Planar SOFC Research Articles

Development, Manufacturing and Deployment of SOFC-Based Products at SOLIDpower

SOLIDpower provides efficient energy solutions based on its proprietary planar SOFC technology. The first commercial product of the company is the EnGen™-2500. The company deploys the CE certified 2.5 kW combined heat and power generator within the FCH-JU project ene.field and with selected European partners. The EnGen™-2500 uses natural gas for combined heat and power generation (CHP) at high electrical and total efficiencies. The product targets multi-family houses and commercial applications. More than 60 units are to be installed within 2015 and field test results will be presented. SOLIDpower operates a first “to scale” production cell in Northern Italy with a production capacity of 2MW/yr. Based on an ordering volume of 50 MW, the stack and balance of stack components match target market requirements, allowing selling and operating systems at grid parity prices. Besides the micro-CHP generator deployment, SOFCpower also pursues strategic development activities to demonstrate biogas and waste-to-energy (WTE) applications on larger than 2.5 kW scale. SOLIDpower progressed on the technological level for SO Electrolysis. Improved electrode materials and production technology open the door for lifetime for electrolysers stacks beyond 16’000 hrs. SOLIDpower expects that SOFC stack cost reductions stemming from SOFC volume manufacturing can be transferred to a large extend also to SOE stacks. In this field, SOLIDpower intends to work with integration partners on the systems level. The paper provides an update of the stack and system development, including operational results of SOFC-based micro-CHP.

An Accurate and Reliable Computational Approach for the Mechanical Stress Analysis of Planar SOFC Stack

Mechanical damage is a major factor limiting the long-term stability of SOFCs. It is a common practice to perform the stress analysis by isolating the interested component of an SOFC stack. This approach is questionable as it is difficult or even impossible to properly define the boundary condition for the isolated component. Here we show that the conventional approach is prone to provide misleading results. Grid settings are tested to reveal the grid dependence of the stress analysis results. A modest grid size is shown to be computationally efficient and satisfactorily accurate, allowing for the stress analysis of 20-40-cell stack by a desktop computer. Tests also show that the common practice with a free boundary condition for the interested component yields the maximal stress that is one order of magnitude too small.

A High Resolution Multi-Physics Numerical Model for Planar SOFC Stack

We report a successful development of a high space resolution multi-physics mathematical model for full scale planar SOFC stacks which is, to the best of our knowledge, the first of its kind. This 3D 30-cell stack model takes into account the coupled physical processes, applicable for both methane and hydrogen based fuels. Every cell consists of 30 channel-rib pitches, i.e., there are 900 repeating units for the electrochemically active area of the 30-cell stack. The computational domain contains all components of the stack, including channels, internal manifolds, PEN assemblies, interconnects, seals and frames. This model is constructed using the CFD software, FLUENT®, supplemented with in-house developed external subroutines to ensure all governing equations and physical properties are correctly coupled and solved successfully. Multi-physics numerical simulations of this truly high resolution computational model of 30-cell stack have been successfully performed in a 2-CPU/16-core PC.

Interdiffusion and Charge Transport Across Surface-Modified Current Collectors in Planar SOFCs

Current collectors are a key component of planar SOFCs, separating air and fuel supplied onto the electrodes and connecting the cells in series. One of main challenges in the SOFC technology developments is to suppress degradation processes, often associated with the interconnect materials, and to provide low contact resistivity in oxidizing atmospheres. The present work is focused on the studies of near-surface interdiffusion phenomena in Crofer 22 APU ferritic steel interconnects with Ni-based protective layers. Particular emphasis was centered on the area-specific resistance (ASR) between the current collectors and La0.8Sr0.2MnO3 (LSM) cathodes, which exhibits time dependencies governed by the protective interlayer composition and interface microstructure alterations. The ASR changes, tested during over 30,000 hours at atmospheric oxygen pressure, can be described in terms of a model assuming that the current across the interconnector | LSM interface is essentially controlled by electron transfer via the interfacial Schottky barrier. The experimental observations validate this approach, explaining the junction resistivity and Schottky barrier height variations as a result of metal interdiffusion between the current collector and Ni-based protective coating.

SOFC Stack Operation: Effects of Electrode-Interconnect Contacts

Planar SOFC multi-cell stacks incorporating metallic interconnects generally exhibit average cell performance lower than that observed for out-of-stack cells under similar conditions. It has been suggested that electrode-interconnect, especially cathode-interconnect, contacts (and their ohmic resistances) are the main cause for this observed difference. In addition, cathode-interconnect contacts can evolve in long-term operation, resulting in stack performance reduction/degradation with time. Experiments using current interruption, AC electrochemical impedance and other characterization techniques have been conducted on single-cell stacks and button cells in contact with interconnect materials. The data obtained to date confirmed the hypothesis that contact ohmic resistance significantly contributes to stack performance losses. This paper summarizes and discusses experimental results on the effects of cathode-interconnect contacts on stack performance.

Dynamic Response Analysis of the Planar and Tubular Solid Oxide Fuel Cells to the Inlet Air Mass Flow Rate Variation

The purpose of present study is to investigate the dynamic response of two conventional types of solid oxide fuel cells to the inlet air mass flow rate variation. A dynamic compartmental model based on CFD principles is developed for two typical planar and tubular SOFC designs. The model accounts for transport processes (heat and mass transfer), diffusion processes, electrochemical processes, anode and cathode activation and ohmic polarizations, among others. Using developed model the dynamic response of the cell to the step change of the air feed stream conditions is investigated. The results show an almost slow electrical response of the cell to the air mass flow rate step variation that is estimated to be about one hour. Moreover, it can be concluded that the effect of the inlet air flow conditions on a tubular solid oxide fuel cell performance is more noticeable than its effects on a planar SOFC. However, the electrical response time of the tubular type SOFC is calculated about ten times more than the planar type.

Performance and long term degradation of 7 W micro-tubular solid oxide fuel cells for portable applications

Micro-tubular SOFCs have shown an astonishing thermal shock resistance, many orders of magnitude larger than planar SOFCs, opening the possibility of being used in portable applications. However, only few studies have been devoted to study the degradation of large-area micro-tubular SOFCs. This work presents microstructural, electrochemical and long term degradation studies of single micro-tubular cells fabricated by high shear extrusion, operating in the intermediate range of temperatures (T∼700 °C). A maximum power of 7 W per cell has been measured in a wide range of fuel utilizations between 10% and 60% at 700 °C. A degradation rate of 360 mW/1000 h (8%) has been observed for cells operated over more than 1500 h under fuel utilizations of 40%. Higher fuel utilizations lead to strong degradations associated to nickel oxidation/reduction processes. Quick thermal cycling with heating ramp rates of 30 °C /min yielded degradation rates of 440 mW/100 cycles (9%). These reasonable values of degradation under continuous and thermal cycling operation approach the requirements for many portable applications including auxiliary power units or consumer electronics opening this typically forbidden market to the SOFC technology.

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.

Numerical investigation on impacts on fuel velocity distribution nonuniformity among solid oxide fuel cell unit channels

In this paper, fuel velocity distribution nonuniformity among channels in planar SOFC units under different working conditions is numerically investigated. A comprehensive three-dimensional electrochemical model is validated and then adopted in a cell unit model with structure of a real cell unit. The model couples interdependent process of species transport, heat transport, chemical reaction, electrochemical reaction, ionic conduction and electronic conduction. A nonuniformity index is proposed to quantitatively evaluate nonuniform degree of fuel velocity distribution among channels in the planar SOFC unit. The effect of the fuel velocity distribution nonuniformity on cell performance and the effects of working voltage, flow rate, flow pattern and fuel type on fuel velocity distribution nonuniformity among channels are investigated. The result shows that an increase in fuel velocity distribution nonunifomtiy can lead to a cell performance drop and fuel velocity distribution is less uniform under lower cell voltage, lower flow rate, using co-flow configuration instead of counter-flow or using syngas as fuel instead of hydrogen. In addition, the CO oxidation should be considered when studying the fuel velocity distribution nonuniformity among channels.

D212 2D Numerical Simulation of Anode Supported Planar SOFC Considering Microstructure of Electrodes

A two-dimensional comprehensive CFD model for anode-supported planner solid oxide fuel cells (SOFCs) is developed to study the performance the cells. The electrode microstructures of the anodes are quantified using FIB-SEM measurements and implemented to the simulation. The model is verified from a short stack experiment with a standard power output of 100W. The simulation results have a good agreement with experimental data, and reveal the relationships between the cell performance and the electrode microstructure. In addition, effects of temperature on the potential loss are presented.

Recent fabrication techniques for micro-tubular solid oxide fuel cell support: A review

Micro-tubular solid oxide fuel cell (MT-SOFC) has recently attracted much attention due to their greater tolerance to thermal cycling, quicker start-up capability, higher output density and better portable characteristics as compared with planar and tubular SOFCs. MT-SOFC requires one of the cell components to act as a support for deposition of remaining cell layers. In fact, the support fabrication step is the most important part as the quality of the support greatly affects performances of the MT-SOFCs. However, there is limited information regarding the fabrication processes especially extrusion of the support for the MT-SOFCs. The aim of this paper is to review on conventional and recent introduced phase inversion-based extrusion techniques, their characteristics and performances. A brief discussion on future direction of a more advanced phase inversion-based extrusion in the MT-SOFCs fabrication, including co-extrusion technique is also included.

The effect of applied control strategy on the current-voltage correlation of a solid oxide fuel cell stack during dynamic operation

Abstract This paper discusses the transient characteristics of the planar type SOFC cell stack, of which the standard output is 300 W. The transient response of the voltage to the manipulation of an electric current was investigated. The effects of the response and of the operating condition determined by the operating temperature of the stack were studied by mapping a current-voltage (I-V) correlation. The current-based fuel control (CBFC) was adopted for keeping the fuel utilization factor at constant while the value of the electric current was ramped at the constant rate. The present experimental study shows that the transient characteristics of the cell voltage are determined by primarily the operating temperature caused by the manipulation of the current. Particularly, the slope of the I-V curve and the overshoot found on the voltage was remarkably influenced by the operating temperature. The different values of the fuel utilization factor influence the height of the settled voltages. The CBFC has significance in determining the slope of the I-V characteristic, but the different values ofthe fuel utilization factor does not affect the slope as the operating temperature does. The CBFC essentially does not alter the amplitude of the overshoot on the voltage response, since this is dominated by the operating temperature and its change is caused by manipulating the current.

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.

Investigating on the Flow Distribution of a Planar Solid Oxide Fuel Cell Stack

In this paper, a realistic 3D numerical model is established to investigate the flow distribution of a 10-cells short planar SOFC stack. The effect of the basic geometric parameters, such as the sub-manifold radii () and the single channel width (), on the stack flow uniformity is examined. And the results and discusses are presented in this paper. This investigation for the SOFC stack holds great significance for the SOFC stack commercialization.

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

In this study, a three-dimensional computational fluid dynamics model has been developed for an anode-supported planar SOFC. The conservation equations of mass, momentum, species/charges and thermal energy are solved by finite volume method for a complete unit cell consisting of 13 parallel channels in both anode and cathode. The simulation results of the developed model are well in agreement with the experimental data obtained at same conditions. In this study, the co-flow arrangement with hydrogen utilization of 60 % and operating voltage of 0.7 V is used as the base case, and compared with the counter-flow arrangement. The predicted results reveals that the maximum temperature obtained in the counter-flow arrangement is about 10 A degrees C lower than that of co-flow, but the counter-flow arrangement has a higher temperature gradient between the respective anodes and cathodes in a cross-section normal to the main flow direction, especially in the air inlet region of the cell (x = 0.04 m),which is very harmful to the lifetime of materials. The current density is very unevenly distributed along and normal to the flow direction for both the co- and counter-flow arrangements, and the maximum values occur at junctions of the electrodes, channels and ribs, which causes higher over-potentials and ohmic heating.

Design and Optimization of an Innovative Solid Oxide Fuel Cell–Gas Turbine Hybrid Cycle for Small Scale Distributed Generation

AbstractDistributed power generation and cogeneration is an attractive way toward a more rational conversion of fuel and biofuel. The fuel cell‐gas turbine hybrid cycles are emerging as the most promising candidates to achieve distributed generation with comparable or higher efficiency than large‐scale power plants. The present contribution is devoted to the design and optimization of an innovative solid oxide fuel cell–gas turbine hybrid cycle for distributed generation at small power scale, typical of residential building applications. A 5 kW planar SOFC module, operating at atmospheric pressure, is integrated with a micro gas turbine unit, including two radial turbines and one radial compressor, based on an inverted Brayton cycle. A thermodynamic optimization approach, coupled with system energy integration, is applied to evaluate several design options. The optimization results indicate the existence of optimal designs achieving exergy efficiency higher than 65%. Sensitivity analyses on the more influential parameters are carried out. The heat exchanger network design is performed for an optimal configuration and a complete system layout is proposed. An example of hybrid system integration in a common residential building is discussed.

Performance of a Solid Oxide Fuel Cell short-stack with biogas feeding

In this study, the main sections required to produce electricity in an SOFC short stack starting from biogas were studied and tested at a lab-scale level. A test-rig including a cleaning unit, a reformer and an SOFC stack was built and used to run a test of 500h. In the plant, an in-house planar SOFC short stack (composed of three nickel based anode-supported cells) was fed by a simulated biogas mixture (60vol.% CH4, 40vol.% CO2) contaminated by 30ppmv of H2S (the main contaminant found in biogas, and deleterious for both the reformer and fuel cells). In the plant, H2S is first removed from the gas stream by adsorption in a fixed bed reactor packed with Na–X zeolites. The mixture was also passed on a ZnO guard bed (heated at 300°C) before feeding the reformer unit in order to guarantee a durable and stable operation of the same. The reformer uses a Ni-based catalyst, and was operated at ∼700°C in a mixed steam-/dry-reforming condition to produce a gas mixture suitable for the Ni-based SOFC with CH4 converted almost entirely to H2 and CO. The 3-cells short stack was operated at 800°C and 0.1Acm−2 for more than 500h. The gas analysis was performed, via a mass spectrometer, in several points of the plant. These measurements indicated that no sulfur breakthrough takes place from the zeolites bed, and furthermore that the catalyst has a quite stable behavior.Results clearly show how a SOFC stack can be successfully operated with a contaminated biogenous gas mixture as fuel. As a matter of fact the cell stack exhibited a regular behavior, with no signs of degradation in the complete test.

Radiation Heat Transfer in Planar SOFC Components: Application of the Lattice Boltzmann Method

Radiation Heat Transfer in Planar SOFC Components: Application of the Lattice Boltzmann Method

Operation Experience with a 20 kW SOFC System

AbstractSystems based on planar SOFC stacks have a great potential to become compact high efficient power plants. At Forschungszentrum Jülich system technology is under development aiming at a 20 kW demonstration plant. An important challenge is to realize a compact and efficient system. To achieve this, Jülich has invented an Integrated Module, which incorporates all hot parts of the system. After a few years of development four stacks with a nominal power of 5 kW were assembled and characterized with proper quality. After integrating them into the system operation was started and 21.3 kW gross power were achieved. Because of a leakage in the periphery the test had to be interrupted after 550 h of operation under load. Because repair would have taken too long, the test was continued with two modules of 5 kW power each. The system was in operation under load for 4,000 h.

SOFC Durability against Standby and Shutdown Cycling

To simulate realistic operating conditions in SOFC systems, we investigate the influence of thermal cycling on the performance of electrolyte-supported planar SOFCs. Thermal cycling is often associated with interruption of fuel supply, with three main modes; hot standby, cold standby, and shutdown. Cell performance degradation is most significant during shutdown cycles. Nickel oxidation and agglomeration are more pronounced when SOFCs are subjected to lower temperatures for longer periods of time, leading to significant performance degradation. Ostwald ripening at the anode leads to degradation as Ni grains increase in size with cycling. Ni particle precipitation on the anode zirconia grains and along electrolyte grain boundaries is found for the first time in shutdown cycling tests. When H2S is mixed with the fuel, the internal reforming reactions and electrode reactions are inhibited by sulfur poisoning of the Ni anodes, accelerating degradation. The SOFC cycling degradation mechanisms are discussed in detail.