and supersaturated conditions of the cathode exhaust gas). Moreover, the exergy destruction ratio of condenser is found to estimate the exergy performance of the condenser. The results show that in the case of supersaturated cathode exhaust gas (CEG) flow, the entropy generation rate due to mass transfer in the film region is found as 0.032 W/(m center dot K) which is 18 times higher than that for the undersaturated CEG flow. However, entropy generation rate due to mass transfer decreases significantly when the hot flow is just over the film region. In the film region, the entropy generation rates originating from heat transfer are found as 0.0055 W/(m center dot K) (for the undersaturated case), 0.0032 W/ (m center dot K) (for the saturated case), and 0.0015 W/(m center dot K) (for the supersaturated case). Moreover, the maximum exergy destruction ratio is found as 0.72 when the CEG is undersaturated and the CEG velocity is 0.18 m/s, while the lowest exergy destruction ratio is calculated as 0.28 when the CEG is saturated.(c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved

A pseudo two-phase model to study effects of non-condensable gases on the water autonomy of a direct methanol fuel cell system

In a direct methanol fuel cell (DMFC) system, water at the condenser outlet is recirculated into the mixing chamber where it mixes with pure methanol and anode outlet stream. The selection of condenser design and operating conditions plays a key role in the successful operation of the system with water autonomy. In this study, a novel pseudo two-phase computational fluid dynamics (CFD) model is proposed to investigate the condensation capability of the condenser under the operating conditions of a DMFC system. The condenser model is simplified by implementing an iterative multi-domain approach where continuity at the decoupled domain interfaces is satisfied by the convergence of boundary conditions. The proposed thermofluids model is validated with experimental data. Through the model, the performance of the condenser is investigated for DMFC operating conditions. The results show that the amount of non-condensable gases, the velocity and the saturation level of cathode exhaust gas (CEG) are the key parameters in these scenarios. The highest condensation effectiveness is calculated for the saturated CEG flow scenario with lower velocity and lower content of non-condensable gases, while the lowest condensation effectiveness is calculated as for the under saturated CEG flow scenario. Another important result is that the increment of the amount of the non-condensable gases throughout the downstream distance is found as 4% which results in the decrement of the saturation and film temperature by 8 K and 4 K, respectively. In addition, it is found that when the inlet velocity of CEG decreases from 0.82 m.s(-1) to 0.18 m.s(-1), the overall condensation rate decreases by 55%. (C) 2020 Elsevier Ltd. All rights reserved

TECHNO-ECONOMIC PERFORMANCE ASSESSMENT OF A REACTOR SYSTEM USED FOR POWER-TO-METHANE PLANT

The interest in methane production from the utilization of electricity and hydrogen, also well-known as power-to-methane (PtM), has increased significantly in the last decades. A typical PtM system mainly consists of an electrolyzer (for hydrogen production) that is powered by a renewable power production technology, such as a solar PV module, a CO2 source and a chemical reactor for methane production. In this study, the sizing of a PtM system is first performed based on the operating strategy that relies on the avoidance of hydrogen storage and selection of appropriate hydrogen content of syngas and catalyst loading in the reactor. Then, thermodynamic analysis is performed for this system. Finally, cost analysis is conducted for new reactor installation considering sizing and thermodynamic performance of the system. The combined simulation shows that the electrical efficiency is found as 50-55 % according to various operating conditions. The syngas production rate decreases even though the methane content of syngas increases when the operating pressure of the reactor decreases. Moreover, the total investment cost decreases from 4,354,200 $to 3,470,200$ when the reactor pressure increase from 1 bar to 7 bar