Fuel Flow Direction Research Articles

Micro solid oxide fuel cell thermal dynamics: Incorporation of experimental measurements and model-based estimations for a multidimensional thermal analysis

Transient behaviour of solid oxide fuel cells during various stages of operation, such as warmup, startup, load fluctuations, and shutdown, must be understood and predicted in order to design an efficient controller and prevent degradation/failure of the cell. The difficulty of measuring and monitoring solid oxide fuel cell thermal features necessitates an evaluation of the solid oxide fuel cell thermal dynamics using a hybrid tool that integrates both experimental and numerical methods. In this study, a hybrid measurement tool consisting of a quasi-two-dimensional microscale model and a test rig was used to investigate the solid oxide fuel cell thermal dynamics. Through the hybrid experiment-model approach, steady-state temperature differences across Positive-Electrolyte-Negative and along fuel flow direction were captured. Additionally, the speed of temperature difference change was estimated in two dimensions, which is crucial when estimating transient thermal stresses. Several thermal measures were used to evaluate the thermal dynamics of solid oxide fuel cells. The impact of the operating voltage regime on solid oxide fuel cell thermal dynamics was studied using the varying operating voltage (Voltage Interrupted Measurement) characterisation approach. The outcomes revealed that optimising the performance of solid oxide fuel cells requires a trade-off between thermal management and fuel utilisation due to the exponential effect of fuel utilisation on steady-state temperature differences. The effects of fuel humidity and oxygen content of the oxidant flow on solid oxide fuel cell thermal dynamics were also examined. The results shed light on the new aspects of fuel cell thermal dynamics that are key in designing future smart controllers.

Fast online diagnosis for solid oxide fuel cells: Optimisation of total harmonic distortion tool for real-system application and reactants starvation identification

This study investigates the potential application of total harmonic distortion (THD), as a fast online monitoring tool, to identify the early signs of failure modes in solid oxide fuel cells (SOFC). Indeed, the timely detection and identification of faults is crucial for applying mitigation strategies to avoid irreversible damage and increase the system lifetime. Two of the most common faulty conditions are considered in this work, namely fuel and air starvation. This experimental work aims at progressively triggering these errors in a controlled manner, by following different realistic scenarios, and recording THD values on a frequent basis. Several well-established laboratory monitoring tools, like electrochemical impedance spectroscopy (EIS), complemented by the distribution of relaxation times (DRT) are used to analyse and support the interpretation of the THD measurements. A sensitivity analysis on the THD parameters, namely the signal amplitude and the number of harmonics, is carried out to identify the optimal combination of parameters and achieve the best faults detectability. Finally, the impact of the fuel flow direction and the operating temperature are examined. The results of this work are summarised as practical guidelines to better employ the THD technique to identify the early warning signs of reactants’ starvation.

Numerical Analysis of the Local Current Efficiency Distribution in a Protonic Ceramic Fuel Cell Considering Gas Concentration Distributions

Protonic ceramic fuel cells (PCFCs) are expected as highly efficient energy conversion devices due to the high proton conductivity at relatively lower operating temperature (400 ºC~600 ºC) compared to solid oxide fuel cells (SOFCs). The water steam is generated at air side so that there is less fuel dilution at fuel side. Therefore, PCFCs could operate with higher fuel utilization. However, besides the main proton conductivity, some other chargers including electron holes, oxygen ions, and electrons can also be conductive in PCFCs leading a performance decrement. Especially, the electron hole conductivity cannot be neglected inducing a decrease of current efficiency under some operating conditions. The conductivities in PCFCs are not only affected by the operating temperature, but the gas concentrations also strongly influence their values [1]. The conductivities of proton and hole are respectively show positive relationship to steam and oxygen concentrations [2-3]. These properties of PCFCs cause complex for investigating the performances.In this study, the defect transports in a PCFC electrolyte with BaZr0.8Yb0.2O3-δ (BZYb20) material were numerical solved with a quasi-two-dimensional Nernst-Planck-Poisson (NPP) model and the effect of gas concentration on the cell performance was discussed. For the NPP model calculation algorithm, the Nernst-Planck equation (or flux continuity equation) and the Poisson equation are successively solved by finite difference method (FDM) until convergence criteria are obtained with the defect concentration boundary conditions. The defect concentrations at electrolyte membrane surfaces (boundary conditions of NPP model) are derived by equilibrium assumptions of the reactions such as hydration and oxidation reactions. The charge neutrality condition is also added to the boundary conditions for the exact solutions [4]. Subsequently, the local defects (i.e., proton and electron hole) concentrations are revealed, so that the local current efficiency distribution in the PCFC can be revealed.The utilizations of fuel and oxygen are set at 80% and 30%, respectively. The vapor concentration at anode and cathode side are assumed as 3%. The electrolyte thickness is 8 μm in the present study. A constant temperature distribution is assumed in the numerical model. An example result under co-flow gas supply including proton current density and current efficiency distributions is shown in the following figure. The proton current density along fuel flow direction is found to increase from inlet to outlet. The reason can be considered as the influences of water steam distributions. The water steam concentration (or partial pressure) at both fuel and air side increase from inlet to outlet. The current efficiency at downstream are higher than that of upstream. This is contributed by the increase of oxygen in gas flow direction. Moreover, the influences of gas flow direction on cell performances have also been investigated. Acknowledgements This research is primarily based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) Grand Number JPNP20003, Japan. We also extend out special thanks to members of AIST Solid State Ionics Materials Group for useful information and discussions. References Li, et al ., Int. J. Hydrogen Energy, 45 (2020), 34139-149.Okuyama, et al ., Int. J. Hydrogen Energy, 39 (2014), 20829-36.Somekawa T, et al ., Int. J. Hydrogen Energy, 41 (2016), 17539-47.Vøllestad E, et al ., Int. J. Electrochem Soc, 161 (2014), F114-24. Figure 1

Three-dimensional modeling of performance degradation of planar SOFC with phosphine exposure

A three-dimensional computational model is developed to predict the performance degradation of planar SOFC anodes exposed to fuel contaminants commonly present in coal syngas. The model parameters are calibrated using data from button cell experiments. The calibrated model is then used to perform simulations to predict performance degradation of planar cells due to phosphine contamination. The results from degradation simulations show that the contaminant coverage alters the current and temperature distributions significantly. The electrochemical characteristics of the degraded cell are analyzed by performing impedance and polarization simulations. The impedance analysis is applied to the cell in three regions along the fuel flow direction to elucidate location specific degradation phenomena. The results show that the degradation rates and the impedance behavior of planar cells are very different and more complicated than those observed in button cells. In such cases, localized impedance analysis may be necessary to elucidate various degradation mechanisms.

De-icing of fuel/oil heat exchange systems via fuel flow direction switching device

Fuel icing of fuel/oil heat exchange systems has been a troublesome issue. In this study, a de-icing method is proposed to manage the fuel icing problem by switching the fuel flow direction of the systems. This method uses the fuel/oil heat exchanger as a passive heat source, heat and momentum of the fuel are delivered to the ice. A device named FFDSD (Fuel flow direction switching device) has been developed for this method. This device consists of two normally open and two closed type solenoid valves, and the proposed configuration consumes electrical power only when switching fuel flow directions (36 W to 0 W). A control algorithm has been developed as a countermeasure against a large amount of ice inflow that conventional methods cannot cope with. FFDSD was designed and fabricated, and the experimental test was conducted. The experimental results show that the proposed method can successfully achieve the de-icing of the fuel system within 1 s. A numerical de-icing process was attempted. Ice in fuel was numerically modeled as a fluidized quasi-solid material based on descriptions in previous studies. Numerical analysis results show a similar tendency to experimental results. The results of this study would be useful to manage fuel icing problem of heat exchange systems.

Simulation and Visualization of Cell Performance Distribution of SOFCs

Introduction It has been reported for SOFC systems that high electrical efficiency beyond 70% could be achieved theoretically in a recent study[1]. It is important to understand the internal characteristics to design highly efficient SOFC. However, it is difficult to visualize internal reactions experimentally. From these reasons, it is desired to develop a simulation technique for visualizing various phenomena within fuel cell stacks. Here, we focus on numerical analysis on electrode kinetics. Exchange current density is an important phenomenological parameter in considering electrochemical reaction kinetics at SOFC electrodes and in simulating cell performance. Here, the exchange current density of fully pre-reformed methane-fueled SOFC anodes is measured and phenomenological equations of anode exchange current density are derived. Furthermore, we combine numerical analysis with the phenomenological relations to simulate the spatial distribution of anode overvoltages. Experimental conditions In this study, electrolyte-supported cells were used to measure the anode exchange current density. The electrolyte material was ScSZ (ScSZ:10mol% Sc2O3–1mol% CeO2–89mol% ZrO2) and the electrode prepared using NiO-ScSZ was applied to the anode. The electrode prepared using LSM（LSM: (La0.8Sr0.2)0.98MnO3）and LSM-ScSZ was applied to the cathode. By changing operation temperature, fuel utilization (Uf), and steam-to-carbon ratio (S/C), anode overvoltage was measured at each current density (0.25, 0.3, 0.35, and 0.4 cm2) for methane-fueled pre-reformed SOFCs by the current interrupt method. The anode exchange current density was then derived from the current density and anode overvoltage values. In simulation, three-dimensional cell model was created using a numerical analysis software. A parallel co-flow model was considered. The inlet gas temperature was 800°C and the anode gas composition was fixed with the S/C of 2.5. Results and discussion The equation of anode exchange current density based on hydrogen-fueled model[2] was revised to consider the equilibrium reforming reaction, the water gas shift reaction, and the conservation law of anode gas partial pressure. It has been reported that the electrochemical reaction, where the adsorbed hydrogen atom (Had) in hydrogen-based fuels is rate-limiting[2], an Arrhenius-type equation shown in Eq. 1 has been proposed based on the assumption that the electrochemical reaction of Had is rate- limiting: i 0,a = a·exp(b/RT)·PH2(A) 0.5·PH2O(A) 0.5 (1) The partial pressure terms were modified considering the equilibrium reforming reaction and the water gas shift reaction. Pre-exponential factor a and activation energy b were fixed using the experimental results. Fig. 1 shows the experimental results of anode exchange current density. The phenomenological values derived from Eq. 1 are shown in Fig. 1. As the fuel utilization varied, the exchange current density increased and reached the maximum at around 40% of Uf. The experimental results could be well fitted by Eq.1. The SOFC cell performance simulation was performed using the three-dimensional SOFC model. Fig. 2 shows the anode overvoltage distribution along the fuel flow direction from the left side to the right side. The overvoltage decreased from the inlet towards the outlet. This is partly because oxygen partial pressure on the anode increased to the outlet and the electromotive force and the exchange current decreased.

Advanced Direct Internal Reforming Concepts for Solid Oxide Fuel Cells Running with Biogas

For the effective utilization of biogas, a CH4-CO2 mixture obtained from the anaerobic fermentation of organic wastes, as an alternative fuel for power generation, high temperature solid oxide fuel cell (SOFC) is one of the most attractive technologies due to its considerably high efficiency of over 50 %. Capability of direct internal reforming (DIR) operation of SOFC can make fuel-processing unit more simple and compact, and can further increase efficiency because heat released during power generation can be recycled for the fuel reforming in the stack. When biogas is supplied directly to SOFC, H2, main reactant for the electrochemical oxidation to generate electricity, is produced within the porous anode material through the simultaneous methane steam and dry reforming reactions, hereafter called methane multiple-reforming (MMR) process (Fig. 1) [1]. However, excess stresses thermally induced by the strong endothermic nature of the MMR process might cause electrolyte fracture. Therefore, the MMR process has to be properly controlled. In this study, advanced concepts to enhance cell performance and reduce thermal stresses accompanied by DIR were studied. Fig. 2-a shows an in-cell reformer concept, a sheet of Ni-loaded paper-structured catalyst (PSC) [2,3] is placed on the anode to partially reduce CH4conversion within the anode volume. Fig. 2-b shows a concept with thin gas-barrier mask made from dense YSZ on the anode surface, which can control reforming rate along fuel flow direction. Current-voltage (I-V) curves for the concepts of Figs. 2-a and b were measured at 800 oC using a 20 x 50 mm2 anode-supported cell (ASC) with the direct feed of simulated biogas mixture (CH4/CO2= 1), and then, a 3-dimensional CFD model of ASC coupling mass- and heat transfer, MMR and electrochemical processes was adjusted so that it can reproduce measured electrochemical behavior. For precisely predicting the consumption and production rates of gaseous species involved in the MMR process, the MMR was modeled using a method based on artificial neural network (ANN) (Inputs: CH4, CO2/CH4, H2O/CH4, Temperature; Outputs: Consumption and production rates of CH4 and H2, respectively). Using the experimentally-verified CFD model, distributions of gaseous species in fuel channel, temperature, thermally-induced stresses in electrolyte and cell deformations were estimated. This study revealed that electrolyte crack can occur at the middle region of the cell length while the minimum electrolyte temperature appears close to the fuel inlet. Performance enhancement with the in-cell reformer concept was confirmed, as shown in Fig. 3. Meanwhile, the concept using gas-barrier mask was found to be effective to reduce temperature gradient within the cell (see Fig. 4), as a result, increasing mechanical stability of DIR-SOFC. References 1 Meng Ni, Int. J. of Hydrogen Energy 38 (36) (2013) 16373–16386. 2 Y. Shiratori, T. Ogura, H. Nakajima, M. Sakamoto, Y. Takahashi, Y. Wakita, T. Kitaoka, K. Sasaki, Int. J. Hydrogen Energy 38 (2013) 10542–10551. 3 Y. Shiratori, M. Sakamoto, J. Power Sources 332 (2016) 170-179. Figure 1

Experimental investigation of spray characteristics of kerosene and ethanol-blended kerosene using a gas turbine hybrid atomizer

Gas turbines have wide application as prime movers in transportation and power generating sectors, most of which are driven by fossil fuels like kerosene. The conventional fuels are associated with problems of air pollution, and the fuel reserves are getting depleted gradually. Addition of ethanol in kerosene leads to better spraying characteristics. The present work deals with the spray characteristics of pure kerosene and 10%-ethanol-blended (by volume) kerosene using a novel gas-turbine hybrid atomizer. Here the inner air and outer air enter in the same and opposite directions, respectively, with respect to the fuel flow direction into the atomizer and a high swirling effect occurs outside the nozzle. The fuel stream is sandwiched between two annular air streams and the flow rate of inner and outer air is varied continuously. Various spray stages like distorted pencil, onion, tulip and fully developed spray regimes have been observed. The breakup length, cone angle and sheet width of the fuel stream are analysed directly from backlit imaging for different fuel and air flow rates. From the image processing, it is observed that breakup occurs at an early stage for 10%-ethanol-blended kerosene due to low viscosity of ethanol. It is also observed that at higher air flow rate, breakup occurs at an early stage due to turbulent nature of the fuel stream.

Electrochemical Behavior of Phosphine Induced Anode Performance Degradation in a Planar SOFC: A Numerical Study

A one-dimensional model was developed to predict the performance degradation of the SOFCs anode exposure of fuel contaminants found in typical coal syngas. This model is extended to a three-dimensional model to predict the phosphine induced performance degradation in relatively large planar cells operating on hydrogen. The model parameters are calibrated using button cell experiments conducted under accelerated tests conditions. These parameters are then used to perform simulations to predict fuel contaminant performance degradation of planar cells. The results from degradation simulations show that the contaminant coverage alter the initial current distribution, as well as the fuel and oxygen distribution inside the anode significantly. The electrochemical characteristic of the degraded cell is analyzed by performing impedance and polarization simulations at cell operating current. The polarization and impedance simulations are implemented by dividing the cell into three regions along the fuel flow direction. The results show that the degradation rates and the impedance behavior of planar cells are very different than those observed in button cells.

Three-dimensional CFD modeling of transport phenomena in multi-channel anode-supported planar SOFCs

In this study, a three-dimensional computational fluid dynamics (CFD) model is developed and applied for anode-supported planar SOFC involving multi-channels. The developed model is first validated in agreement with the experimental data obtained at same conditions. Three different flow arrangements (co-, counter- and cross-flow) are simulated and compared in terms of cell overall performance and various transport phenomena occurred inside the SOFC single cell functional components. Local distribution of temperature, mass flow rate, current density, gas concentrations of reactants and products in both fuel and air sides under different flow arrangements is predicted and presented. It is found that the co-flow and counter-flow arrangements have a better performance than that of the cross-flow arrangement at the same operating conditions. It is also found that the temperature for the three flow arrangements is unevenly distributed and the significant temperature gradients exist along the length of the cell. The mass flow rate of fuel at the inlet of each channel is uniform, however its difference between the side channel and the channel at the center is increasing along the fuel flow direction, which reaches a maximum value at the outlet region. It is also predicted that the maximum current density is located at the interfaces between the channels, ribs and the electrodes resulting in a large over-potential and a heat source in the electrodes, which is harmful to the cell overall performance and working life time.

A Numerical Investigation Into the Interaction Between Current Flow and Fuel Consumption in a Segmented-in-Series Tubular SOFC

A typical segmented-in-series tubular solid oxide fuel cell (SOFC) consists of flattened ceramic support tubes with rows of electrochemical cells fabricated on their outer surfaces connected in series. It is desirable to design this type of SOFC to operate with a uniform electrolyte current density distribution to make the most efficient use of the available space and possibly to help minimize the onset of cell component degradation. Predicting the electrolyte current density distribution requires an understanding of the many physical and electrochemical processes occurring, and these are simulated using the newly developed SOHAB multiphysics computer code. Of particular interest is the interaction between the current flow within the cells and the consumption of fuel from an adjacent internal gas supply channel. Initial simulations showed that in the absence of fuel consumption, ionic current tends to concentrate near the leading edge of each electrolyte. Further simulations that included fuel consumption showed that the choice of fuel flow direction can have a strong effect on the current flow distribution. The electrolyte current density distribution is biased toward the upstream fuel flow direction because ionic current preferentially flows in regions rich in fuel. Thus the correct choice of fuel flow direction can lead to more uniform electrolyte current density distributions, and hence it is an important design consideration for tubular segmented-in-series SOFCs. Overall, it was found that the choice of fuel flow direction has a negligible effect on the output voltage of the fuel cells.

Optimization of Manifold Positions and Fuel Flow Direction in a DMFC Stack

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The benefit of solid oxide fuel cells with integrated air pre-heater

A new design has recently been proposed in the field of solid oxide fuel cells, consisting of a traditional electrochemical cell integrated with a pre-heater. In this paper a simulation model for the rectangular planar solid oxide fuel cell with integrated air pre-heater is presented. A two-dimensional stack simulation is presented as well, one axis coincides with the fuel flow direction, the other with the stack height. Local quantities such as current density, gas and solid temperatures are reported and cell characteristics predicted. In a parameter study, effects of oxygen utilisation and heat-transfer conditions in the pre-heater on the lcoal temperature distribution of the solid structure are considered. As a result, the benefit of the new cell design becomes evident when low air flow rates are applied. A further advantage associated with the reduced flow rate is the low air temperature at the inlet.

Superadiabatic Combustion in Porous Media: Wave Propagation, Instabilities, New Type of Chemical Reactor

This paper discusses theoretical and experimental results of filtration combustion of methane, hydrogen, and acetylene in porous media. Two general cases were studied: linear propagation of a slow, thermal combustion wave during fast fuel filtration, and a reverse unsteady-state combustion process, when the fuel flow direction is periodically switched from one end to the other. The intensive heat transfer between the heat releasing filtrating gas and the high thermal capacity, porous medium (through the highly developed internal solid surface) results in energy accumulation in the solid body and in the so called superadiabatic effect, when gas temperatures in the porous combustor can significantly exceed the adiabatic temperature of a feeding gas fuel. It was found that in such a superadiabatic combustor (SAC): (1) a very fuel lean gas mixture (e.g. very small concentration of CH4, C2H2, or H2 in air) can be burned, and (2) conversely even a very small amount of oxygen in such gas fuels as CH4 or H2S can support a combustion process associated with the high temperature pyrolysis, hydrogen production and hydrocarbon or hydrogen sulfide partial oxidation. The physical phenomena of superadiabatic combustion examined included: superadiabatic (SAC) wave propagation, SAC-wave velocity dependence on fuel concentration and gas flow rate, gasdynamic and kinetic thermodiffusion instabilities of SAC-waves, and cellular and double wave structures occurring as a result of the overheating instabilities. Possible SAC applications are discussed. These include: applications in chemistry and energy systems (particularly for a lean gas fuel burning), partial oxidation of very fuel rich gas mixtures, hydrogen production from fossil fuels, and applications for environmental control, in particular for air purification of volatile organic compounds (VOC).