Thermal stress and thermo-electrochemical analysis of a planar anode-supported solid oxide fuel cell: Effects of anode porosity

A 3D integrated numerical model is constructed to evaluate the thermal-fluid behavior and thermal stress characteristics of a planar anode-supported solid oxide fuel cell (SOFC). Effects of anode porosity on performance, temperature gradient and thermal stress are investigated. Using commercial Star-CD software with the es-sofc module, simulations are performed to obtain the current-voltage (I-V) characteristics of a fuel cell as a function of the anode porosity and the temperature distribution within the fuel cell under various operating conditions. The temperature field is then imported into the MARC finite element analysis (FEA) program to analyze thermal stresses induced within the cell. The numerical results are found to be in good agreement with the experimental data. It is shown that the maximum principal stress within the positive electrode-electrolyte-negative electrode (PEN) increases at a higher current and a higher temperature gradient. It is recommended that the temperature gradient should be limited to less than 10.6 °C mm⁻¹ to maintain the structural integrity of the PEN.     

On the source terms of species equations in fuel cell modeling

By checking the unit of the source terms, it is found that the source terms used in some fuel cell modeling studies are incorrect. The correct source terms of species equations are proposed in this communication.     

Multi-scale design simulation of a novel intermediate-temperature micro solid oxide fuel cell stack system

This paper presents a multi-scale simulation technique for designing a novel intermediate-temperature planar-type micro solid oxide fuel cell (mSOFC) stack system. This multi-scale technique integrates the fundamentals of molecular dynamics (MD) and computational fluid dynamics (CFD). MD simulations are carried out to determine the optimal composition of a potential electrolyte that is capable of operation in the intermediate-temperature region without sacrifice in performance. A commercial CFD package plus a self-written computational electrochemistry code are employed to design the fuel and air flow systems in a planar five-cell stack, including the preheating manifold. Different samarium-doped ceria (SDC) electrolyte compositions and operating temperatures from 673 K to 1023 K are investigated to identify the maximum ionic conductivity. The electrochemical performance simulation using an available 5-cell yittria-stablized-zirconia (YSZ) mSOFC stack shows good agreement with our experimental results. The same stack design is used to predict a novel SDC-mSOFC performance. Feasibiulity studies of this intermediate-temperature stack are presented using this multi-scale technique.     

Model-based development of low-level control strategies for transient operation of solid oxide fuel cell systems

The exploitation of an SOFC-system model to define and test control and energy management strategies is presented. Such a work is motivated by the increasing interest paid to SOFC technology by industries and governments due to its highly appealing potentialities in terms of energy savings, fuel flexibility, cogeneration, low-pollution and low-noise operation.The core part of the model is the SOFC stack, surrounded by a number of auxiliary devices, i.e. air compressor, regulating pressure valves, heat exchangers, pre-reformer and post-burner. Due to the slow thermal dynamics of SOFCs, a set of three lumped-capacity models describes the dynamic response of fuel cell and heat exchangers to any operation change.The dynamic model was used to develop low-level control strategies aimed at guaranteeing targeted performance while keeping stack temperature derivative within safe limits to reduce stack degradation due to thermal stresses. Control strategies for both cold-start and warmed-up operations were implemented by combining feedforward and feedback approaches. Particularly, the main cold-start control action relies on the precise regulation of methane flow towards anode and post-burner via by-pass valves; this strategy is combined with a cathode air-flow adjustment to have a tight control of both stack temperature gradient and warm-up time. Results are presented to show the potentialities of the proposed model-based approach to: (i) serve as a support to control strategies development and (ii) solve the trade-off between fast SOFC cold-start and avoidance of thermal-stress caused damages.     

Spatially smoothed fuel cell models: Variability of dependent variables underneath flow fields

Spatial smoothing can be applied to reduce a three-dimensional (3D) fuel cell model to a computationally cost-efficient two-dimensional (2D) counterpart. In this study, the averaged dependent variables in the streamwise direction are augmented with a statistical measure of their distribution in the spanwise direction in the form of standard deviations. A functional form of the latter is postulated and justified a posteriori by comparing the predictions from the 2D spatially-smoothed model with the original 3D counterpart: good agreement is found. This measure of the variability thus complements the average predictions of existing spatially smoothed models.     

A hierarchical modeling approach to the simulation and control of planar solid oxide fuel cells

The main aim of the paper is to propose hierarchical modeling as a suitable methodology to perform control-oriented analysis of planar solid oxide fuel cells (P-SOFC).     

A CFD model with semi-empirical electrochemical relationships to study the influence of geometric and operating parameters on DMFC performance

A three-dimensional computational fluid dynamics (CFD) model is developed to investigate the influence of geometric and operating parameters on performance of a direct methanol fuel cell (DMFC). Semi-empirical relationships are introduced to describe the electrochemical behaviors required in the CFD governing equations. Coefficients in these semi-empirical relationships are fitted using experimental data. Two geometric configurations with serpentine channels at the anode and cathode are considered in this work. Temperature, methanol concentration, and methanol flow rate are selected as the operating parameters. Due to the computational effort of CFD, an adaptive metamodeling method is developed to reduce the number of data-fitting iterations for obtaining the coefficients in the semi-empirical relationships. The effectiveness of the method is demonstrated by fitting the model using the experimental data collected from the first geometric configuration of the DMFC and comparing the predicted performance of the second configuration with its experimental performance. A commercial CFD system, Fluent 12.0, was used in this research.     

Performance evaluation of an air breathing polymer electrolyte membrane (PEM) fuel cell in harsh environments – A case study under Saudi Arabia's ambient condition

A key parameter that determines the efficiency of proton exchange membrane fuel cells is their operating conditions. Optimization of various components in these fuel cells is pivotal in improving cell performance, as their performance is directly related to the operational conditions the cells are subjected to.This investigation examined the viability of an air breathing fuel cell subjected to ambient conditions in Riyadh in Saudi Arabia. A validated three-dimensional air breathing 5-cell stack, modelled in ANSYS was utilised to generate the results. Furthermore, the work also considered the feasibility of deploying a humidifier unit for the hydrogen inlet, so as to ascertain the physical behaviour of the PEMFC stack. It was observed that the performance of the stack reaches its peak during the summer time (June–August), and hydrogen humidification improves output performance by 40%.     

The importance of fuel variability on the performance of solid oxide cells operating on H2/CO2 mixtures from biohydrogen processes

Biologically produced mixtures of H₂ and CO₂ (biohydrogen) from processes such as dark fermentation or photo-fermentation are versatile feedstocks which can potentially be utilised in solid oxide cell (SOC) devices. In this work, solid oxide electrolysis of biohydrogen has been investigated for the first time and is compared directly with fuel cell mode utilisation. The performance and fuel processing of SOCs utilising biohydrogen have been characterised in greater detail than has been achieved previously through the use of experiments which combine electrochemical techniques with quadrupole mass spectrometry (QMS). The effects of fuel variability on SOC overpotentials and outputs have been established and it is shown that cell performance is not significantly affected provided the fuel composition stays within 40–60 vol% H₂. QMS measurements indicate H₂O and CO production takes place in-situ via the reverse water-gas shift (RWGS) reaction. Electrical power production in fuel cell mode is predominantly through H₂ oxidation, whilst CO is converted in the WGS reaction to regenerate CO₂ but does not contribute to electrical power production. In electrolysis mode, CO is produced simultaneously through electrochemical CO₂ reduction and the RWGS reaction; H₂O is electrochemically reduced to regenerate H₂.     

Fabrication and characterization of functionally-graded LSCF cathodes by tape casting

Functionally graded cathodes for solid oxide fuel cells are prepared using a tape casting process. The microstructures of the cathodes are gradually changed from a finer LSCF layer with smaller grains (to increase the number of active sites for oxygen reduction) to a coarser LSCF layer with larger grains and higher porosity (for efficient current collection and fast gas transportation). The microstructure and electrochemical properties of the porous electrodes are characterized using scanning electron microscopy and electrochemical impedance spectroscopy, respectively. The cathodic polarization resistance of test cells with functionally-graded LSCF cathodes fired at 1050 °C is reduced to 0.075 Ω cm² at 700 °C and 0.036 Ω cm² at 750 °C, demonstrating peak power densities of 371.5, 744.6, and 1075.3 mW/cm² at 700, 750, and 800 °C, respectively.