Opportunities and Challenges in Using SOFCs in Waste to Energy Systems

Introduction With the recent enhancement of renewable energy utilization, large scale energy conversion and storage become more important to control the surplus electric power due to the unsteady power generation. Reversible hydrogen energy storage system via solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC) are expected as a high efficient device because of its high operating temperature (600℃~1000℃). It is possible to unify them to one device as reversible system (RSOFC/EC), but it is necessary to understand the mechanism of FC/EC reactions and design the device with a good balance between FC and EC in order to apply them for an actual energy system. While SOFC reaction models and many electrochemical data of electrodes have been developed and reported by many researchers, there are few reports to investigate the reaction model of SOEC. Thus, it is possible to predict the performance of SOEC based on a comprehensive SOFC/EC models and experimental data of SOFC, because the reactions of SOFC and SOEC are a reversible relationship and the equal value of IR loss can be applied. However, the comprehensive SOFC/EC electrochemical model has not been established. A series of experimental data of anodic and cathodic current density vs. the potential should be investigated at various conditions to develop the SOFC/EC model. In this work, a series of anodic and cathodic electrode performance was taken using a SOFC in various conditions. Furthermore, attempting to predict SOEC performance from SOFC performances, we proposed and discussed a new model of electrolysis based on our developed SOFC model. Experimental The Disks of 8 mol% Y2O3-ZrO2 (YSZ; 0.25 mm thickness, 20 mm diameter) were used as the electrolyte and the anode was a Ni/YSZ or Ni/GDC composite, and the cathode was a La0.8Sr0.2MnO3/ScSZ composite. We measured the dependence of the current density of anodes on oxygen activity with wide range to get both power generation and electrolysis data for various gas composition (PH2=0.3~0.99 atm, PH2O=0.01, 0.03, 0.05 atm). Results and Discussion 1. Method to fit parameters on the SOFC/EC model We have developed the electrochemical reaction model on the fuel electrode (=anode) based on the equilibrium of competitive adsorption and oxygen activity, ao , specified by anode potential (Eq.1, Table.1) [1]. The SOFC model was constructed by assuming competitive adsorption equilibrium of H2, H2O and O on Ni surfaces at three-phase boundary and the rate-determining step to be Langmuir-type reactions of H with O (Table.1). We obtained an equation of the electrical current density, i, as a function of the oxygen activity (ao ) calculated from the anode potential which is simply measured from experiments. To confirm effectiveness of the model, we fitted the model to actual experimental data described by i vs. ao . From the equations of model on Table.1, each current density of SOFC (ia ) and SOEC(ic ) are described as Eq.2. To draw the plot of i vs. ao on the model, the rate constants (ka, kc ) and the equilibrium constants (KO, KH, KH2O ) need to be defined as reliable value. Thus, from the equilibrium potential vs. 1 atm O2 (Eq.3), we defined the rate constant of kc as Eq.4. When the kc became a dependent variable of ka , then we fitted ia to power generation experimental data described by i vs. ao so that the values of KHKO/KH2O were fitted approximately. Finally, i c model could be drawn on the plot of i vs. ao and the maximum value of i c was obtained by Eq.5 which is independent of ao and Eq.6 was obtained using Eq.4. 2. Experimental results and parameter fitting on the SOFC/EC model As the expreimental results, the maximum power density of SOFC with Ni/YSZ was 588.5 mW/cm2 for FC mode at PH2=0.99 atm, PH2O=0.01 atm, and ic(max) was 0.285 A/cm2 for EC mode at PH2O=0.03 atm. However, ic(max) calculated by the model (Eq.6) was 0.050 A/cm2. Also, in case of SOFC with Ni/GDC, ic(max) was 0.400 A/cm2 for EC mode at PH2O=0.03 atm, while ic(max) calculated by the model (Eq.6) was 0.055 A/cm2. The electrolysis current density measured by our RSOFC/EC showed 6 times higher than model values approximately. This result suggested the possibility that additional parallel reaction path was existed. In this presentation, we will discuss the possible electrochemical paths to explain the disagreement between the model and the experimental data. Finally, the comprehensive SOFC/EC model will be proposed to predict EC performance using SOFC data.

The nickel(Ni)-patterned electrode enables researchers to quantify the triple-phase boundary (TPB) length and Ni surface area, exclude the influence of bulk gas diffusion and clearly separate the active regions of the chemical/electrochemical reactions. This tool has been studied for identifying the reaction mechanism in SOFC but rarely in SOEC. In this study, the Ni-patterned electrodes with the stripe width of 100 μm were tested in both the solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes at the atmosphere of H2O/H2 and further H2O/CO2/H2/CO. The experimental test shows the stability of the Ni-patterned electrode in the H2O/H2 atmosphere was much poorer than that in the CO/CO2 atmosphere due to the nickel reacting with steam. Thus, the Ni-patterned electrode needed to operate at a temperature of ≤700oC with the inlet H2/H2O molar ratio of >7 and the current-applied time shouldn’t be over 5 h to keep the accuracy of the TPB length and Ni surface area. The effects of the temperature, partial pressure of H2O and H2 are investigated. The activation energy of the Ni-patterned electrodes was 0.386 eV. The electrochemical performance had a positive correlation with both the partial pressure of H2 and H2O, especially showed higher sensitivity to the partial pressure of H2O. Further, a possible mechanism of the H2O/H2 electrochemical conversion was proposed according to the experimental data and existing literature, which contains two-step charge-transfer reaction: H(Ni)+O2-(YSZ)OH-(YSZ)+(Ni)+e- and H(Ni)+OH-(YSZ)H2O(YSZ)+(Ni)+e-. Based on the experiments, an analytical calculation was performed to investigate the rate-determining steps in the Ni-patterned electrodes by bridging the connection between the kinetic parameters and operating conditions. The results indicate the rate-limiting steps may be different for the SOFC and SOEC modes. In SOFC mode, the H2 electrochemical oxidation could depend more on the charge-transfer reaction H(Ni)+O2-(YSZ)→OH-(YSZ)+(Ni)+e-, however in SOEC mode, the rate-determining step of the H2O electrochemical reduction should be H2O(YSZ)+(Ni)+e-→OH-(YSZ)+H(Ni). In this way, a reversible mechanism could be applied for reversible SOFC (RSOFC) to describe the conversion between hydrogen and steam by different charge-transfer-reaction domination in the SOFC and SOEC mode. Figure 1

This paper proposes a novel solar thermal power generation system that employs a proton conducting reversible solid oxide fuel cell (RSOFC-H) and a hybrid photovoltaic thermal module. The photovoltaic thermal module supplies electrical power to the solid oxide fuel cell (SOFC) while operating in electrolysis mode. The stored fuel (Hydrogen), generated by electrolysis mode is utilised to generate power in fuel cell mode. Electrochemical modelling and analysis of RSOFC-H is presented in this paper. The working condition of the system is: as a solid oxide steam electrolyser (SOSE) mode during the day time and solid oxide fuel cell (SOFC) mode during night time. Performance analysis of RSOFC-H has been conducted under varying operating and design parameters, such as current density and cell temperature. In SOSE mode of operation, efficiency has been observed to be almost constant with the variation of current density and it is obtained to be 78.31%. During SOFC mode of operation, maximum efficiency obtained is to be 64.62% at cell temperature of 873K and current density of 500A/m2.

Techno-Economic Analysis of Reversible Solid Oxide Fuel Cell System Couple with Waste Steam

Developing safe, reliable, cost-effective, and efficient hydrogen-electricity co-generation systems is an important step in the quest for national energy security and minimized reliance on foreign oil. This project aimed to, through materials research, develop a cost-effective advanced technology cogenerating hydrogen and electricity directly from distributed natural gas and/or coal-derived fuels. This advanced technology was built upon a novel hybrid module composed of solid-oxide fuel-assisted electrolysis cells (SOFECs) and solid-oxide fuel cells (SOFCs), both of which were in planar, anode-supported designs. A SOFEC is an electrochemical device, in which an oxidizable fuel and steam are fed to the anode and cathode, respectively. Steam on the cathode is split into oxygen ions that are transported through an oxygen ion-conducting electrolyte (i.e. YSZ) to oxidize the anode fuel. The dissociated hydrogen and residual steam are exhausted from the SOFEC cathode and then separated by condensation of the steam to produce pure hydrogen. The rationale was that in such an approach fuel provides a chemical potential replacing the external power conventionally used to drive electrolysis cells (i.e. solid oxide electrolysis cells). A SOFC is similar to the SOFEC by replacing cathode steam with air for power generation. To fulfill the cogeneration objective, a hybrid module comprising reversible SOFEC stacks and SOFC stacks was designed that planar SOFECs and SOFCs were manifolded in such a way that the anodes of both the SOFCs and the SOFECs were fed the same fuel, (i.e. natural gas or coal-derived fuel). Hydrogen was produced by SOFECs and electricity was generated by SOFCs within the same hybrid system. A stand-alone 5 kW system comprising three SOFEC-SOFC hybrid modules and three dedicated SOFC stacks, balance-of-plant components (including a tailgas-fired steam generator and tailgas-fired process heaters), and electronic controls was designed, though an overall integrated system assembly was not completed because of limited resources. An inexpensive metallic interconnects fabrication process was developed in-house. BOP components were fabricated and evaluated under the forecasted operating conditions. Proof-of-concept demonstration of cogenerating hydrogen and electricity was performed, and demonstrated SOFEC operational stability over 360 hours with no significant degradation. Cost analysis was performed for providing an economic assessment of the cost of hydrogen production using the targeted hybrid technology, and for guiding future research and development.

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

Since the solid oxide fuel cell (SOFC) has fuel flexibility, high electrical efficiency, and environmental benefits, it is considered a promising electrochemical energy-conversion device. However, it is important to include a waste heat recovery (WHR) unit in SOFC due the high operating temperature. In the present work, the integration of SOFC and WHR is developed. Accordingly, a parametric evaluation of the critical variables from energy, exergy, exergoeconomic, and environmental (4E) viewpoints is carried out. To determine the optimal decision variables, a tri-objective optimization is performed on the novel integrated SOFC-WHR system; the Pareto frontier solution is also provided. On the Pareto curve, the ultimate solution is selected employing the TOPSIS method. The objective functions are cost rate, output power, and CO2 emission, and the corresponding optimal values are identified as 28.5 $/GJ, 1368 kW, and 0.205 kg/kWh, respectively. Moreover, the scattered distribution of nine design variables is explored, and the behavior of decision variables is obtained.

The objective of this paper was to design energy integrated solid oxide fuel cell (SOFC) based microcombined heat and power (micro-CHP) systems using syngas derived from lignocellulosic biomass waste. The novel contributions of this work include (1) integration of syngas between a community-scale biomass gasification plant and SOFC-based micro-CHP systems in buildings; (2) heat integrated designs of SOFC-based micro-CHP systems; and (3) integration between SOFC and other heat-led renewable technologies, such as, syngas boilers, ground source heat pump (GSHP), and air source heat pump (ASHP). Conceptual process design frameworks including detailed heat recovery strategies were developed using Aspen plus. It is demonstrated that increases in heat integration between the SOFC inlet and outlet gases enhance the power-to-heat generation ratio from the SOFC, albeit at a higher cost of heat exchanger area. A straw (14.6 MJ/kg LHV) based community-scale gasification plant can generate 50−100 kWe of electricity at an overall CHP generation efficiency of ∼42%, while if integrated to SOFC-based micro-CHP systems (1 kWe) in individual dwellings via syngas, the overall energy efficiency can be greatly enhanced to ∼85%. It is envisaged that the SOFC should be operated at the highest electrical efficiency based on optimal heat integration; however, this needs to be coupled to other heat-led renewable technologies in order to meet the high residential heat to power demand ratio in the UK. Around 2.2 times more syngas may be required in boilers for supplementing residential heating. In integration with a GSHP loop, the flue gas from SOFC can itself be used as a refrigerant, and energy integration between the two systems can achieve an overall efficiency of ∼90%.

Solid oxide fuel cells are fuel flexible in the sense that they can convert a variety of fuels into electricity because of the use of an oxygen ion conducting electrolyte. Traditional solid oxide fuel cells utilize gaseous reactants, such as hydrogen and carbon monoxide, derived from hydrocarbon based fuels, but also ammonia has been reported. A challenge is still the application of this type of cell for the conversion of solid, carbon containing fuels such as coal or biomass derived carbon in terms of pyrolized wood , e.g. charcoal (direct carbon fuel cell (DCFC)). However, several high power densities based on carbon as fuel have been reported for so called hybrid solid oxide fuel cells. Hybrid direct carbon fuel cells may employ a classical solid oxide fuel cell e.g. using Ni-cermet anodes together with carbon dispersed in a carbonate melt on the anode side. In a European project, the utilization of various coals has been investigated with and without addition of an oxidation catalyst to the carbon-carbonate slurry or anode layer. The nature of the coal and the catalyst composition affect both open circuit voltage and power output. Highest open circuit potentials and power densities were observed for bituminous coal and by adding manganese oxide or praseodymium-doped ceria to the carbon/carbonate mixture. Comparing the performance of an anode supported (315 micron thick anodes) and cathode supported cells (15 micron thick cathode) indicates a superior performance of the latter cell type when using carbon black as carbon source. Using biomass derived carbon (charcoal) as fuel without an additional catalyst resulted in an open circuit voltage of 984 mV and a maximum power density of 121 mW/cm2, similar to the power output of bituminous coal and using ceria or manganese based catalysts. Further outlook on the kinetics of the carbon oxidation processes occurring in the vicinity of the anode of these cells will be provided.

Efficiencies of olive kernel gasification combined cycle with solid oxide fuel cells (SOFCs)

Development of nickel based cermet anode materials in solid oxide fuel cells – Now and future

SOFC fuelled with reformed urea

Chapter 6 - Synthesis, integration, and intensification of solid oxide fuel cell systems: process systems engineering perspective

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.

Thermodynamic analysis of a food waste plasma gasification-based multigeneration system with dehumidification and carbon capture