SOFC Model Research Articles

Thermodynamic Evaluation of Integrated Heat-Pipe Reformer SOFC System

In the presented work a medium-sized integrated system based on a sodium heat pipes powered allothermal fluidized bed gasification and SOFC is introduced, simulated and validated in Aspen Plus. Suitable feedstocks for such fluidized bed gasification systems are for example waste and biomass, especially wood. The concept development and evaluation have been conducted as part of the EU FCH-JU project SOFCOM. During the allothermal heat pipes gasification process heat for the endothermic gasification reactions is transported into the gasification chamber via the heat pipes. The heat pipes allow low temperature differences using mainly latent heat transport. Inside the heat pipes sodium is evaporated on the hot side by a heat source (i.e. combustion chamber) and condensed on the cold side (i.e. gasification chamber). Heat pipes based gasification has proven to be a reliable technology at different installations by the company Agnion energy GmbH and others. The gasifier model has been validated against data from this type of gasifier. The SOFC model applied is a thermodyamic model, which has been developed in the frame of SOFCOM and already described in detail in other works. In the presented system configuration the product gas from the gasifier is cleaned during several process steps at an intermediate temperature of 350°C. After the cleaning procedure the gas is introduced into the SOFC anode where it is converted into electricity and high temperature heat at a fuel utilization rate of 80%. At the same time preheated air is fed to the cathode of the SOFC. The SOFC exhaust is fed into a fluidized bed post combustion chamber where residual fuel is burned with residual oxygen. Heat is extracted via the heat pipes from the combustion chamber, as well as the SOFC itself. Combustion exhaust gases are used to preheat the cathode air. Residual exhaust heat is used to generate high pressure superheated steam, which in turn is expanded in a steam turbine. At this point, the steam necessary for the fluidization of the gasifier and gasification agent is not produced in a separate steam generator but instead extracted from the steam turbine. Especially by means of utilization of excess heat from the SOFC and its exhaust gases via the heat pipes and full integration of the steam cycle the electrical efficiency of the combined system reaches up to 62%LHV. Figure 1

3D Multiscale-Multiphysics SOFC Modelling Status at the Institute of Electrochemical Process Engineering, FZ Jülich

The Institute of Electrochemical Process Engineering (IEK-3), SOFC modelling research group has been focusing on the 3D multiphysics modelling of SOFCs [1], auxiliary components [2-4], as well as whole fuel cell systems [5] during the last seven years; thereby giving particular attention to various geometrical scales including, micro, meso and macro level of solid oxide fuel cells [6-9], as well as cell, stack and system level high-end modelling. Currently, emphasis of research has been given to the material, process and 3D design optimisation of APU fuel cell stacks, including the thermomechanical behaviour during the heating-up, operation and shut-down stages for safe SOFC operation. Micro modelling of cells, including the thermomechanical fatigue behaviour has been another research area to understand the cyclic durability of the critical fuel cell stack regions. The full scale 3D system level multiphysics modelling of the Forschungszentrum Jülich integrated SOFC system has been successfully continued and important results have been achieved and experimentally validated to improve the knowledge about the overall multiphysics system in full detail. Moreover, the interacting component behaviour of the fuel cell stack, afterburner, heat exchanger, pre-reformer and baffle plates have been intensively investigated to improve their design and thermofluid flow, thermomechanical performance. Coupled CFD/FEM is used to predict their thermofluid flow, heat radiation, electrochemical and chemically reacting species transport, creep, elastoplasticity and cyclic behaviour. M. Peksen, Progress in Energy and Combustion Science, 48, 1-20 (2015)M. Peksen, L. Blum and D. Stolten, Int. J. Hydrogen Energy, 37, 12540-12547 (2012)Q. Fang, L. Blum, R.Peters, M. Peksen, P. Batfalsky, D.Stolten, Int. J. Hydrogen Energy 2, 1128-1136 (2015)M. Peksen, Ro. Peters, L. Blum, D. Stolten, Int. J. Hydrogen Energy, 36 , 6851-6861 (2011)A. Al-Masri, M.Peksen, L.Blum, D. Stolten Applied Energy, 135, 539-547 (2014)M. Peksen, Int. J. Hydrogen Energy, 39, 5137 (2014).M. Peksen, Int. J. Hydrogen Energy, 38, 13408-13418 (2013)M. Peksen, Int. J. Hydrogen Energy, 36, 11914 (2011).M. Peksen, A. Al-Masri, L. Blum and D. Stolten, Int. J. Hydrogen Energy, 38, 4099 (2013).

3D Multiscale-Multiphysics SOFC Modelling Status at the Institute of Electrochemical Process Engineering, FZ Jülich

The Institute of Electrochemical Process Engineering (IEK-3), SOFC modelling research group has been giving particular attention to the 3D multiscale-multiphysics modelling of SOFC systems during the last seven years. Focus has been the coupled thermochemical fluid flow and thermomechanical interactions of SOFCs, auxiliary components, as well as complete fuel cell systems. Currently, emphasis of research has been given to the material, process and 3D design optimisation of APU fuel cell stacks, micro modelling of cells and the full scale high-end 3D system level multiphysics modelling and optimisation. Therby, an important investigation field has been the various geometrical component scales, including macro, meso, and micro level by coupling the experimentally validated numerical predictions from system level down to the component and cell level. Coupled CFD/FEM is used to predict the thermochemical fluid flow, heat radiation, electrochemical and species transport, creep deformation, elastoplasticity and cyclic behaviour.

SOFC and MCFC system level modeling for hybrid plants performance prediction

In this paper a generalized model, based on system-level approach, for predicting the High Temperature Fuel Cells (HTFCs) behavior and performance is presented.The system-level model allows to forecast the HTFC performance under different operating conditions (cell temperature, anode off-gas recirculation, reactants temperatures, fuel and oxidant utilization factors, etc.) and cell design (tubular and planar configurations and with co-flow, counter-flow and cross-flow arrangements).Mass and energy balances are solved by considering both the electrochemical (i.e. electro-oxidation of hydrogen) and thermochemical reactions (i.e. reforming and shifting reactions) which occur in the anode and cathode sides and by applying different equations systems to take into account the type of fuel cell (MCFC or SOFC).The ability of the proposed model in the HTFCs performance prediction is pointed out by the model validation carried out by using experimental data and by analyzing the impact of the model calibration parameters on the cell voltage calculation carried out by means of a sensitivity analysis.Numerical results show that the model allows to characterize the behavior of the HTFCs with a good approximation so, thanks to the simplicity of the simulation procedure and to the small computational time efforts, it can be a useful tool for predicting the performance of hybrid power plants or more complex systems in which the fuel cell is one of the main components.

Experimental analysis of the flow structure in the laboratory model of SOFC fuel cell channels

In the presented paper a flow structure in the gas channel of planar SOFC fuel cell is presented. The model taken for analysis was constructed based on the channel geometry manufactured by SOFC Power company. The shape of a channel was rectangular filled with large number of obstacles which role is to divide the flow into segments with possibly homogenous velocity distribution. The model itself was constructed from Plexiglas and the reactant gases flow was modelled by water motion. To investigate and visualize the flow structures a PIV technique was applied. Three different flow rates were taken for investigations and the flow uniformity and time dependence was studied.

FDI oriented modeling of an experimental SOFC system, model validation and simulation of faulty states

We present here a model we have established for a laboratory-size SOFC system. Model equations are presented and discussed, together with an extensive model validation, carried out in both steady state and transient operating conditions. The validated model is then used to simulate four classes of system faults, i.e. air leakage, fuel leakage, SOFC degradation and reformer degradation. When the different faults occur, the physico-chemical operating parameters of the system vary in a different manner, and our main objective is to understand their behavior. For example, for the system under analysis, SOFC degradation and fuel leakage cause a decrease of the gas discharge temperature, while reformer degradation causes an increase of the same parameter. The results of the fault analysis are described and discussed, and are reported in such a way to provide a basis for the development of an FDI tool based on pattern recognition techniques.

Parameter identification of an SOFC model with an efficient, adaptive differential evolution algorithm

An efficient, adaptive differential evolution (DE) algorithm is proposed in which DE parameter adaptation is implemented. A ranking-based vector selection and crossover rate repairing technique are also presented. The method is referred to as IJADE (Improved Jingqiao Adaptive DE). To verify the performance of IJADE, the parameters of a simple SOFC electrochemical model that is used to control the output performance of an SOFC stack are identified and optimized. The SOFC electrochemical model is built to provide the simulated data. The results indicate that the proposed method is able to efficiently identify and optimize model parameters while showing good agreement with both simulated and experimental data. Additionally, when compared to other DE variants and other evolutionary algorithms, IJADE obtained better results in terms of the quality of the final solutions, robustness, and convergence speed.

Modeling of Direct H2S Fueled Solid Oxide Fuel Cells with Reforming Chemical Kinetics

AbstractA direct H2S fueled SOFC model is developed based on Ni‐YSZ/YSZ/YSZ‐LSM button cell test stand. The model considers the detailed reforming chemical processes of H2S and multi‐physics transport processes in the fuel cell and fuel supply tubes. The model is validated using experimental data. Extensive simulations are performed to study the complicated interactions between multi‐physics transport processes and chemical/electrochemical reactions. The results elucidate the fundamental mechanisms of direct H2S fueled SOFCs. It is found that suitably increasing the H2O content in the supplied H2S fuel can improve SOFC electrochemical performance; high operating temperature may facilitate the reforming of H2S and improve the electrochemical performance. The sulfur poisoning effect may be mitigated by increasing the H2O content in the fuel, increasing the operating temperature, decreasing the flow rate, and/or making the cell work at low voltage (or high current) conditions.

Sulfur Poisoning of SOFCs: A Model Based Explanation of Polarization Dependent Extent of Poisoning

Several experimental studies have shown that, 1) the extent of the poisoning effect due to trace amounts of sulfur compounds in the fuel is lower if a SOFC is operated at a higher current density, and 2) the performance drop due to sulfur poisoning is much lower for Ni-GDC or Ni-ScSZ anodes when compared to Ni-YSZ anodes. This work presents a first principles numerical model that simulates experimental studies of sulfur poisoning on SOFC button cells. The exchange current densities for the electrodes are determined using sulfur-free polarization data for cells fueled by humidified mixtures of H2 and N2. A detailed surface reaction model that predicts the fractional coverage of all adsorbed species at the three phase interface is coupled to the SOFC model and the sulfur coverage is used to alter the anode exchange current density. The resulting model predictions match experimental observations during both galvanostatic and potentiostatic operation. Our analysis shows that the observed lower performance drop at higher current density is due to the non-linear nature of the electrochemical rate equations, and that the lower impact of sulfur poisoning on Ni-GDC and Ni-ScSZ anodes (compared to Ni-YSZ anodes) is due to their higher electrochemical activity.

Developing 3-D Model of Intermediate-Temperature SOFC with GDC Electrolyte

SOFCs with GDC electrolytes present challenges for stack modeling due to the variation in conductivity and chemical activity of GDC as a function of effective oxygen partial pressure P O2, which varies significantly down the channel in SOFC anodes with increasing fuel utilization. This paper presents a 3-D model for an intermediate-temperature SOFCs with GDC electrolytes, Ni/GDC anodes, and LSCF/GDC cathodes. The model uses fitted kinetics to capture, internal reforming and water-gas-shift in the anode as well as variations in open circuit voltage and activation overpotentials as a function of flow composition and temperature along the length of the channel. The model is validated against button cell data. The results show that for 600 and 650°C inlets, super-equilibrium concentrations of CH4 remain after 5 cm of channel. For the same fuel stoichiometry and cell voltage, humid H2 provides over 30% more power than reformate with more than 5% CH4.

Recent Developments of 3D Coupled Multiphysics SOFC Modelling At Forschungszentrum Jülich

The Jülich SOFC modelling group has been performing 3D coupled multiphysics modelling of SOFCs, auxiliary components, as well as whole fuel cell systems since five years. Currently, emphasis has been given to the thermomechanical robust design and process optimisation of full-scale fuel cell stacks and whole fuel cell systems, including all auxiliary components. Particular attention is given to improve the understanding of the steady and transient processes, effecting the robustness and life-time duration of the components that directly influence the performance. Coupled CFD/FEM is used to mimic the thermofluid flow, electrochemical and chemical reactions, as well as thermo- mechanics.

Multi-physicochemical modeling of direct methane fueled solid oxide fuel cells

Both H2–H2O and CO–CO2 electro-oxidation processes are considered in direct methane fueled SOFC model and incorporated into the comprehensive couplings between multi bulk transport processes, diffusion processes, surface chemical processes and the cell polarization performance. The model is validated using the experimental data of polarization performance at 600 °C, 650 °C and 700 °C respectively. Upon the model validation, extensive simulations are carried out to elucidate the complicated interaction mechanisms with emphasis on surface reaction processes and surface adsorbates under SOFC operating conditions. Results show that the surface reactions of methane are strongly dependent on the cell operating conditions and increase with increasing the operating temperature and cell current. The bulk gas species and surface adsorbates show different degree of sensitivities to cell operating conditions. Surface carbon deposition can be mitigated through increasing the operating temperature, the cell operating current, and the exchange current of electrodes as well as suitable increasing H2O content in the fuel. Surface carbon deposition may also be mitigated through suitable reducing the content of CH4, CO, and/or CO2 in the fuel.

Simulation and Control for Load Change Process on a Solid Oxide Fuel Cell

This paper addresses the dynamic characteristics of SOFC with an emphasis on control strategy development for optimized temperature control during load change. A methodology on regulating fuel utilization (Uf) and air excess ratio (λ) is investigated to validate alleviation of temperature fluctuation. An integrated dynamic model of SOFC, composed of mass balance, temperature balance, and electrochemistry, collaborates with Model Predictive Controller (MPC) in MATLAB/Simulink to verify the dynamic characteristic during load change process in the paper. A variable λ strategy is proposed and compared with traditional fixed λ strategy. Simulation results show the variable λ strategy can reduce the temperature variation for a safe operation, in comparison with that by the fixed λ strategy.

Investigation of thermal radiation effects on solid oxide fuel cell performance by a comprehensive model

Thermal radiation in the air and fuel channels has the potential to dramatically influence the overall operating conditions and performance of solid oxide fuel cells. A 3D comprehensive model is developed, with emphasis on quantifying the radiative heat transfer process and its effects. The radiosity method is used for the thermal radiation in the air and fuel channels. The thermal radiation heat transfer is coupled to the overall energy conservation equation. Commercially available COMSOL CFD software is used as a platform for the global thermal-fluid modeling of the SOFC. The effects of the operating voltage, emissivity, ambient temperature and flow arrangement (Co- and Counter-flow) on the performance of SOFC are investigated. The predicted results reveal that the radiative heat transfer should be considered in SOFC modeling simulation, and the effects of the thermal radiation on the performance of SOFC under a different flow arrangement is sometimes quite significant.

Scrutiny of MT-SOFC Stack Manifolding Design Using CFD

In this work we investigated the use of Computational Fluid Dynamics (CFD) in order to describe the behav- iour of a single Micro Tubular Solid Oxide Fuel Cell (MT-SOFC) and a bundle thereof. It is the first step before building a rather necessarily complicated experimental apparatus in order to compare the predictions with experimental measure- ment. The first goal of this study was to test the suitability of commercially available CFD & SOFC modelling software, with some modified features. The second goal was to predict the effects of various fuel and oxidant manifolding tech- niques regarding temperature, species and current density distributions. A result of this paper showed that CFD is a very useful tool, when a SOFC module is incorporated for MT-SOFC stack modelling. A second result showed that the oxidant flow regime was much more important than the fuel regime in order to manipulate a single MT-SOFC's temperature pro- file. The cases investigated had a radiation model included and the differences in temperature profiles, when radiation was included and neglected, especially for MT-SOFCs with view factors to the reactor housing, was shown to be important. The CFD predictions clearly showed the benefits and advantages associated with the different forms of fuel and oxidant manifoldings. A future experimental analysis is currently being designed.

SOFC modelling based on discrete Bayesian network For system diagnosis use

Abstract We propose in this paper a diagnosis method that is aimed to detect and isolate SOFC system fault by using the FC stack as a sensor. A discrete Bayesian network (BN) was established to illustrate the input-output causal relations of the stack. In order to examine the generalizability of the network structure, the BN was parameterized to fit the experimental data from two different SOFC systems. The models showed reasonable accuracy of state estimation for 6 operating variables. Finally, the BN model was experimented for diagnosing a specified system fault.

SOFC modeling considering electrochemical reactions at the active three phase boundaries

It is expected that fuel cells will play a significant role in a future sustainable energy system, due to their high energy efficiency and the possibility to use renewable fuels. A fully coupled CFD model (COMSOL Multiphysics) is developed to describe an intermediate temperature SOFC single cell, including governing equations for heat, mass, momentum and charge transport as well as kinetics considering the internal reforming and the electrochemical reactions. The influences of the ion and electron transport resistance within the electrodes, as well as the impact of the operating temperature and the cooling effect by the surplus of air flow, are investigated. As revealed for the standard case in this study, 90% of the electrochemical reactions occur within 2.4 μm in the cathode and 6.2 μm in the anode away from the electrode/electrolyte interface. In spite of the thin electrochemical active zone, the difference to earlier models with the reactions defined at the electrode–electrolyte interfaces is significant. It is also found that 60% of the polarizations occur in the anode, 10% in the electrolyte and 30% in the cathode. It is predicted that the cell current density increases if the ionic transfer tortuosity in the electrodes is decreased, the air flow rate is decreased or the cell operating temperature is increased.

Optimal integration strategies for a syngas fuelled SOFC and gas turbine hybrid

This article aims to develop a thermodynamic modelling and optimization framework for a thorough understanding of the optimal integration of fuel cell, gas turbine and other components in an ambient pressure SOFC–GT hybrid power plant. This method is based on the coupling of a syngas-fed SOFC model and an associated irreversible GT model, with an optimization algorithm developed using MATLAB to efficiently explore the range of possible operating conditions. Energy and entropy balance analysis has been carried out for the entire system to observe the irreversibility distribution within the plant and the contribution of different components. Based on the methodology developed, a comprehensive parametric analysis has been performed to explore the optimum system behavior, and predict the sensitivity of system performance to the variations in major design and operating parameters. The current density, operating temperature, fuel utilization and temperature gradient of the fuel cell, as well as the isentropic efficiencies and temperature ratio of the gas turbine cycle, together with three parameters related to the heat transfer between subsystems are all set to be controllable variables. Other factors affecting the hybrid efficiency have been further simulated and analysed. The model developed is able to predict the performance characteristics of a wide range of hybrid systems potentially sizing from 2000 to 2500 W m −2 with efficiencies varying between 50% and 60%. The analysis enables us to identify the system design tradeoffs, and therefore to determine better integration strategies for advanced SOFC–GT systems.

Modeling of a SOFC Fuelled by Methane: Influence of the Methane Steam Reforming Kinetics

Methane appears to be a fuel of great interest for SOFC systems because it can be directly converted into hydrogen by Gradual Internal Reforming (GIR) within the SOFC anode with a new cell configuration. This study proposes a model of SOFC which takes into account the kinetics of the reforming reaction on the catalyst in order to assess the limitations under current of this new configuration of the SOFC cell.

A DFT Based Coupled Transport-Reaction Model for H2 Fueled SOFC Model Anodes

not Available.