Real Stack Research Articles

Stability of Glass Ceramic Sealants in Atmospheres with Increased Water Contents

For Solid oxide fuel cells (SOFC), operating at temperatures above 800 °C, glass ceramic sealants are state of the art. The properties of the glass ceramics are adapted to the metallic and ceramic join partners, operation conditions, as also to the atmospheres: fuel gas and air. In SOFCs, glass ceramic sealants offer reliable, long term stable operation over several 10,000 hours of service. The demand to operate solid oxide cells (SOC) in fuel cell- as also in electrolysis cell mode increases the demands of the stability of the sealants against atmospheres. In the electrolysis mode, the SOC-stacks were impinged by gas with high water contents to produce H2. Water contents of up to 95 % and temperatures above 800 °C are hard conditions for all applied materials of SOC stacks. In addition, glass ceramic sealants have to electrical insulator under applied electric voltage. In the electrolysis mode, the applied voltage is even higher than in fuel cell mode. In conclusion, the electrolysis mode is much harsher as fuel cell mode and glass ceramic sealants have to be adapted to this. Glasses in the system BaO-CaO-Al2O3-B2O3-SiO2 with La2O3-additions have the potential to form glass ceramics with extraordinary high coefficient of thermal expansion (CTE)values, which are suitable for use as sealants compatible with Crofer 22 APU interconnect. Their characteristics have been generally described in various studies and are also in focus of our own work and have proven their suitability for use under SOFC operating conditions. However, achieving such high CTE values is especially challenging when these glasses must be designed for use in stacks operating with electrolyte supported cells because sealing and operating temperature are usually higher than in other stack designs. Their behavior under simulated electrolysis conditions together with a direct correlation of test by using these glasses as sealants in real stack designed to operate with electrolyte supported cells has not been investigated yet and presents the scope of this study.For this study, a series of glasses has been prepared as glass powders and characterized in terms of relevant thermophysical properties such as softening behavior, thermal expansion and formation of crystalline phases. La2O3-free sealing glasses established as standards for use in commercial stacks served as references and results of both groups of sealing glasses have been compared. The new La2O3-containing glasses showed CTE-values up 10.8 ppm K-1 at a glass transition temperature ranging between 640°C and 650°C for a partially crystallized microstructure. The softening and crystallization characteristics analyzed by hot stage microscopy and DSC/TGA allow sealing processes of stacks components in the temperature range between 900°C and 950°C. Helium-leakage rates of sealed sandwich type samples made of Crofer 22 APU showed values below 10-8 mbar l-1 s-1. From suitable glasses sandwich type sample sealings made of two Crofer 22 APU coupons have been prepared and exposed to a dual atmosphere in a special test rig under simulated electrolysis conditions at 850 °C with different electrical voltages applied across the sealants and a water content of 50 % in the steam atmosphere. In this out-of-stack environment, the resistance of the glass sealants can be recorded while being treated with high water atmosphere and electric voltage of up to 5 V at 850 °C. The development of resistance over 300 hours of aging show which glass composition fit to the conditions and which exhibit high degradation rates. Afterwards cross sections have been prepared and analyzed by SEM. Effects observed in the interfacial regions glass-Crofer 22 APU and glass-atmosphere have been correlated with the electrical measurements. A 30 layers stack was tested under typical SOEC conditions for approx. 3200 hours. Four different glass types were used and compared in terms of their degradation rate and the ASR value. The selection of the glasses was made according to their thermal expansion and softening behavior as well as their insulating properties.

NO Selective Catalytic Reduction over Atom-Pair Active Sites Accelerated via In Situ NO Oxidation.

Selective catalytic reduction (SCR) of NOx with NH3 is the most efficient technology for NOx emissions control, but the activity of catalysts decreases exponentially with the decrease in reaction temperature, hindering the application of the technology in low-temperature SCR to treat industrial stack gases. Here, we present an industrially practicable technology to significantly enhance the SCR activity at low temperatures (<250 °C). By introducing an appropriate amount of O3 into the simulated stack gas, we find that O3 can stoichiometrically oxidize NO to generate NO2, which enables NO reduction to follow the fast SCR mechanism so as to accelerate SCR at low temperatures, and, in particular, an increase in SCR rate by more than four times is observed over atom-pair V1-W1 active sites supported on TiO2(001) at 200 °C. Using operando SCR tests and in situ diffuse reflectance infrared Fourier transform spectra, we reveal that the introduction of O3 allows SCR to proceed along a NH4NO3-mediated Langmuir-Hinshelwood model, in which the adsorbed nitrate species speed up the re-oxidation of the catalytic sites that is the rate-limiting step of SCR, thus leading to the enhancement of activity at low temperatures. This technology could be applicable in the real stack gas conditions because O3 exclusively oxidizes NO even in the co-presence of SO2 and H2O, which provides a general strategy to improve low-temperature SCR efficacy from another perspective beyond designing catalysts.

Development and evaluation of a novel 3D simulation software for modelling wood stacks.

Assessing the solid wood content is crucial when acquiring stacked roundwood. A frequently used method for this is to multiply determined conversion factors by the measured gross volume. However, the conversion factors are influenced by several log and stack parameters. Although these parameters have been identified and studied, their individual influence has not yet been analyzed using a broad statistical basis. This is due to the considerable financial resources that the data collection entails. To overcome this shortcoming, a 3D-simulation model was developed. It generates virtual wood stacks of randomized composition based on one individual data set of logs, which may be real or defined by the user. In this study, the development and evaluation of the simulation model are presented. The model was evaluated by conducting a sensitivity and a quantitative analysis of the simulation outcomes based on real measurements of 405 logs of Norway spruce and 20 stacks constituted with these. The results of the simulation outcomes revealed a small overestimation of the net volume of real stacks: by 1.2% for net volume over bark and by 3.2% for net volume under bark. Furthermore, according to the calculated mean bias error (MBE), the model underestimates the gross volume by 0.02%. In addition, the results of the sensitivity analysis confirmed the capability of the model to adequately consider variations in the input parameters and to provide reliable outcomes.

Investigation and Quantification of Irreversible Degradations and Their Sensitivities during the Start-up Phases of PEMFC By an Accelerated Emulation Approach

In automotive application, frequent Start-up/Shut-down (SU/SD) phases of PEMFC system are prone to cause severe degrading conditions. Many studies have been achieved over the past decades to better understand and to mitigate MEA aging and performance decay due to the well-identified reverse current mechanism1. Indeed, the H2|Air front during H2 injection at the air-filled anode creates two sub-cells: H2/Air sub-cell operating in Fuel Cell mode and Air/Air sub-cell operating in Electrolysis Cell mode. This second sub-cell is subjected to high potential at cathodic side (about 1.6 V vs RHE), leading to harsh corrosion of both Pt nanoparticles and carbon support within the cathode catalyst layer. Two main mitigating solutions are generally proposed in the literature: (i) development of more stable materials and (ii) improvement of system control 2,3. Experimental DOE AST4 is usually performed only on small single cell to evaluate MEA component durability. Nevertheless, these tests cannot be fully correlated with real automotive operating conditions to predict PEMFC stack lifetime.In this work, more realistic AST protocols in regard to PEMFC stack/system configuration during start-up phase are proposed. Our experimental approach consists in emulating a representative transient cathodic potential profile (Figure a and b) during the H2|Air front formation and propagation along anodic channel using a 100 cm² single cell with a real stack active area design5. A comprehensive investigation and quantification of degradations caused by reverse current was carried out under SU cycling. Additional sensitivity study on the main operating parameters (potential profile, relative humidity and temperature) was performed to evaluate their respective impact on the cathode degradation and the performance losses. Periodic measurements such as polarization curves and cyclic voltammetry (Figure c) are coupled with CO/CO2 gas emission analysis (Figure d) during cycling at cathode to monitor and quantify the physical and electrochemical modifications within the MEA during our aging SU cycles.On the whole, this original experimental study proposes an overall degradation mechanism and these results are compared with model prediction to validate the most impactful parameters during reverse current phases at the real cell size and design. Ultimately, it will provide key-data to propose and optimize finely different mitigation strategies to be used in PEMFC stack and systems. References Reiser, C. A. et al. A reverse-current decay mechanism for fuel cells. Electrochemical and Solid-State Letters 8, A273–A276 (2005).Zhang, T., Wang, P., Chen, H. & Pei, P. A review of automotive proton exchange membrane fuel cell degradation under start-stop operating condition. Applied Energy 223, 249–262 (2018).Yu, Y. et al. A review on performance degradation of proton exchange membrane fuel cells during startup and shutdown processes: Causes, consequences, and mitigation strategies. Journal of Power Sources 205, 10–23 (2012).Fuel Cell Tech Team Accelerated Stress Test and Polarization Curve Protocols for PEM Fuel Cells. Energy.gov https://www.energy.gov/eere/fuelcells/downloads/fuel-cell-tech-team-accelerated-stress-test-and-polarization-curve.Randrianarizafy, B. Multi-physics modeling of startup and shutdown of a PEM fuel cell and study of the carbon support degradation : mitigation strategies and design optimization. (Université Grenoble Alpes, 2018). Figure 1

Evaluation of electrical load estimation in Diyala governorate (Baaquba city) based on fuzzy inference system

The evaluation of electrical load estimation is requisitely of any electrical power system. This manner is needed for system obligation, economical distribution and maintenance time of electrical system. In this paper, we propose electrical load estimation method based on fuzzy inference system which gives accurate results for estimated loads in Iraq (Diyala governorateBaaquba city). And it can assist the electrical generation and distribution system that depends on important parameters (temperature, humidity and the speed of the wind). By considering the parameters temperature, humidity and the speed of the wind. These parameters are applied as inputs to the fuzzy logic control system to obtain the normalize estimated load as output by electing membership functions. It is exceptionally valuable to form a choice by taking into consideration these assessed readings that come to from the proposed FIS that displayed in this paper with precision of 0.969 from the real stack request.

TEM Characterization of Long Term Aged Interconnect Oxide Layers from Real Stack

TEM Characterization of Long Term Aged Interconnect Oxide Layers from Real Stack

TEM Characterization of Long Term Aged Interconnect Oxide Layers from Real Stack

Steel interconnect robustness under SOFC conditions for extended operation remains a challenge. The observation and analysis of the interconnect microstructure aged in real conditions for long periods provides analysis data to help to understand degradation and potentially to design accelerated testing.In this study, TEM observations of the cross section of interconnects aged in real conditions (9000h and 20000h stacks) are presented. The interconnects are Crofer22APU with the air side coated with manganese cobalt oxide (MCO) to prevent chromium evaporation and increase oxidation resistance. FIB-prepared lamella from the inlet and outlet air side are analysed by TEM imaging and EDX. A major difference between inlet and outlet is the position of Mn-rich grains in the chromia scale: at the inlet, Mn-rich grains are located at the metal/chromia interface (figure 1(a)) while at the outlet, Mn-rich crystals are located close but not directly adjacent to the metallic bulk (figure 1(b)). The inlet microstructure has been observed in the literature in an oxidation study of MCO coated steel [1]. However, the outlet case which was observed for both the 9000h and 20000h aged stack samples was not yet reported in the literature, to the best of our knowledge. The difference between the inlet and outlet position was observed as well on a 15000h aged sample by SEM, and may be explained by differences between the inlet and outlet conditions: higher temperature at the outlet, as well as lower oxygen content. In addition to this particular Mn position, Fe particles are also observed in the chromia scale, confirming results previously described in the literature[2]. The analysis of the MCO coating also shows an inner denser layer, as expected after ageing at high temperature[3]. Analysis of the segregation of cobalt oxide at the MCO grain boundaries is also presented.More investigations are necessary to explain the complete oxidation mechanisms of the MCO-coated Crofer interconnect. However, the important role of the coating in interconnect oxidation is shown. The understanding of all reactions and the analysis of interconnects aged in real conditions, on different locations, should help designing more reliable combinations, better predictive models and possibly accelerated tests.[1] S. Molin et al., « Microstructural and electrical characterization of Mn-Co spinel protective coatings for solid oxide cell interconnects », J. Eur. Ceram. Soc., vol. 37, no 15, p. 4781-4791, déc. 2017, doi: 10.1016/j.jeurceramsoc.2017.07.011.[2] L. V. Gambino, N. J. Magdefrau, et M. Aindow, « Microstructural effects of the reduction step in reactive consolidation of manganese cobaltite coatings on Crofer 22 APU », Mater. High Temp., vol. 32, no 1-2, p. 142-147, janv. 2015, doi: 10.1179/0960340914Z.00000000090.[3] B. Talic, V. Venkatachalam, P. V. Hendriksen, et R. Kiebach, « Comparison of MnCo2O4 coated Crofer 22 H, 441, 430 as interconnects for intermediate-temperature solid oxide fuel cell stacks », J. Alloys Compd., vol. 821, p. 153229, avr. 2020, doi: 10.1016/j.jallcom.2019.153229. Figure 1

TEM Characterization of Long Term Aged Interconnect Oxide Layers from Real Stack

Interconnects from real SOFC stacks operated for 9000h and 20000h are characterized by means of SEM and TEM. This study focuses on the air side of the interconnect, which has been coated with manganese cobalt spinel oxide (MCO) doped with iron. Differences between the zones of air channel and interconnect rib are highlighted. Iron particles are found in the thermally grown oxide (TGO) exposed to the air channel. Manganese-rich crystals are observed close to the steel bulk for the rib contact zone, and in the bottom part of the TGO for the air channel zone. Cobalt is segregating at grain boundaries in the MCO after 20000h. More investigation is necessary to fully understand all mechanisms but the additional information gathered in the present study should help in better designing accelerated testing and modelling.

Detailed insight into processes of reversible solid oxide cells and stacks using DRT analysis

This study demonstrates applicability of the distribution of relaxation times (DRT) methodology to reveal more precise analysis of processes that occur in solid-oxide fuel (SOFC) and electrolysis (SOEC) cells of industrial-size as well as 10-layer stacks. Initially, we show that before starting to interpret the data observed, it is necessary to verify the quality of the impedance data measured in order to be used for further analysis. For the purpose of experimental study, operating parameters were varied and their impact on the system performance was determined. When operating cells and stacks very similar specific frequencies were identified, whereby for the stack one additional peak was observed. In the fuel cell mode the main frequency ranges to link with the processes that occur were identified to be 0.01–5 Hz and 50–100 Hz. In the electrolysis mode, the DRT amplitudes observed are lower, but specific frequencies higher, even by a factor of 10. In addition, specific behaviour of DRT-peaks could be correlated to the morphological changes of the Ni-based fuel electrodes that occurred during the stability tests performed. Summarizing this, the results presented in this study show how to: (i) distinguish different processes, (ii) link the knowledge gained from the analysis of single-cell level and subsequently, (iii) transfer it to real stack system. Thus, this study also represents a guideline for a detailed electrochemical performance characterization of high temperature fuel cell and electrolysis systems using the sophisticated DRT tool.

“Good Robot!”: Efficient Reinforcement Learning for Multi-Step Visual Tasks with Sim to Real Transfer

Current Reinforcement Learning (RL) algorithms struggle with long-horizon tasks where time can be wasted exploring dead ends and task progress may be easily reversed. We develop the SPOT framework, which explores within action safety zones, learns about unsafe regions without exploring them, and prioritizes experiences that reverse earlier progress to learn with remarkable efficiency. The SPOT framework successfully completes simulated trials of a variety of tasks, improving a baseline trial success rate from 13% to 100% when stacking 4 cubes, from 13% to 99% when creating rows of 4 cubes, and from 84% to 95% when clearing toys arranged in adversarial patterns. Efficiency with respect to actions per trial typically improves by 30% or more, while training takes just 1-20k actions, depending on the task. Furthermore, we demonstrate direct sim to real transfer. We are able to create real stacks in 100% of trials with 61% efficiency and real rows in 100% of trials with 59% efficiency by directly loading the simulation-trained model on the real robot with no additional real-world fine-tuning. To our knowledge, this is the first instance of reinforcement learning with successful sim to real transfer applied to long term multi-step tasks such as block-stacking and row-making with consideration of progress reversal. Code is available at https://github.com/jhu-lcsr/good_robot .

An Optimized Filtering Method of Massive Interferometric SAR Data for Urban Areas by Online Tensor Decomposition

The filtering of multi-pass synthetic aperture radar interferometry (InSAR) stack data is a necessary preprocessing step utilized to improve the accuracy of the object-based three-dimensional information inversion in urban area. InSAR stack data is composed of multi-temporal homogeneous data, which is regarded as a third-order tensor. The InSAR tensor can be filtered by data fusion, i.e., tensor decomposition, and these filters keep balance in the noise elimination and the fringe details preservation, especially with abrupt fringe change, e.g., the edge of urban structures. However, tensor decomposition based on batch processing cannot deal with few newly acquired interferograms filtering directly. The filtering of dynamic InSAR tensor is the inevitable challenge when processing InSAR stack data, where dynamic InSAR tensor denotes the size of InSAR tensor increases continuously due to the acquisition of new interferograms. Therefore, based on the online CANDECAMP/PARAFAC (CP) decomposition, we propose an online filter to fuse data and process the dynamic InSAR tensor, named OLCP-InSAR, which performs well especially for the urban area. In this method, CP rank is utilized to measure the tensor sparsity, which can maintain the structural features of the InSAR tensor. Additionally, CP rank estimation is applied as an important step to improve the robustness of Online CP decomposition - InSAR(OLCP-InSAR). Importing CP rank and outlier’s position as prior information, the filter fuses the noisy interferograms and decomposes the InSAR tensor to acquire the low rank information, i.e., filtered result. Moreover, this method can not only operate on tensor model, but also efficiently filter the new acquired interferogram as matrix model with the assistance of chosen low rank information. Compared with other tensor-based filters, e.g., high order robust principal component analysis (HoRPCA) and Kronecker-basis-representation multi-pass SAR interferometry (KBR-InSAR), and the widespread traditional filters operating on a single interferometric pair, e.g., Goldstein, non-local synthetic aperture radar (NL-SAR), non-local InSAR (NL-InSAR), and InSAR nonlocal block-matching 3-D (InSAR-BM3D), the effectiveness and robustness of OLCP-InSAR are proved in simulated and real InSAR stack data. Especially, OLCP-InSAR can maintain the fringe details at the regular building top with high noise intensity and high outlier ratio.

Optimal estimation of the Proton Exchange Membrane Fuel Cell model parameters based on extended version of Crow Search Algorithm

This paper presents a new method for optimal model parameters estimation of the Proton Exchange Membrane Fuel Cell (PEMFC). The purpose is to find the optimal parameters such that the integral of the absolute error (IAE) between the output voltage of the real stack and the achieved data has been minimized. Different metaheuristics can be used for the minimization. Most of them have their shortcomings. Therefore, in this study, a new improved version of the Crow Search Algorithm (ICSA) is suggested to increase the algorithm diversity and to prevent trapping in the local minimum. The study has been then applied to two practical case studies, Nexa PEMFC and NedSstack PS6 PEMFC and the results are compared with some state of art methods, COA, (N + λ) - ES algorithm and basic CSA to show the better fitting of the suggested algorithm toward the others. The results show that IAE value for the Nexa PEMFC in ICSA, CSA, COA, and EA are 14, 16, 15, and 23, respectively and the IAE value for the NedStack PS6 PEMFC in ICSA, CSA, COA, and EA are 1.52, 1.76, 1.63, and 2.08, respectively that indicate the higher performance of the suggested algorithm when applying to both case studies.

Comparison of off-gas utilization modes for solid oxide fuel cell stacks based on a semi-empirical parametric model

Solid oxide fuel cells (SOFCs) provide a clean and efficient pathway to use a variety of carbon containing fuels for power generation. Reuse or recycle of the high-temperature off-gas can help to improve the fuel or air utilization of SOFC stacks, as well as to improve system efficiency and reduce operating costs. The off-gas utilization mode and its effect on the stack performance need to be further clarified. In this study, a novel semi-empirical parametric stack model was established based on the experimental data of a 3-cell SOFC stack. This parametric model can accurately predict the output performance of real stacks, which was verified by multi-condition experiments. Based on this stack model, one basic process and four off-gas utilization modes were designed and constructed. Comparative analysis of these modes under different operation parameters was carried out. The effects of key operation parameters, including fuel flow rate, operating current, stack temperature, pre-reformer temperature, split ratio and recycle ratio on the stack performance were studied, which were characterized by the electrical efficiency and exergy efficiency of the power module. It was found that the mode with two-stage stacks can get the maximum electrical efficiency (>45%) with the operating current less than 20 A, while the mode with anode off-gas recycled to the pre-reformer instead obtained the maximum electrical efficiency (>35%) when the specified current was larger than 25 A. A comprehensive and in-depth perspective on different stack off-gas utilization modes is provided, which can supply feasible guidance for the next-step overall system design. Besides, the stack modeling and process simulation method developed in this study can provide reference for related research.

(Invited) Small Angle Scattering As Unique Probe to Quantify Water Distribution and Ionomer Structure in Operating PEMFC

The Proton Exchange Membrane Fuel Cell (PEMFC) is a highly complex energy conversion device coupling several electrochemical and transport phenomena occurring on a wide range of length-scales within a multilayered structure, named Membrane Electrodes Assembly (MEA). The fundamental description of all phenomena involved in PEMFC operation and degradation, and more particularly their interplay, is a contemplated pivotal requirement for improvements. This is a grand challenge, as a number of unsolved questions remain. In fact, all relevant physical and electrochemical phenomena are not fully identified or quantified. In the PEMFC, water plays a crucial role, since it is not only the main product of the electrochemical reaction but, above all, it is the fundamental common denominator to all the phenomena. In a real stack, as an open system converting H2 and O2, the operation and degradation are intrinsically heterogeneous resulting especially in a complex 3D distribution of water in the volume of the MEA. Among the most important component of the MEA is the ionomer, made of a PerFluoroSulfonic Acid (PFSA) polymer, which acts as a solid electrolyte to transfer proton and water to, from, and within the electrodes. The presence of highly hydrophilic sulfonic acid groups makes it, most probably, the most sensitive MEA component toward water activity/content. Indeed, it governs its swelling, structure as well as transport and mechanical properties. This property of the ionomer, along with its role, explains why PEMFC behaviour is so dependent on water activity. Still very recently, a work of Kongkanand et al.[1] emphasizes how ionomer is a crucial electrodes' constituent in view of enhancing performance. For these reasons, many researches are still devoted to studying the ionomer in relation with water activity and as well as to quantifying the 3D water distribution during PEMFC operation. It is well known that Small Angle Scattering (SAS) is the most suitable technique to characterize the structure of the ionomer[2]. A side from that, neutron is the probe of choice to quantify water within operating PEMFC[3]. For these reasons, we have been developing for several years SAS techniques, either with Neutrons or X-Ray, in order to measure simultaneously and operando, the water distribution and the ionomer nanostructure. Spatial and time resolutions offew µm² and 50 ms, respectively, have been achieved with the last generation of X-Rays sources such as the one of European Synchrotron Radiation Facility (ESRF), and ~4 mm² and 30 s with high neutron flux on D22 beamline at Laue Langevin Institute (ILL)[4],[5]. Thus, it is possible to follow the kinetics during transient. In particular, we have shown that, within the membrane, local water content is greater in front of the current collecting rib than in front of the gas supply channel. We have also evidenced some evolution of the physical structure of the membrane upon aging in real conditions depending on the local water content. We thus pushed forward these techniques, which now allows us to have unique information on the local water activity and ionomer structure within the CL. Our results demonstrate how SAS techniques are powerful to assess the main current scientific and technological issues of the PEMFC. [1] Kongkanand et al., ACS Energy Lett. 2018, 3, 618−621 [2] Fumagalli et al., J. Phys. Chem. B 2015, 119, 7068−7076 [3] Boillat et al., Current Opinion in Electrochemistry 2017, 5, 3–10 [4] Morin et al., J. Electrochem. Soc. 2016, 163, F9-F21 [5] Martinez et al., ACS Advanced Energy Materials 2019 Figure 1

Operating limitation and degradation modeling of micro solid oxide fuel cell-combined heat and power system

The micro Solid Oxide Fuel Cell-Combined Heat and Power (SOFC-CHP) System is one of the most promising technologies of distributed generation and has been commercialized in Japan and Europe. However, the performance degradation of an SOFC stack still exists, causing the ratio of power and heat to change and the Joule heat to increase in the stack. This phenomenon should not have been neglected. A zero-dimension SOFC model including basic Nernst, ohmic, activation and concentration voltage loss was used to simulate the micro SOFC-CHP system, and a degradation model was also applied to modeling the power change with respect to operating time in different situations. Both of the models were verified by real stacks experiment data and the effectiveness was proved in system-level modeling. Three key operating parameters of the system were promoted: stack current I, excess air factor α and fuel utilization Uf. Given the safe temperature ranges of an SOFC stack, combustor and reformer, the most appropriate parameter range was illustrated to be limited in a shape of a small triangle. Within the safe parameter range, while the excess air factor α was fixed at 2.64, the electrical efficiency reached a maximum 44.9% (HHV) at 120 A, while the electric power output of SOFC and hot water were both 5.58 kW. As for the dynamic modeling, by comparing constant current, constant voltage and constant power three operating patterns, results showed that the constant current pattern had the longest lifetime and the constant power pattern followed.

Development of Large-Scale and Quasi Multi-Physics Model for Whole Structure of the Typical Solid Oxide Fuel Cell Stacks

Although the performance and corresponding manufacturing technology of solid oxide fuel cells (SOFC) units have greatly improved and have met commercial requirements over the past decades, they are constructed such that they perform poorly and lack strong duration outputs. Therefore, achieving high performance and extending duration at a stack level are challenges faced by the development process. This paper develops a large-scale and multiphysics model for the complete structure of a typical 10-cell SOFC stack. It includes solid components, flow paths, and porous sections—solid ribs, interconnectors, anode support, anode function layer, electrolyte layer, cathode layer, air/fuel feed manifolds, feed header, rib channels, exhaust header and outlet manifolds. The multiphysics application includes momentum, mass, energy and quasi electrochemical transporting; and their mutual coupling processes within the stack. This new model can help us understand the working specifics of the large-scale stack, obtaining distribution details of static pressure, species fraction, and temperature gradient; further addressing optimization of structure and operation parameters. These details serve as guidelines for practical structural designs and parameters in real stack levels.

CO2 utilization via a novel anaerobic bioprocess configuration with simulated gas mixture and real stack gas samples

ABSTRACTCO2, which is considered to be one of the major causes of climate change, has reached to critical levels in the atmosphere due to tremendous consumption of fossil fuels all over the world. In this study, anaerobic bioconversion of CO2 into bio-methane using a novel bioprocess configuration (HYBRID bioreactor) was studied under mesophilic conditions. Varying ratios of H2/CO2 gas mixture and volumetric feeding rates were investigated and no additional organic matter and trace element were needed throughout the study. The maximum methane production of 19 m3 CH4/m3 reactor/d was achieved at a H2/CO2 ratio of 4:1 and feeding rate of 24 m3 gas/m3 reactor/d. It was determined that H2 conversion rate is about 96%. For demonstration purpose, real stack gas sample from a petrochemical industry was also tested under optimized operational conditions. No inhibitory effect from stack gas mixture was observed. This study provided an environmentally friendly and sustainable solution for industries such as petrochemical industry in order to produce extra energy while capturing their waste CO2. Thereby, a sustainable and environmentally friendly model solution was presented for industries with high CO2 emissions.

Glass ceramics sealants for SOFC interconnects based on a high chromium sinter alloy

AbstractThe operation of solid oxide fuel cells requires gas tight and stable sealing materials. Glass ceramic materials have been established to seal different stack designs and can be optimized to match the requirements of components and joining conditions. It is known that glass‐based sealants are sensitive to the formation of undesired chromia species in contact with ferritic steels used as interconnects. Their formation depends on the earth alkaline type and content in the glass as well as the chromium content of the interconnect. A robust interconnect material is the chromium‐based sinter alloy CFY with &gt;93 wt.% chromium what in turn enforces the formation of chromia phases. SOFC stacks with chromium‐based interconnects and electrolyte supported cells with 10SCSZ electrolyte are candidates for SOFC systems with high efficiency in a wide power range. This study presents results on the development of sealing glasses adapted for the CFY by optimizing the glass components BaO, CaO, SiO2, and Al2O3. Depending properties like crystallization behavior, thermal expansion, adhesion and reactivity in contact with CFY have been regarded. The glass has been tested under operating conditions of SOFC (dual atmosphere, electric potentials) in samples as well as in real stacks. It has been shown that optimized glass compositions, especially regarding a limited BaO content combine a good joining behavior and minimized BaCrO4 formation in contact with the CFY alloy.

Influence of Operating Parameters and System Design on Efficiency of Biomass and Biogas Based SOFC Systems

With the recent COP 21 declaration many countries have dedicated themselves to a clear path towards decarbonization. This requires major installation of wind and solar power. However, these only provide intermittent electricity. Thus, balance power has to be provided, ideally from renewable resources. Currently only biomass seems to be available in large enough quantity to provide significant balancing power. A wide range of biomass feedstocks can be made available for power generation for example via anaerobic digestion or thermo-chemical gasification. However, resources are still limited. Hence, in order to maximize its potential the biomass has to be used at the highest possible efficiency. Electrochemical conversion, especially in SOFC, in general offers very high efficiency. However, also in SOFC systems the choice of the optimal system design and operating parameters can make a big difference. The presented work investigates utilization of biogas and syngas from biomass gasification in different system designs. This is done using a thermodynamic SOFC model built in Aspen Plus, which has been validated against literature data. A comparison to cell and stack performance of the well known manufacturers Fuel Cell Energy and Forschungszentrum Jülich is presented. It is shown that at identical operating conditions the model shows slightly worse performance than the real stacks. With the model, the influence of different fuel pre-treatment and pre-reforming options are compared at a fixed stack size. Furthermore the effect of anode offgas recirculation, as well as the choice of fuel utilization, oxidant, operating temperature and pressure are investigated. During the analysis operating conditions prone to the risk of carbon deposition are excluded. Also the impact on the balance of plant is taken into account. Results show, that depending on the fuel, system design and parameters the electrical efficiency of the fuel cell alone can vary between 35 and 71%. From this, depending on the system parasitic consumption and losses of up to 8% points have to be subtracted. Thus, net AC efficiencies reach 30 to 63%. Finally a new system design and operating regime is proposed, which can significantly raise the DC efficiency while reducing the parasitic consumption by adjusting the gas treatment and operating parameters. Modelling results of new design show net AC system efficiencies of up to 72%.

Development of a Segmented Single SOFC Test Equipment to Simulate Stack-Like Operating Conditions

A common problem when it comes to stack CFD modelling is the lack of detailed local information about temperatures and gas compositions. For the calibration and validation of the CFD models local measurements of these parameters are required. However, it is fairly difficult implementing such measurements in commercial stacks without changing the stack design to allow for additional thermocouples or gas extraction channels. Furthermore, such implementations would automatically change the stack behavior compared to a stack without additional instrumentation due to heat and gas losses caused by the measurement itself. On the other hand standardized single cell tests do not represent stack like conditions due to typically very homogenous temperature distributions. Therefore, a new single cell test equipment based on a segmented single SOFC which enables an operation of the cell under stack like conditions was developed. The aim of this development is to provide a single cell test equipment which provides comprehensive data about temperature distribution, cell voltage distribution and local gas compositions to be used for the calibration of a commercial CFD code [1]. The complete test equipment was simulated by means of CFD and the results were compared with experimental data. The single cell consists of a commercial anode supported SOFC with 4 segmented cathodes (40.5x44 mm2 each) and 1 anode (120x120 mm2). The segmented cathode allows the measurement of local cell voltages. Furthermore, several gas extraction channels along the anode flow channels were implemented to measure the gas conversion with respect to the electrochemical and steam reforming reactions taking place. As the steam reforming reaction has a significant impact on the temperature profile, in real stacks it is important to consider the conversion rate properly along the anode to allow for a correct simulation of the temperature distribution. The temperature profile was measured with 34 thermocouples which were distributed along the anode flow field below the anode surface. Several test cases to calibrate the CFD model regarding the thermal behavior were carried out by placing the test equipment in an electric oven and heating it up to 600 °C. The cell test equipment was additionally equipped with 4 electrical heaters placed below the anode channels to produce temperature profiles similar to real stack operating conditions. Therefore, also inhomogeneous temperature profiles can be produced. The 4 electrical heaters can be controlled individually to heat each of the 4 segments differently. The electrical heaters provide enough heat to increase the temperature of the cell up to 800 °C. Therefore, temperature profiles along the cell with gradients of 150 K between the hottest and coldest spot along the active cell area were achieved. By changing the set oven temperature this gradient can be further modified. Existing stack temperature profiles were used to calibrate the test equipment which enables similar temperature distributions as they occur for a stack under real operation. The CFD simulation meets the measured temperature profile within 10 K which is seen to be a very accurate result. Furthermore, this work will show the comparison of experimental and CFD simulation results for the operation of the cell at 700, 750 and 800 °C in H2 and CH4/H2O operation. The comparison covers local cell voltages, the temperature distribution and the gas compositions. For both gas feed mixtures a fitting of the CV curve within the CFD software was carried out. It can be seen that the electrochemical behavior can be simulated fairly accurately by the software. The software allows also for the consideration of steam reforming and shift reactions along the complete anode flow path. By adjusting the activation energy as well as the rate-constant for both reactions, the same behavior as it was measured in the experiment could be achieved. As expected it can be seen that the conversion of methane by steam reforming is higher at the inlet of the single cell. This effect correlates very well with the observed temperature decrease in this region caused by endothermic steam reforming which is in the range of 10 K. It has been proven that this kind of test equipment is a very powerful tool to calibrate the CFD simulation models as basis for further full scale stack simulations. A further potential utilization of this approach is the possibility to transfer the validated temperature profiles into a FE simulation to enable an experimentally supported investigation of thermo-mechanical stresses within the cell or sealing. [1] FIRETM v2017, User Manual, AVL List GmbH, 2017.