3D transient thermomechanical behaviour of a full scale SOFC short stack

Hermetic sealing of planar solid oxide fuel cell components is a critical issue. The long term operation and structural reliability of the fuel cell stacks depend strongly on the thermomechanically induced stress–strain behaviour of the fuel cell stack. These are especially affected through the thermal transients, which the fuel cell stack is subjected to, over time. Hence, the thermomechanical characterisation of the fuel cell stack during thermal cycling is indispensable. The current paper elucidates a fully three dimensional thermomechanical analysis of a planar type SOFC short stack over a whole thermal cycle. A coupled computational fluid dynamics and computational structural mechanics analysis has been performed. Typical stack components i.e., cell component, wire-mesh, metal frame, interconnector plates and sealant materials have been considered. The model represents the physical resolution of the air channels and the manifold regions. The non-linear elasto-plastic behaviour of the metal components as a function of temperature is considered. The study gives an insight about the transient thermal behaviour of a full scale fuel cell stack and its thermomechanical response, determining the mechanisms that trigger the thermomechanically induced stress during the heating-up, operation and shut-down stages.     

Development of cube-type SOFC stacks using anode-supported tubular cells

Tubular SOFCs have shown many desirable characteristics such as high thermal stability during rapid heat cycling and large electrode area per unit volume, which can accelerate to realize SOFC systems applicable to portable devices and auxiliary power units for automobile. So far, we have developed anode-supported tubular SOFCs with 0.8–2 mm diameter using Gd-doped CeO₂ (GDC) electrolyte, NiO-GDC anode and (La, Sr)(Co, Fe)O₃ (LSCF)-GDC cathode. In this study, a newly developed cube-type SOFC stack which consists of three SOFC bundles was designed and examined. The bundle consists of three 2 mm diameter tubular SOFCs and a rectangular shaped cathode support where these tubular cells are arranged in parallel. The performance of the stack whose volume is less than 1 cm³ was shown to be 2.8 V OCV and over 1 W at 1.6 V under 500 °C. Cathode loss factor due to current collection from cathode matrix was also estimated using a proposed model.     

Cube-type micro SOFC stacks using sub-millimeter tubular SOFCs

Fabrication and characterization of tubular SOFCs under sub-millimeter (0.8 mm), bundles and stacks for low temperature operation were shown. The materials used in this study were Gd doped CeO₂ (GDC) for electrolyte, NiO–GDC for anode and (La, Sr)(Co, Fe)O₃ (LSCF)–GDC for cathode, respectively, and LSCF for supports of the tubular cells for bundle fabrication. After applying a sealing layer and current collector for each bundle of five micro tubular SOFCs, each bundle was stacked vertically, to build a four-storey cube-type stack with volume of about 0.8 cm³. The performance of the stack was shown to be 3.6 V OCV and 2 W maximum output power under 500 °C operating temperature. Preliminary quick start-up test was also conducted at the condition of 3 min start-up time from 150 to 400 °C for 5 times, and the results showed no degradation of the performance during the test.     

Influence of current densities in SOFC and PEFC stacks on a SOFC–PEFC combined system

A solid oxide fuel cell (SOFC)–polymer electrolyte fuel cell (PEFC) combined system was investigated by numerical simulation. Here, the effect of the current densities in the SOFC and the PEFC stacks on the system's performance is evaluated under a constant fuel utilization condition. It is shown that the SOFC–PEFC system has an optimal combination of current densities, for which the electrical efficiency is highest. The optimal combination exists because the cell voltage in one stack increases and that of the other stack decreases when the current densities are changed. It is clarified that there is an optimal size of the PEFC stack in the parallel-fuel-feeding-type SOFC–PEFC system from the viewpoint of efficiency, although a larger PEFC stack always leads to higher electrical efficiency in the series-fuel-feeding-type SOFC–PEFC system. The 40 kW-class PEFC stack is suitable for the 110 kW-class SOFC stack in the parallel-fuel-feeding type SOFC–PEFC system.     

Computational analysis of thermo-fluid and electrochemical characteristics of MOLB-type SOFC stacks

A 3D simulation tool for solid oxide fuel cells (SOFCs) was described to simulate the mass, momentum and energy conversions in the mono-block layers built (MOLB)-type SOFC system. Considering the co-flow and counter-flow cell designs, the temperature distributions, variations of reaction species and current densities of the single-unit cell were calculated under the different working conditions. The simulation results show that the co-flow case has more uniform temperature and current density distributions. Similar to the planar SOFC, in co-flow case, increasing fuel delivery rate or hydrogen mass fraction in the fuel, average temperatures of PEN (positive/electrolyte/negative) and current densities rise, but the average temperatures of PEN decrease with increasing the delivery rate of air. In particular, MOLB-type SOFC has some advantages such as: higher hydrogen utilizations, lower temperature difference and higher current density. However the current density distributions are less uniform in MOLB-type SOFC, which is a disadvantage in this type SOFC.     

Solar absorption behaviour of multilayer stacks

In this paper, the absorption spectra of multilayer structures have been obtained and analysed in the wavelength range 0.5–10 μm. The multilayer system is an SMS type in which M is a thin metallic layer sandwiched between two semiconducting layers. The systems (i) B.S./Cu/B.S. and (ii) Cu₂S/Cu/Cu₂S (where B.S. = black sulphide) are developed by the methods of electrodeposition/chemical dipping, whereas the ZnS/Cu/ZnS system is fabricated in vacuo. The /ε ratios are also calculated using the experimentally observed absorption spectra, and found to be higher in the case of the ZnS/Cu/ZnS system, in comparison to the other two systems. The absorption spectra have also been calculated using our theoretical model of the above-mentioned systems.     

SOFC stacks for mobile applications with excellent robustness towards thermal stresses

Solid oxide fuel cells (SOFC) are attractive power units for mobile applications, like auxiliary power units or range extenders, due to high electrical efficiencies, avoidance of noble metals, fuel flexibility ranging from hydrogen to hydrogen carriers such as ammonia, methanol or e-gas, and tolerance towards CO and other fuel impurities. Among challenges hindering more wide-spread use are the robustness under thermal cycling. The current study employs short stacks containing anode or metal supported SOFCs, which were subjected to thermal cycles in a furnace and under more realistic conditions without external furnace. Heating from 100 °C to operating temperature was accomplished by sending hot air through the cathode compartment and heating from bottom (and top) of the stack, reaching a fastest ramping time of ca. 1 h. The stacks remained intact under severe temperature gradients of at least 20 °C/cm for anode supported and 30 °C/cm for metal supported SOFCs.     

External temperature field test and leakage fault diagnosis for SOFC stacks

The uniformity of Solid Oxide Fuel Cell (SOFC) stacks largely affects the stability and performance of the SOFC power generation system with multiple stacks. However, the health conditions of the stacks are usually assessed by the long-time electrochemical performance test, and some small faults are not able to be quickly identified when the I–V performances have small differences. In this paper, a non-contact temperature sensor array constructed by thermocouples is proposed for NDE (non-destructive evaluation) application in SOFC stacks. The temperature sensor array is fixed near the outer surfaces of the 1 kW-class stack to measure the external temperature field of the stack. Based on the experimental results, it is found that the proposed method is feasible for the examination of the external temperature field of stacks under different operation conditions, which helps to screening the stacks before assembling to the system. At the same time, a real-time online monitoring of fuel leakage is performed using the temperature sensor array, and stacks with unusual temperature distribution due to the leakages can be quickly recognized by comparing with the healthy stack.     

Development of MnCoO coating with new aluminizing process for planar SOFC stacks

Chromia-forming ferritic stainless steels find widespread use as interconnect materials in SOFCs at operating temperatures below 800 °C, because of their thermal expansion match and low cost. However, volatile Cr-containing species originating from this scale can poison the cathode material in the cells and subsequently cause power degradation in the devices. To prevent this, a conductive manganese cobaltite spinel coating has been developed, but unfortunately; this coating is not compatible with glass-based seals between the interconnect or cell frame components and the ceramic cell due to reactions between the coating and the glass. Thus, a new aluminizing process has been developed to improve the stability of the sealing regions of these components, as well as for other metallic stack and balance-of-plant components.     

Chemical interaction between glass–ceramic sealants and interconnect steels in SOFC stacks

In order to gain insight into the mechanism causing performance degradation and/or failure, stacks of planar solid oxide fuel cells (SOFC) are routinely dismantled and examined after operation at Forschungszentrum Jülich. The post-operation inspection focuses in particular on the chemical and mechanical compatibility aspects of cell and stack materials.In the present work a short-term degradation effect is addressed, which was found to be caused by unwanted chemical interactions between glass–ceramic sealants and ferritic steel interconnects. The post-operation inspection revealed severe steel corrosion along the seal rims. Under SOFC stack conditions rapidly growing oxide nodules were observed bridging the 200 μm seal gap between the metallic components after a few hundred hours of operation. These oxide nodules, rich in iron, gave rise to local short-circuiting effects eventually resulting in stack failure.The present study, combined with recent model investigations triggered by the stack results, indicates that severe degradation only occurs in the case of glass–ceramic sealants which contain minor amounts of PbO. Furthermore, the rate of corrosion attack of the metallic components strongly depends on the silicon (Si) content of the ferritic steel. The stack tests suggest that increasing the Si content increases the corrosion rate, and thus detrimentally influences the stack performance.     

Performance of a glass-ceramic sealant in a SOFC short stack

The development of sealants for solid oxide fuel cells (SOFCs) is a significant challenge as they must meet very restrictive requirements; they must withstand the severe environment of the SOFC (i.e., be resistant to oxidative and reducing gas environments) and be thermo-chemically and thermo-mechanically compatible with the materials to which they are in contact with. This work discusses the design and the operation of two SOFC short stacks (based on planar anode-supported cells) along with the performance of a glass ceramic sealant inside the stack.     

Mechanical reliability and durability of SOFC stacks. Part II: Modelling of mechanical failures during ageing and cycling

Intricate relationships between mechanical and electrochemical degradation aspects likely affect the durability of solid oxide fuel cell stacks. This study presents a modelling framework that combines thermo-electrochemical models including degradation and a contact thermo-mechanical model that considers rate-independent plasticity and creep of the components materials and the shrinkage of the nickel-based anode during thermal cycling. This Part II investigates separately or together the contributions of mechanical and electrochemical degradation on the behaviour during long-term operation and thermal cycling.     

Dynamic tank in series modeling of direct internal reforming SOFC

A dynamic tank in series reactor model of a direct internally reforming solid oxide fuel cell is presented and validated using experimental data as well as a computational fluid dynamics (CFD) model for the spatial profiles. The effect of the flow distribution pattern at the inlet manifold on the cell performance is studied with this model. The tank in series reactor model provides a reasonable understanding of the spatio‐temporal distribution of the key parameters at a much lesser computational cost when compared to CFD methods. The predicted V–I curves agree well with the experimental data at different inlet flows and temperatures, with a difference of less than ±1.5%. In addition, comparison of the steady‐state results with two‐dimensional contours from a CFD model demonstrates the success of the adopted approach of adjusting the flow distribution pattern at the inlet boundaries of different continuous stirred tank reactor compartments. The spatial variation of the temperature of the PEN structure is captured along with the distributions of the current density and the anode activation over‐potential that strongly related to the temperature as well as the species molar fractions. It is found that, under the influence of the flow distribution pattern and reaction rates, the dynamic responses to step changes in voltage (from 0.819 to 0.84 V), fuel flow (15%) and temperature changes (30 °C), on anode side and on cathode side, highly depend on the spatial locations in the cell. In general, the inlet points attain steady state rapidly compared to other regions. Copyright © 2017 John Wiley & Sons, Ltd.     

Dynamic temperature modeling of an SOFC using least squares support vector machines

Cell temperature control plays a crucial role in SOFC operation. In order to design effective temperature control strategies by model-based control methods, a dynamic temperature model of an SOFC is presented in this paper using least squares support vector machines (LS-SVMs). The nonlinear temperature dynamics of the SOFC is represented by a nonlinear autoregressive with exogenous inputs (NARXs) model that is implemented using an LS-SVM regression model. Issues concerning the development of the LS-SVM temperature model are discussed in detail, including variable selection, training set construction and tuning of the LS-SVM parameters (usually referred to as hyperparameters). Comprehensive validation tests demonstrate that the developed LS-SVM model is sufficiently accurate to be used independently from the SOFC process, emulating its temperature response from the only process input information over a relatively wide operating range. The powerful ability of the LS-SVM temperature model benefits from the approaches of constructing the training set and tuning hyperparameters automatically by the genetic algorithm (GA), besides the modeling method itself. The proposed LS-SVM temperature model can be conveniently employed to design temperature control strategies of the SOFC.     

Dynamic thermal behaviour of a wall

A new technique for dealing with the problem of the numerical simulation of heat conduction in solids is presented. This method is based on that of Milne but does not have any limitation in the time step amplitude. It has, therefore, the advantage of being more accurate and requires less computational time than the traditional methods. The mathematics of the method are shown and a comparison with the Crank-Nicolson technique is made.     

Mechanical reliability and durability of SOFC stacks. Part I : Modelling of the effect of operating conditions and design alternatives on the reliability

Electrochemical and mechanical aspects in solid oxide fuel cell stack must be understood to meet the reliability targets for market implementation. This study presents a stack modelling framework that combines thermo-electrochemical models, including degradation and a contact finite-element thermo-mechanical model. It considers rate-independent plasticity and creep of the component materials and proposes periodic boundary conditions to model the stacking of repeating units. This Part I focuses on the effects of the operating conditions and design alternatives.     

Dynamic behaviour of baffled solar air heaters

A mathematical model describes the dynamic thermal behaviour of a baffled solar air heater is presented. The transient behaviour of the heater results from sudden changes in the intensity of the incident solar radiation and the inlet fluid temperature. In terms of the presented model, analytical solutions for the fluid and solid domains are derived. Also, expression for the thermal efficiency of the heater is presented. The effects of different design parameters on the thermal performance of the heater are investigated. The validity of the theoretical model is verified experimentally where it is found that both theoretical and experimental results are in a good agreement.     

Low-cost air-cooled PEFC stacks

The general objective of the work was to adapt the existing technology of polymer electrolyte fuel cells (PEFC) to the demands for commercial applications and to make it competitive with respect to performance and costs compared to the present day technology of combustion engines. This should be realised by the use of optimised electrodes which enable a highly efficient operation with air at low overpressure and a simplified construction of the electrochemical cell which is capable of mass production and low-cost. The initial project cost goal (materials and fabrication, extrapolated to a production of 100,000 fuel cell systems per year) was 100–200 ECU/kW. The feasibility of this goal was planned to be ascertained by the development of a PEFC stack in the size of 3.5 kW rated power.     

Ageing of anode-supported solid oxide fuel cell stacks including thermal cycling,and expansion behaviour of MgO–NiO anodes

This paper reports on medium term tests of anode-supported five-cell short stacks, as well as on some separate anode development. Two stacks were operated under steady-state conditions: one with unprotected metal interconnects, H₂ fuel and 0.35 A cm⁻² (40% fuel utilisation) polarisation current showed an average cell voltage degradation of 56 mV per 1000 h for 2750 h; one with coated metal interconnects, synthetic reformate fuel and 0.5 A cm⁻² (60% fuel utilisation) polarisation current showed an averaged cell voltage degradation slope of 6.6 mV per 1000 h for 800 h before a power cut prematurely interrupted the test. A third stack was subjected to 13 complete thermal cycles over 1000 h, average cell voltage degradation was evaluated to −2 mV per cycle for operation at 0.3 A cm⁻², open circuit voltage (OCV) remained stable, whereas area specific resistance (ASR) increase amounted on average to 0.008 Ω cm² per cycle.