Development of 1 kW class solid oxide fuel cell stack using anode-supported planar cells

In this paper the operation and performance of a high temperature solid oxide fuel cell (SOFC) stack on biomass syn-gas from a demonstration biomass gasification combined heat and power (CHP) plant is investigated. The objective of this work is to develop a computer simulation model of a biomass-SOFC CHP system, flexible enough for use in industry, capable of predicting system performance under various operating conditions and using diverse fuels. The biomass gasifier is of the dual fluidised bed (DFB) type with steam as the gasifying agent and is operated at atmospheric pressure. The tubular SOFC configuration, developed by Siemens Power Generation Inc (SPGI), is selected. It is considered to be the most advanced design and is approaching commercialisation. The SOFC stack model, developed using the chemical process flowsheet simulator Aspen Plus, is of equilibrium type and is based on Gibbs free energy minimisation. The SOFC model performs heat and mass balances and considers the ohmic, activation and concentration losses for the voltage calculation. Data available in the literature on the SPGI SOFC operating on natural gas is used to validate the model. The system model predicts thermodynamic condition and composition of all internal flow streams, the heat generated by the SOFC stack, voltage (V), current (I) and efficiency. Operating parameters are varied over a wide range, parameters such as fuel utilisation factor (Uf), current density (j) and steam to carbon ratio (STCR) have significant influence. The results indicate that there must be a trade-off between voltage, efficiency and power with respect to j and the SOFC stack should be operated at low STCR and high Uf, within certain limits. SOFC stack operation on biomass syn-gas is compared to operation on natural gas and as expected there is a drop in performance, which is attributed to increased input fuel and air flow due to the lower quality of the fuel gas. The optimum realistic operating conditions with regard to SOFC stack performance are identified. High electrical efficiencies are predicted making these systems very attractive for CHP applications.

Parametric exergy analysis of a tubular Solid Oxide Fuel Cell (SOFC) stack through finite-volume model

Solid oxide fuel cell (SOFC) stacks were operating with metallic interconnects that separate each cells. Metallic interconnects play an important role in gas flow system and heat transfer mechanism, because its geometry makes gas flow channel and it has a high thermal conductivity. Many researcher design the flow channel for optimizing the cell performance like a ratio of height and width, channel to channel distance, or new type channels. For high conversion efficiency, most solid oxide fuel cell in power generation application operate at high fuel utilization and low current condition. However, high fuel utilization condition makes a large gradient of chemical potential by fuel depletion and it induces a low cell performance and a large chemical stress in flow direction. This phenomena can be observed at high performance cell or high operating temperature condition, because almost fuel is consumed at upstream of fuel channel by high electrochemical reaction kinetics and most cell area has a low chemical potential environment. Therefore, even distribution of electrochemical reaction rate is important. In this study we reduce the gradient of chemical potential by using diffuser-shape fuel channel, and optimize the diffuser-shape by design study. In chemical species conservation law, higher flow velocity at upstream makes higher convective flux in flow direction compared to diffusive flux in cell-thickness direction, and it can reduce the electrochemical reaction rate at reacting electrode. Optimized diffuser-shape fuel flow channel from this study may enhance the performance and the durability of solid oxide fuel cell stacks.

This study presents a comparative evaluation of Molten Carbonate Fuel Cell (MCFC) and Solid Oxide Fuel cell (SOFC) stacks coupled with a micro gas turbine (MGT). For the analysis, it is assumed that the fuel supply to the stacks is constant in all the analyzed conditions. The components of the system have been sized using the first law of thermodynamics to meet the thermal conditions required to maintain fuel cell stack operation, while an exergetic analysis has been implemented in order to assess the components in terms of irreversibilities. Furthermore, an economic analysis to estimate the Levelized Cost of Electricity (LCOE) has been carried out to indicate the most feasible option between the two analyzed systems. The capacity of the fuel cell stacks is 500kWdc operating at 350 kPa and 600 °C/650 °C. The results indicate that MCFC stacks are more efficient than SOFC stacks, but are considerably more expensive. Nevertheless, the SOFC/MGT system has a global efficiency higher than that of the MCFC/MGT; also its Total Capital Investment (TCI) is 2.5–3.5 times lower, thus making the SOFC/MGT coupling more attractive. The only product considered coming out from the systems to determine the LCOE is electricity. The LCOE for the SOFC/MGT system is 0.339–0.402$/kWh and for the MCFC/MGT is 0.875–0.897$/kWh, which for both configurations is still higher than the current electricity prices. One aspect that increases the investment and consequently the LCOE is the high cost of the stacks and their replacements which is around 19 % and 10 % of the total Purchased Equipment Cost (PEC) for MCFC and SOFC stacks, respectively.

Generally, the manufacturing technology of fuel cell units is considered to satisfy the current commercialization requirements. However, achieving a high-performance and durable stack design is still an obstacle in its commercialization. The solid oxide fuel cell (SOFC) stack is considered to have performance characteristics that are distinct from the proton exchange membrane fuel cell (PEMFC) stacks. Within the SOFC stack, vapor is produced on the anode side instead of the cathode side and high flow resistance within the fuel flow path is recommended. In this paper, a 3D multiphysics model for a one-cell SOFC stack with the interdigitated channels for fuel flow path and conventional paralleled line-type rib channels for air flow path is firstly developed to predict the multiphysics distribution details. The model consists of all the stack components and couples well the momentum, species, and energy conservation and the quasi-electrochemical equations. Through the developed model, we can get the working details within those SOFC stacks with the above interdigitated flow channel features, such as the fuel and air flow feeding qualities over the electrode surface, hydrogen and oxygen concentration distributions within the porous electrodes, temperature gradient distribution characteristics, and so on. The simulated result shows that the multiphysics field distribution characteristics within the SOFC and PEMFC stacks with interdigitated flow channels feature could be very different. The SOFC stack using the paralleled line-type rib channels for air flow path and adopting the interdigitated flow channels for the fuel flow path can be expected to have good collaborative performances in the multiphysics field. This design would have good potential application after being experimentally confirmed.

The paper presents the scheme of mini CHP with methane reformer and planar solid oxide fuel cells (SOFC) stack. The mini CHP produces electricity, superheated steam, hot air and methane for the reformer and it also preheats cathode air used in the SOFC stack as an oxidant. Moreover, the scheme’s mathematical model is constructed. The thermochemical reactor with impeded fluidized bed for autothermal steam reforming of methane (reformer) studied experimentally is the key element in producing synthesis gas – fuel for the SOFC stack. The research indicates that synthesis gas containing 55% of hydrogen can be derived using the reactor. Through mathematical modeling, the entire reactor’s principal dimensions as well as flow rates of air, water and methane were adjusted to the methane reforming for the mini CHP studied. The paper includes the heat balances of the reformer, SOFC stack and waste heat boiler that produces the superheated steam, hot air and methane for the reformer as well as preheated cathode air. These balances were instrumental for calculating the useful product fraction in the reformer, fraction of hydrogen oxidized in the SOFC anode channel, electric gross efficiency, anode temperature, exothermic effect of synthesis gas hydrogen oxidation by air oxygen, excess entropy and Gibbs free energy change at standard conditions, SOFC electromotive force (EMF), specific flow rate of standard fuel for heat, and power generation. The simulation study has shown that the hydrogen oxidation products temperature in the SOFC anode channel is 850ºС, electric gross efficiency 61.0%, single fuel cell EMF 0.985 V, fraction of hydrogen oxidized in the SOFC anode channel 64.6%, specific flow rate of standard fuel for power generation 0.16 kg/(kW·h), for heat generation 44.7 kg/GJ. All the specific parameters are in good agreement with other publications results.

The Solid oxide fuel cell (SOFC) is an electrochemical energy-conversion device which offers tremendous promise for delivering high electrical efficiency and significant environmental benefits in terms of fuel flexibility, as well as clean and efficient electric power generation. However, the state-of-the-art SOFC cell currently involves expensive and time-consuming multi-step manufacturing processes including tape casting, screen printing, shaping, lamination, sintering, etc. In addition, SOFC stacks are usually assembled with pre-fabricated cells, metal interconnects, and glass-ceramic sealing. These indispensable components add extra layers of complexity to the stack structure’s fabrication and maintenance, significantly increasing the size and weight of the final product. These intrinsic limitations on the current planar stack cannot be easily removed by traditional manufacturing processes. However, as additive manufacturing (AM) technologies start to mature and show increasing market penetration, it lends itself admirably to breaking the limits on high-power-density SOFC stack manufacturing. Storagenergy’s SOFC project is developing an all-ceramic AM-SOFC by a high-speed AM process of multiple materials. The developed technology allows the fabrication of robust and compact AM-SOFC stacks with unique features such as monolithic architecture with embedded novel micro flow channels and thinner elements (anode, cathode, electrolyte, and interconnect) by only three production steps (3D printing, sintering, and catalyst infiltration) without the need for subsequent assembly of individual layers and indispensable components. This simplification of the fabrication process is expected to reduce the cost of the SOFC and increase SOFC design flexibility and manufacturing reliability.

Simulation of a tubular solid oxide fuel cell stack using AspenPlusTM unit operation models

This paper reports the results from the European project PROSOFC (funded by FCH-JU, FP7). Within the project an innovative methodology for a production and reliability oriented SOFC cell and stack design is developed. In particular the PROSOFC project aims at improving the robustness, manufacturability, efficiency and cost of SOLIDpowers state-of-the-art SOFC stacks so as to reach market entry requirements. The key issues are the mechanical robustness of solid oxide fuel cells (SOFCs), and the delicate interplay between cell properties, stack design, and operating conditions of the SOFC stack. The novelty of the project lies in combining state of the art methodologies for cost-optimal reliability-based design (COPRD) with actual production optimization. To achieve the COPRD beyond state of the art multi-physical modeling concepts were developed and validated for significantly improved understanding of the production and operation of SOFC stacks. The most critical failure modes for stack operation were identified in order to align the experimental investigations to generate suitable data for modelling these failures. The multi-physics model allows a probabilistic approach to consider statistical variations in production, material and operating parameters for the optimization phase. The project provides a methodology for 3D description of spatial distribution of material properties based on a random field models. The key for the whole methodology are validating experiments, homogenized models on multiple levels of the SOFC system (cell, interconnect, sealings) and introduction of extensive test programs specified by the COPRD methodology. The probabilistic models were related to the experimentally obtained properties of base materials to establish a statistical relationship between the material properties and the most relevant load effects in view of potential damage/failure based on multi-physics simulation can be established. Software algorithms for meta models that allow the detection of relationships between input parameters (materials, loading, etc.) and output parameters and to perform a sensitivity analysis were developed and implemented. The capabilities of the methodology for COPRD being developed is illustrated on two practical cases. A procedure based on meta models has been developed to estimate elastic, primary and secondary creep model parameters from 4-point bending tests. The goal is to (i) account for the effects of the friction between the sample and the rollers, anticlastic curvature, contact point shift and large deflection, which can affect the accuracy of the measurements, and (ii) measure properties which cannot be obtained using closed-form analytical solutions. This could be achieved by model-based parameter estimation with a 3-D continuum finite-element (FE) model, at the cost of very high running time. The proposed approach is computationally much more efficient. It starts with a sensitivity analysis to generate distributed meta models of the displacement at the inner rollers during loading-unloading cycles and creep measurements simulated by the 3-D FE model. The optimization problem for parameter estimation is then solved using the meta models. The accuracy of the approach was tested using a set of numerical experiments and comparison with results from 3-D imaging and computational homogenization. The experiments on Ni-YSZ samples were then analyzed. For the stochastic investigation of the thermo-electrochemical behavior of the stack a multi-physics stack model implemented in gPROMS and Fluent was linked and controlled automatically by optiSlang to perform a sensitivity analysis. The outcome is (i) the identification of the most relevant production and operating parameters which can have a significant impact on the stack lifetime and (ii) meta models of the efficiency and of the 3-D temperature distribution generated in the view of optimization. The tests show for instance that the accuracy of the 3-D meta model of the temperature distribution is sufficient to identify the minimum of variations in the spatial distribution of the temperature during cycling between full and part load by using sets of operating conditions for optimization.

Recent developments in control and integration of solid oxide fuel cells: From stack to system

Over the past several years Nexceris has advanced its solid oxide fuel cell (SOFC) stack technology through innovations in materials, design, and operation. Sustainable fuel cell operation of Nexceris’ lightweight stack at high current density (0.5 – 0.7 A/cm²) and fuel utilization (70 – 80%) yields both high gravimetric power density and electrical efficiency. Stack robustness against thermal gradients, thermal cycling, and one-off events has been achieved through a combination of design modifications, sealing advancements, and operational protocols. Meanwhile, Nexceris’ stack technology maintains the key advantages of its patented FlexCell planar cell design, internal reforming capability, and tolerance to sulfur impurities in military logistic fuels.Recent and ongoing work at Nexceris includes stack demonstrations in breadboard systems and at customer sites, scale-up of stack manufacturing capabilities, and the development of a 10-kW stack module, opening the door to large-scale distributed generation and energy storage applications. This presentation will provide an update on the status and capabilities of Nexceris’ SOFC stacks and cost-saving approaches to stack fabrication.

Solid Oxide Fuel Cells (SOFCs) offer the potential for compelling thermo-economic value propositions in wide range of stationary and transportation applications through their potential for high conversion efficiency and their inherent fuel flexibility. Furthermore, many of the core SOFC materials, component, and system technologies have tremendous potential in electrolysis applications where fuels or chemicals could be produced using renewable electricity. However, to date, the wide-spread commercialization of SOFC/SOEC systems has been hampered by their high cost and durability challenges.Fortunately, high temperature solid oxide-based energy conversion systems are well suited for integration with thermal-mechanical energy conversion systems in configurations that offer the potential for significant cost and thermodynamic performance synergies. Specifically, their integration with gas turbines or internal combustion engines offers the potential for ultra-high (>70%) efficiency at an attractive manufacturing cost (<$1/W). In these hybrid systems, engines or turbines are used to convert stack waste exergy to additional useful work resulting in higher overall efficiency and lower power-specific cost (e.g., $/kW) compared to SOFC-only systems.Furthermore, in the case of SOFC and gas turbine hybrid systems, the SOFC stacks are typically located downstream of the gas turbine compressor and are at elevated pressure—enabling increased Nernst potential and decreased overpotentials and thereby higher power density at constant voltage. This benefit translates into a significant power-specific stack cost reduction. Additionally, the gas turbine compressor and recuperator may serve as the SOFC stack blower and recuperator—yielding further cost synergies via the elimination of duplicative balance of plant components.In 2017, ARPA-E launched the INTEGRATE program to develop hybrid system concepts and enabling component technologies that if realized would result in 100kW-scale distributed generation systems with >70% fuel to electricity conversion efficiencies and system manufacturing costs of <$1/W. In the initial two-year Phase I portion of the program, nine teams developed a suite of hybrid system concepts. Given these concepts, the teams then proceeded to develop 1) durable SOFC stacks that are capable of operating at high (~4 bar) pressure, 2) high temperature (>600 °C) and low-cost heat exchangers, and 3) innovative control system approaches. In 2019, three of the original nine teams were selected to design, build and demonstrate 100kW-scale systems by 2023.In the interest of focusing limited financial resources on foundational development risks, while avoiding the weight, volume, and fuel flexibility challenges associated with transportation applications and mitigating the financial risks associated with adopting new technologies at utility (e.g., >100MW) scale, natural gas fueled distributed generation (DG) systems were selected as the initial INTEGRATE application target. These DG systems would offer a fuel to electric power conversion efficiency that is roughly double that of the fossil fueled portion of the US electric grid. Furthermore, as renewable fuels such as hydrogen, ammonia, and bio- or electro- derived hydrocarbons (e.g., renewable natural gas or synthetic kerosene) become available, these systems would be capable of operating on them with relatively minor fuel processing system adjustments.Moving beyond stationary DG applications, SOFC hybrid systems can leverage their fuel flexibility in the pursuit of a decarbonized of long-distance transportation sector—if the systems can be made both light and small enough. To this end, ARPA-E recently launched three programs that have taken on the challenge of developing ultra-efficient, yet compact and light weight carbon-neutral electrified propulsion systems for commercial aviation.One of these programs is focused on carbon-neutral fuel to electric power conversion sub-systems—Range Extenders for Electric Aviation with Low Carbon and High Efficiency (REEACH). In the two-year Phase 1 of this program, six of nine selected teams will develop SOFC/gas turbine/battery hybrid systems that are fueled with renewable natural gas or synthetic kerosene. Phase 1 focuses on system conceptual design and fuel conversion component risk reduction, while full system design and prototype demonstration of a sub-scale fuel-to-electric power conversion device using a carbon-neutral liquid fuel is planned for Phase 2.If the REEACH-envisioned systems are successfully developed, they would enable economically attractive all-electric commercial aircraft up to a narrow body airframe with dramatically reduced (ideally zero) net carbon emissions. Furthermore, INTEGRATE/REEACH hybrid system technologies would also have attractive value propositions in maritime, rail, or heavy-duty vehicle applications.

Recently, the desire of decarbonization is increasing on the part of the general public worldwide as many companies take part in RE100 initiative. Tokyo Gas Co., Ltd. declared to reduce 10 million tons carbon dioxide emission by 2030 in “Compass 2030” which is company vision of Tokyo Gas group in November 27th, 2019. We drive CO2 net-zero including emission from customer and lead the transition to a decarbonized society. To achieve CO2 net-zero, we will take two actions. The first one is expansion of use of renewable energy. Second is development of decarbonization technologies for gaseous energy, which include effective use of natural gas, use of carbon capture, utilization and storage (CCUS).Fuel cell system is expected to use natural gas effectively. In Japan, residential fuel cell system has already installed extensively since the first model was sold in 2009. Tokyo Gas have also studied fuel cell technologies for over three decades. These days, we developed and reported mono generation system which had AC-power generation efficiency of over 65.0%LHV and output of 5kW. In this system, anode lines of solid oxide fuel cell (SOFC) stacks are connected in series. A fuel regenerator is placed between two-stage SOFC stacks, which remove H2O from the anode off-gas of first-stage SOFC stack. Then, the regenerated anode off-gas is used as a fuel of the second-stage SOFC stack. Hence, this system can be operated at high total fuel utilization (Uf) value of over 90% and enable each SOFC stack to be operated at safe Uf condition for each, simultaneously. We could make easy flow system by using condenser as fuel regenerator.This system can reduce much CO2 than existing power resource because of its higher efficiency, but still exhausts some amount of CO2 gas. To achieve decarbonized society, further reduction of CO2 emission for fuel cell system will be required. Therefore, we focused on CO2 separation membrane as a fuel regenerator which has high selectivity for both H2O and CO2. In previous work, we performed DC-power generation of 75.5%LHV by connecting membrane module as the fuel regenerator of the two-stage SOFC system. CO2 recovery rate from membrane reached to about 40%. To improve the CO2 recovery rate, we invent the new system as figure 1. The new system recycles regenerated anode off-gas which was removed H2O and CO2 by CO2 separation membrane. The membrane has high selectivity of H2O and CO2. In addition, a separation at the membrane was driven on the differential pressure by decompressing without sweep gas. As a result, high concentration of CO2 was recovered. In ideal situation, the new system will be operated under Uf of 100% and recovered CO2 of 100%. In this study, we demonstrated this system as shown in figure 1. Membrane was put in fuel cell module. It was operated at a total Uf of 97.1%, and power generation efficiency reached DC 77.3%LHV at power output of 5.65 kW under the thermal self-sustainable condition without the electric heater. Assuming the efficiencies of the auxiliary machines and the power conditioner to be respectively 94% and 95%, the estimated AC-power generation efficiency of the hot module was 69.0%LHV and CO2 emission factor reached to 294 g-CO2/kWh. At the same time, CO2 recovery rate was 85%. We achieved high efficiency and low in this system. In the future, we have to discuss about the use of recovery CO2. Figure 1

Thermal stress analysis of a planar SOFC stack

Solid oxide fuel cells (SOFCs), as highly efficient energy conversion devices, are anticipated to play a significant role in the realization of a low-carbon society. ENE-FARM type S (EF), equipped with SOFC stack, has been commercialized since 2012 by Osaka Gas. To further expand the market of EF, higher electrical efficiency, higher durability, lower cost of SOFC stack, and downsizing of EF are needed. For realizing these issues, two model changes were conducted in 2016 and 2020. The SOFC stack installed in the current EF, which is composed of Anode-supported SOFC (ASC), has established durability of over 12 years. For further widespread adoption in the EF market, we have focused on Metal-supported SOFCs (MSC), which have the potential to reduce costs by using fewer expensive ceramics.However, the use of a metal substrate in MSCs introduces unknown degradation phenomena that differ from those in ASCs. These phenomena include the degradation of the metal substrate and the interfacial reaction between the metal substrate and anode due to interdiffusion from the metal substrate and anode. Understanding and addressing the degradation phenomena originating from a metal substrate are significant challenges for the commercialization of MSCs. In this study, we investigated the degradation phenomena of MSCs, especially at the metal substrate/anode interface.The degradation phenomena are influenced by evaluation conditions such as durability time, temperature, and gas composition. The concentration of water vapor at the fuel gas outlet is much higher than at the fuel gas inlet throughout electrochemical reactions. To confirm the differences in the degradation phenomena due to gas composition, two MSC pieces were prepared. MSCs were fabricated at a lower temperature compared to ASCs, utilizing ceramic electrodes and electrolytes on a 16 cm2 ferritic stainless-steel substrate. Single cell tests were conducted using air at the cathode and fuel at the anode. The operating conditions for the evaluation were as follows: current density was varied from 0 to 0.42 A/cm², with H₂/H₂O used as the anode gas at two different dew points (DP). Test 1 was conducted at a DP of 30°C, and Test 2 was conducted at a DP of 88°C, while air served as the cathode gas. Both the anode gas and air flow rates were fixed during the evaluation. The operation was conducted with a fuel utilization factor (Uf) of approximately 10% and an air utilization factor (Ua) of approximately 20% at 0.42 A/cm².Figure 1 shows the results of the durability tests of 16 cm2-MSC cells conducted at 800°C. The change in voltage from the initial value is denoted as ΔV. The reduction in voltage on the fuel outlet side was approximately 25 mV greater than that on the fuel inlet side after 1000 hours. During these durability tests, the IR and electrode overvoltage (η) were measured by the current interruption method. The open circuit voltage remained stable without significant changes throughout the 1000 hours durability tests for both the fuel inlet and outlet sides. Figure 2 shows the results of IR and η during the durability tests conducted at 800°C. The observed reduction in voltage was attributed to increases in both IR and η, with similar increments noted for both the fuel inlet and outlet sides in relation to IR. It was found that no abnormally high resistance layer formation or significant increase in oxidation layer occurred on the outlet side after 1000 hours at 800°C.The more pronounced voltage drop observed at the outlet side is primarily due to a more significant increase in η. Considering that the evaluation was conducted under uniform conditions, except for the water vapor concentration on the anode gas side, it is plausible that the factors contributing to the increase in η are associated with the sintering or poisoning of the anode. These findings suggest that the reduction in reaction area and decreased gas diffusivity are significant contributors to the observed effects. In the presentation, we will discuss the insights derived from the electrochemical measurement results along with detailed cross-sectional analyses conducted after the durability tests. Figure 1