Experimental and modeling investigations on local performance and local degradation in solid oxide fuel cells

Zacharie Wuillemin
EPFL, 2009
Thèse EPFL

Résumé

This thesis focuses on performance, reliability and degradation in solid oxide fuel cells (SOFC), which currently represent the three key technical challenges for the deployment of this technology. By its development of dedicated modeling and experimental tools, this thesis offers a new access to, and comprehension of, local electrochemical performance, degradation and reliability. The experimental tools developed have, for the first time, allowed in-situ measurements of local performance degradation with time, and this in a real SOFC prototype. Giving access to local electrochemistry on 18 measurement points in a repeat-element of 200 cm2 active area, the spatial distribution of the electrochemical reaction, as well as its degradation with time, could be monitored and analyzed. The intrinsically local character of degradation could clearly be revealed, a point of central importance for future investigations on stack degradation and for the interpretation of post-experiment analyses. Using impedance spectroscopy, it was possible to identify the affected electrochemical processes and to study the spatial distribution of their degradation. The result was put in relation with post-experiment analyses, allowing to identify pollutants on the air side as major source of degradation. To understand the highly coupled phenomena leading to performance, degradation and reliability issues, a 3D computational fluid dynamic model (CFD) was developed. Based on the key idea to include the non-ideal properties of the used components and materials in the model, it was possible to obtain an excellent match between experimental observations and modeling outputs. Besides the identification of performance limitations, one result of crucial importance was in addition obtained by the identification of the principal cause of failure for the prototypes tested in the laboratory. The model revealed the presence of detrimental local redox-cycling of the cells upon changes of the operating point, as a result from an inadequate combination of slightly porous seal materials and certain aspects of the stack construction. This analysis, validated by experimental observations, led to solutions permitting an important gain in efficiency and reliability. Based on the identified and analyzed performance limitation, degradation and failure sources obtained from the tools developed in this thesis, two stack prototypes were successively designed, manufactured and tested in collaboration with the industrial partner HTceramix-SOFCpower. Starting from a predecessor design limited at 250 Wel and an electrical efficiency (LHV) inferior to 40%, the first of the designed prototypes attained a power output of 1.1 kWel (72 cells of 50 cm2), as well as a maximal efficiency of 53% in short stack configuration. The second designed stack, which represents the major achievement of this thesis, had, at the time of writing, reached a power output of 1.84 kWel and a maximal efficiency of 53% in a 20-cell stack configuration (200 cm2 cells). Both results were obtained using dilute hydrogen as fuel; in other words, future operation on reformed natural gas should lead to an electrical efficiency exceeding 60% (LHV). With the successful resolution of the main failure source and a demonstrated gain in efficiency, the chosen design iterations confirmed the predicting capabilities and the accuracy of the CFD model for a design towards the mandatory reliability and the high performance expected from SOFC stacks. Finally, the degradation issue, which was found to be strongly correlated with pollution from different sources, was addressed in a prospective study showing the capability of the CFD model to predict the internal generation, transport and deposition of pollutants inside of a stack. The good match obtained with experimental observations supports therefore the development of such types of models, both for model-based diagnostics and for future design iterations.

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https://infoscience.epfl.ch/record/140999?ln=fr

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Zacharie Wuillemin
EPFL, 2009
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/140999?ln=fr

À propos de ce résultat

Concepts associés (30)

Concepts associés

Chargement

Publications associées (24)

Publications associées

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Solid oxide fuel cell stack simulation and optimization, including experimental validation and transient behavior

Diego Larrain

This thesis presents the development of models for the simulation and optimization of the design of a planar solid oxide fuel cell (SOFC) stack. Fuel cells produce electric power directly from a fuel by electrochemical reactions. The high efficiencies demonstrated make them a promising technology for energy conversion. The main challenges lie with reliability and cost reduction. Some applications demand at the same time strong requirements on the compactness of the system and its ability to load following. The models have been developed to represent the novel stack proposed by HTceramix SA (Yverdon, Switzerland) which is tested and partly developed at the Laboratoire d'Energétique Industrielle (LENI). The model has been created in a way which allows its use for design optimization: this requires detailed and validated outputs to gain insight in the behavior of a new stack design and computational efficiency to allow sensitivity studies and optimization. Electrochemistry, mass and heat transfer phenomena are combined with a 2D fluid motion description to obtain a generalized model which can be applied to a large range of geometries. An efficient stack modeling approach is proposed. Validation of the model has been carried out with measurements and a 3D computational fluid dynamics model. A methodology based on parameter estimation has been used to identify kinetic parameters and other uncertain parameters. Local temperature measurements and a local current density measurement have been performed and also used for model validation. The 2D model has been successfully validated showing good agreement with both the experiments and the detailed 3D model. Simulation of the novel stack geometry (counter-flow) has allowed to identify the main problems arising from this compact geometry where the non-homogeneous velocity field creates stagnant zones which limit the operation at high efficiency. The simulated temperatures are characterized by important gradients and excessive level values (>850°C) for an intermediate temperature SOFC (700-800°C). This motivated to work on an alternative geometry, which based on simulation results, solves most of the problems previously identified. The thesis presents several examples of the influence of design on the system performance and reliability. Transient simulations have been performed and the design choice had only a small impact on the transient behavior which presents intrinsically an important thermal inertia. On the contrary, degradation behavior is dependent on the design. Stack degradation has been simulated by including the metal interconnect degradation into the stack model. The approach has allowed to identify a new criterion to express degradation consistently for different test conditions. To assist stack design, new approaches are necessary. The geometry of a stack was initially determined by a number of decision variables (such as cell area, thickness of the channels and interconnects) on which extensive sensitivity analysis were conducted. This method is of limited use as each of the objectives on stack design led to different solutions. To overcome this limitation, multi-objective optimization has been applied to the stack design problem. Application of this method is new in this field and different optimization strategies are tested. The results from the optimization allow to identify a clear trade-off between the compactness of the stack and the temperature level (and therefore the degradation).

EPFL2005

Chargement

The solid oxide fuel cell (SOFC) is a direct energy conversion device, which allows the production of electricity with high efficiency while maintaining pollutant emissions at a low level. It offers better fuel flexibility and lower vulnerability to impurities in the oxidising and fuel gases than other fuel cells. The lack of reliability and durability of SOFC devices currently impedes their large-scale commercialisation. The mitigation of mechanical and electrochemical degradation that together limit the lifetime, requires the precise understanding of the phenomena. The underlying processes act and interact at different spatial and temporal scales, which complicates characterisation and, consequently, the identification of the dominant contributions to the observed overall degradation in field conditions. This thesis has developed a modelling framework to investigate electrochemical and mechanical reliability and durability issues in planar, intermediate-temperature SOFC stacks based on anode-supported cells. The approach consisted in coupling thermo-electrochemical and thermo-mechanical continuum models, spanning from the electrode micro-scale to the stack macro-scale, to include in the analysis the detrimental interactions between the different phenomena that provoke failure after combined prolonged operation and cycling conditions. To achieve this aim, the existing cleavages between the research fields had to be bridged. At each sub-scale and for each aspect of interest, data was first gathered to identify the needed and achievable level of complexity of the description, and to calibrate the models using parameter estimation. Then, specific studies to understand the key dependences on the local conditions and limitations of the proposed approaches were carried out. Finally, the sub-models were implemented together in SRU/stack models to capture the multi-factorial and progressive nature of immediate or delayed failures in SOFCs. The implementation of a calibrated electrochemical model with degradation phenomena in SOFC stack models shed light on the micro- and macro-scale interactions that cause the progressive activation of the electrochemical degradation phenomena. Dynamic optimisation identified the critical decision variables, in terms of operating conditions, whereas disparities in the electrochemical and mechanical properties of the materials, stack design and constraints from the system were further included, to propose case-specific mitigation procedures. The results ascertained the predominant effect of overpotential, rather than current density, on the electrochemical degradation, as it governs the chromium contamination and the formation of undesirable insulating phases in the cathode. In the most striking cases, the lifetime could be extended by a factor of up to five, by the sole adjustments of the operating conditions. Counter-flow configuration, with low methane conversion in the reformer, is more favourable than co-flow. The thermo-mechanical contact model enlarged the analysis with insights into the complex failure modes, which ultimately cause the mechanical failure of the cell directly or indirectly, through a succession of deleterious events. The model explains the difficulty to ensure the integrity of the cells, the electrical contact and gas-tightness of the compartments, while preventing thermal buckling, during load following and thermal cycling. The effects of design and history were analysed in light of thermo-electrochemical and mechanical degradation, combined with rate-independent plasticity and creep, and stacking conditions. For the first time, to our knowledge, the modelling framework developed here has encompassed both electrochemical and mechanical degradation, along with calibration procedures, for lifetime predictions and identification of complex failure modes. This capability is expected to contribute significantly to improve the durability of SOFC devices in the future, since most of the modelled issues are currently addressed by progressive empirical adjustments.

EPFL2011

Chargement

Approaches on the Thermal Management and Parameter Estimation of Solid Oxide Fuel Cells and their Systems

Leonidas Tsikonis

Solid Oxide Fuel Cells (SOFCs) are electrochemical devices that convert chemical energy from fuel directly into electrical and thermal energy. They present high electrical efficiency and because of this feature they are considered to be an important power source alternative. However, due to their high temperature of operation, SOFCs are subject to degradation, that is, their ability to produce electricity deteriorates in time. The reasons and the conditions that contribute to degradation are multiple. Among the studied factors, operating temperature is an important one. It has been found that a stable and controllable thermal environment can mitigate this undesirable phenomenon. Therefore, the pursuit of appropriate thermal conditions during their operation is of paramount importance for their lifetime and consequently for their commercialisation. Studying and obtaining such conditions is also known as thermal management. Another important point regarding the study of SOFCs is the development and use of adequate mathematical models that the engineer and generally the researcher will use as tools to analyse their behaviour and will determine conditions for safe and efficient operation. For practical applications, these models must be validated against experimental data. The result of such validation is the determination, or calibration, of the model parameters so that the output of the model fits measurements taken in the laboratory to the best possible extent. This process is called parameter estimation or system identification. This thesis tackles both aspects. In its first part it works on the thermal management of a specific product, an SOFC autonomous unit fed with methane, producing electrical power and liberating hot off-gases that can be used as a source of thermal power. A dynamic model of the physical system is developed and validated against experimental data. During this process, it was found that measurement of gas temperatures using thermocouples may be severely biased due to radiation effects of surrounding solids to the thermocouples. In order to overcome this hurdle, the phenomena that take place around the thermocouple were incorporated into the system's mathematical model. Such a systematic error may have important consequences on the thermal management and generally on the control of the SOFC system. Indeed, an investigation effectuated in this thesis revealed how incorrect gas temperature measurements affect the system's control to such an extent that, depending on the conditions, may lead to system failure. The validation of the system model proved to be particularly challenging. This fact incited the author to investigate the factors that contribute to reliable parameter estimations. This is the subject of the second part of this thesis. A first study was performed on model-based Design of Experiments (DoE) for SOFC button cells, i.e. on how measurements on small cells may be optimised. To our knowledge the study treats for the first time the issue of repetitive measurements and their impact on the quality of parameter estimation systematically. It distinguishes the notion of measurements, which can be repetitive, from that of measurement points (or design points), which are defined once for an experiment. Introducing a new type of graph that provide information on quality criteria as functions of the number of measurement points for constant number of measurements, it stresses the importance of repetitions of measurements during experiments, which up to now has been neglected. A mathematical formula is also given that helps the experimenter define the necessary number of repetitions for a specific degree of precision of the information matrix. The theoretical findings on Design of Experiments are validated experimentally with measurements on button cells. The thesis introduces a novel method with which repetitive parameter estimations are obtained using data from polarisation curves. This method allows the calculation of histograms for the model parameters approximating this way their stochastic behaviour. Standard deviations, covariance and correlation matrices are calculated directly from the available data, avoiding the approximate calculation based on sensitivity (Jacobian) matrices. The experimental results showed again the importance of repetitions on the quality of parameter estimation. A rule-of-thumb is introduced suggesting that the number of repetitions of measurements needs to be at least equal to the number of calculated parameters plus three in order to avoid high correlations. However, repetitions do not influence the values of parameters or the fit to the experimental data, but their precision. These values are influenced by the number of measurement points. Another point of importance shown in this thesis is that there is a counterbalance between the parameters' variances and covariances and their correlations, that is, if the one improves, the other deteriorates. This is demonstrated to be in accordance with the Cramèr-Rao theorem in statistics. A consequence of this is the conclusion that parameter estimation cannot be obtained to an arbitrarily high precision. Another consequence is that assessment of system identification based on only one of the widely known criteria may not be adequate, a result that concurs with the findings of other authors in the field of DoE. Finally the available results from the afore-mentioned method revealed a potential relation between correlations of parameters and the non-uniformity of their histograms. Among the investigated examples, a case of fast degradation exhibits how the introduced parameter estimation method may be used as a diagnostics tool. However such a use requires the employment of models that describe the phenomena adequately, especially after degradation. The thesis ends with the calculation of the histograms of the parameters of the SOFC unit's heat exchanger providing a stochastic perspective of parameter estimation at an SOFC system level.

EPFL2011

Chargement

Approaches on the Thermal Management and Parameter Estimation of Solid Oxide Fuel Cells and their Systems

Leonidas Tsikonis

EPFL2011

Solid oxide fuel cell stack simulation and optimization, including experimental validation and transient behavior

Diego Larrain

EPFL2005

EPFL2011