Effects of transport scale on heat/mass transfer and performance optimization for solid oxide fuel cells

A three-dimensional thermo-fluid–electrochemical model is developed to study the heat/mass transport process and performance of a solid oxide fuel cell (SOFC). The main objectives are to examine the transport channel size effects and to assess the potential of a thin-film-SOFC. A parametric study was performed to evaluate the channel scale effects on the temperature, species concentration, local current density and power density. The results demonstrate that decreasing the height of flow channels can lower the average solid temperature and improve cell efficiency. However, this improvement is rather limited for the smallest channels. Compared with the conventionally sized SOFC, the miniaturized SOFC with a thin-film electrolyte has the advantages of a lower operating temperature and a better performance. Based on our simulation results, the power density of a miniaturized SOFC could reach up to 5.461 W cm⁻³. However, an extremely small structure will lead to severe thermal stress induced by a large temperature gradient, a cell with a thicker rib width would have a higher efficiency and a lower average temperature. Numerical simulation is expected to help optimize the design of a solid oxide fuel cell.     

The Gaston Planté Medal Presentation Address

The paper shows and discusses a procedure of parameter estimation applied to the evaluation of some operating parameters of a proton-exchange membrane fuel cell (PEMFC). First, a brief literature review about the main parameters (exchange current density, cell resistance, internal current density and limiting current density) has been done. Then the analytical model adopted to describe the polarization curve has been discussed. Based on this model, a parameter analysis has been done, and it has been shown that three parameters of the cell polarization curve model can be simultaneously estimated: the cathode exchange current density, the cell resistance and the internal current density. To evaluate these parameters both a set of our measurements on a PEM single cell (active area of 25 cm² and Nafion 115 membrane) and data from other authors has been considered.The cell has been fed with pure hydrogen and air, the cell temperature has been varied from 50 °C to 80 °C, and accordingly the reactants have been introduced in the cell humidified at the same temperature. The parameters have been estimated in each operating conditions of the cell, and their behavior, as a function of the cell operating temperature, has been discussed.     

Equilibrium and kinetic modeling of char reduction reactions in a downdraft biomass gasifier: A comparison

The thermodynamic and kinetic modeling of char reduction reactions in a downdraft (biomass) gasifier has been presented. Mass and energy balance are coupled with equilibrium relations or kinetic rate parameters (using varying char reactivity factor) in order to predict status of un-converted char in addition to gas composition, calorific value, conversion efficiency, exit gas temperature, endothermic heat absorption rate and gasifier power output. Both modeling predictions are compared against experimental data for their validity. The influence of char bed length and reaction temperature in reduction zone has been examined. CO and H₂ component, calorific value of product gas and the endothermic heat absorption rate in reduction zone are found to be sensitive with reaction temperature, while char bed length is less sensitive to equilibrium predictions. For present case, all char conversion takes place at critical reaction temperature of 932 K for equilibrium, while for kinetic modeling critical reaction temperature and critical char bed length of 950 K and ～25 cm have been identified, comparing the predictions. The critical reaction temperatures and critical char bed length also depend on inlet components composition and initial temperature supplied to the reduction reaction zone model.     

Fuel flexibility study of an integrated 25 kW SOFC reformer system

The operation of solid oxide fuel cells on various fuels, such as natural gas, biogas and gases derived from biomass or coal gasification and distillate fuel reforming has been an active area of SOFC research in recent years. In this study, we develop a theoretical understanding and thermodynamic simulation capability for investigation of an integrated SOFC reformer system operating on various fuels. The theoretical understanding and simulation results suggest that significant thermal management challenges may result from the use of different types of fuels in the same integrated fuel cell reformer system. Syngas derived from coal is simulated according to specifications from high-temperature entrained bed coal gasifiers. Diesel syngas is approximated from data obtained in a previous NFCRC study of JP-8 and diesel operation of the integrated 25 kW SOFC reformer system. The syngas streams consist of mixtures of hydrogen, carbon monoxide, carbon dioxide, methane and nitrogen. Although the SOFC can tolerate a wide variety in fuel composition, the current analyses suggest that performance of integrated SOFC reformer systems may require significant operating condition changes and/or system design changes in order to operate well on this variety of fuels.     

Gasification of hazelnut shells in a downdraft gasifier

The potential offered by biomass to reduce greenhouse gas production is now being more widely recognised. The energy in biomass may be realised either by direct combustion use, or by upgrading into more valuable and useable products such as gas, fuel oil and higher value products for utilisation in the chemical industry or for clean power generation. Up till now, gasification work has concentrated on woody biomass but recently sources of other biomass with large energy production potential have been identified, namely hazelnut shells. Therefore, a pilot scale downdraft gasifier is used to investigate gasification potential of hazelnut shells. A full mass balance is reported including the tar production rate as well as the composition of the produced gas as a function of feed rate. Additionally, the effect of feed rate on the CV/composition of the product gas and the associated variations of gasifier zone temperatures are determined with temperatures recorded throughout the main zones of the gasifier and also at the gasifier outlet and gas cleaning zones. Pressure drops are also measured across the gasifier and gas cleaning system because the produced gas may be used in conjunction with a power production engine when it is important to have low pressure drop in the system. The quality of the product gas is found to be dependent on the smooth flow of the fuel and the uniformity of the pyrolysis, and so the difficulties, encountered during the experiments are detailed. The optimum operation of the gasifier is found to be between 1.44 and 1.47 N m³/kg of air fuel ratios at the values of 4.06 and 4.48 kg/h of wet feed rate which produces the producer gas with a good GCV of about 5 MJ/m³ at a volumetric flow of 8–9 N m³/h product gas. It was concluded that hazelnut shells could be easily gasified in a downdraft gasifier to produce good quality gas with minimum polluting by-products. It is suggested that, in view of ease of operation, small-scale gasifiers can make an important contribution to the economy of rural areas where the residues of nuts are abundant. It is also suggested that gasification of shell waste products is a clean alternative to fossil fuels and the product gas can be directly used in internal gas combustion engines, thus warranting further investment/encouragement by authorities to exploit this valuable resource.     

Biomass integrated gasification fuel cell systems-Concept development and experimental results

The link-up of wood gasification with high temperature solid oxide fuel cells (Biomass-Integrated Gasification Fuel Cell System, B-IGFC) is a promising approach to reach high electrical efficiencies in small-scale biomass fuelled combined heat and power plants (CHP). The main technical challenge is the adjustment of the three main system components gasification, gas processing and fuel cell. A B-IGFC concept has been developed based on the findings that producer gas originating from the updraft gasification of wood can be electrochemically converted in a SOFC whereby tars are degraded to hydrogen and carbon monoxide and contribute to the electrochemical reactions and power generation. Important unit operations of the B-IGFC system were characterised experimentally. During a long-term test of the complete B-IGFC demonstration unit, the gasifier and the catalytic partial oxidation could be operated without problems, delivering a fuel gas to the SOFC system with relatively constant composition and properties. Compared to methane operation, the SOFC system delivered approx. 40% less current. With the chosen operating conditions, carbon deposition was effectively prevented. Ash deposits were identified as major obstacle for a smooth SOFC system operation.     

Integration of solid oxide fuel cell and palladium membrane reactor: Technical and economic analysis

This paper presents a technical and economic analysis of a solid oxide fuel cell system equipped with a palladium membrane reactor (PMR–SOFC) with the aim of determining the benefits of such an integrated unit over the conventional reformer module (CON-SOFC). The performance of both SOFC systems under the conditions for energetically self-sustaining operation (Q_NET = 0) was achieved by varying the fuel utilization for each operating voltage. Two types of fuels, i.e., methane and desulphurized biogas, are considered. The simulation results show that the maximum power density of the CON-SOFC fuelled by methane (0.423 W/cm²) is higher than that of the CON-SOFC fuelled by biogas (0.399 W/cm²) due to the presence of CO₂ in biogas. For the PMR–SOFC, it is found that the operation at a higher permeation pressure offers higher power density because lower fuel utilization is required when operating the SOFC at the energy self-sustained condition. When the membrane reactor is operated at the permeation pressure of 1 bar, the methane-fuelled and biogas-fuelled PMR–SOFCs can achieve the maximum power density of 0.4398 and 0.4213 W/cm², respectively. Although the PMR–SOFC can offer higher power density, compared with the CON-SOFC, the capital costs of supporting units, i.e., palladium membrane reactor, high-pressure compressor, and vacuum pump, for PMR–SOFC need to be taken into account. The economic analysis shows that the PMR–SOFC is not a good choice from an economic viewpoint because of the requirement of a large high-pressure compressor for feeding gas to the membrane reactor.     

Modular biomass gasification-based solid oxide fuel cells (SOFC) for sustainable development

The integration of biomass gasification with SOFCs offers the potential of highly efficient and renewable power generation, primarily in modular solutions. SOFC seems to be the most promising fuel cell technology of biomass gasifier producer gases. Solid oxide fuel cells, because of their high operating temperature, do not require pure hydrogen as fuel, exhibiting high fuel flexibility. Sufficient amounts of cereal, cotton, corn, olive, coffee or palm tree residues are available in Mediterranean areas, while the climatic conditions are favorable for energy crops cultivations. Their residues can be utilized for electricity production by modular biomass gasification-based solid oxide fuel cells (SOFC).     

Simulation analysis of a system combining solid oxide and polymer electrolyte fuel cells

We evaluated the performance of system combining a solid oxide fuel cell (SOFC) stack and a polymer electrolyte fuel cell (PEFC) stack by a numerical simulation. We assume that tubular-type SOFCs are used in the SOFC stack. The electrical efficiency of the SOFC–PEFC system increases with increasing oxygen utilization rate in the SOFC stack. This is because the amount of exhaust heat of the SOFC stack used to raise the temperature of air supplied to it decreases as its oxygen utilization rate increases and because that used effectively as the reaction heat of the steam reforming reaction of methane in the stack reformer increases. The electrical efficiency of the SOFC–PEFC system at 190 kW ac is 59% (LHV), which is equal to that of the SOFC-gas turbine combined system at 1014 kW ac.     

Numerical simulation of proton exchange membrane fuel cells at high operating temperature

A three-dimensional, single-phase, non-isothermal numerical model for proton exchange membrane (PEM) fuel cell at high operating temperature (T ≥ 393 K) was developed and implemented into a computational fluid dynamic (CFD) code. The model accounts for convective and diffusive transport and allows predicting the concentration of species. The heat generated from electrochemical reactions, entropic heat and ohmic heat arising from the electrolyte ionic resistance were considered. The heat transport model was coupled with the electrochemical and mass transport models. The product water was assumed to be vaporous and treated as ideal gas. Water transportation across the membrane was ignored because of its low water electro-osmosis drag force in the polymer polybenzimidazole (PBI) membrane. The results show that the thermal effects strongly affect the fuel cell performance. The current density increases with the increasing of operating temperature. In addition, numerical prediction reveals that the width and distribution of gas channel and current collector land area are key optimization parameters for the cell performance improvement.     

Dependence of polarization in anode-supported solid oxide fuel cells on various cell parameters

The performance of solid oxide fuel cells (SOFCs) is affected by various polarization losses, namely, ohmic polarization, activation polarization and concentration polarization. Under given operating conditions, these polarization losses are largely dependent on cell materials, electrode microstructures, and cell geometric parameters. Solid oxide fuel cells (SOFC) with yttria-stabilized zirconia (YSZ) electrolyte, Ni–YSZ anode support, Ni–YSZ anode interlayer, strontium doped lanthanum manganate (LSM)–YSZ cathode interlayer, and LSM current collector, were fabricated. The effect of various parameters on cell performance was evaluated. The parameters investigated were: (1) YSZ electrolyte thickness, (2) cathode interlayer thickness, (3) anode support thickness, and (4) anode support porosity. Cells were tested over a range of temperatures between 600 and 800 °C with hydrogen as fuel, and air as oxidant. Ohmic contribution was determined using the current interruption technique. The effect of these cell parameters on ohmic polarization and on cell performance was experimentally measured. Dependence of cell performance on various parameters was rationalized on the basis of a simple analytical model. Based on the results of the cell parameter study, a cell with optimized parameters was fabricated and tested. The corresponding maximum power density at 800 °C was ∼1.8 W cm⁻².     

Two-dimensional transient model of a cascaded micro-tubular solid oxide fuel cell fed with methane

A two-dimensional transient simulation model for a cascaded (10 cascades) micro-tubular solid oxide fuel cell (SOFC) and a common micro-tubular SOFC with a preheater tub was developed to calculate the distribution of gas species, the local current, voltage and temperature. The goal was to compare these two different SOFC designs under steady state and load change conditions with respect to power density. Steady state simulation results have shown that the cascaded cell concept is able to operate at higher average cell voltages in principle. Compared to the cascaded cell concept the average ohmic anodic and cathodic resistance of the common cell has to be 84 times lower to reach the same average power density at an average voltage of 0.7 V. The cascaded cell concept gives the opportunity to operate at a higher average voltage than the lowest Nernst voltage offers. This could be interesting to realize high fuel utilization (>85%). Transient simulation results have shown that both cell concepts respond within 10 s on an electrical load change. The cascaded cell concept has shown a lower temporary voltage drop than the common cell concept. This paper describes the theory, solution techniques and results.     

Fabrication and performance evaluation of 3-cell SOFC stack based on planar 10 cm × 10 cm anode-supported cells

This study reports the development of planar-type solid oxide fuel cell (SOFC) stacks based on an internal gas manifold and a cross-flow type design. A single-columned, 3-cell, SOFC stack is assembled using 10 cm × 10 cm anode-supported unit cells, metallic interconnects and glass-based compression-seal gaskets. The power-generating characteristics of the unit cell and stack are characterized as a function of temperature. The practical viability of the stack and stack components is investigated via long-term operation and thermal cycling tests. According to performance evaluation at 700 °C, the short stack produces about 100 W in total power at an average cell voltage of around 0.7 V. There are, however, some scale-up problems related to multi-cell stacking. This work addresses key issues in stack fabrication and performance improvement.     

System modeling of an air-independent solid oxide fuel cell system for unmanned undersea vehicles

To examine the feasibility of a solid oxide fuel cell (SOFC)-powered unmanned undersea vehicle (UUV), a system level analysis is presented that projects a possible integration of the SOFC stack, fuel steam reformer, fuel/oxidant storage and balance of plant components into a 21-in. diameter UUV platform. Heavy hydrocarbon fuel (dodecane) and liquid oxygen (LOX) are chosen as the preferred reactants. A maximum efficiency of 45% based on the lower heating value of dodecane was calculated for a system that provides 2.5 kW for 40 h. Heat sources and sinks have been coupled to show viable means of thermal management. The critical design issues involve proper recycling of exhaust steam from the fuel cell back into the reformer and effective use of the SOFC stack radiant heat for steam reformation of the hydrocarbon fuel.     

Effect of operating parameters on performance of an integrated biomass gasifier,solid oxide fuel cells and micro gas turbine system

An integrated power system of biomass gasification with solid oxide fuel cells (SOFC) and micro gas turbine has been investigated by thermodynamic model. A zero-dimensional electrochemical model of SOFC and one-dimensional chemical kinetics model of downdraft biomass gasifier have been developed to analyze overall performance of the power system. Effects of various parameters such as moisture content in biomass, equivalence ratio and mass flow rate of dry biomass on the overall performance of system have been studied by energy analysis.It is found that char in the biomass tends to be converted with decreasing of moisture content and increasing of equivalence ratio due to higher temperature in reduction zone of gasifier. Electric and combined heat and power efficiencies of the power system increase with decreasing of moisture content and increasing of equivalence ratio, the electrical efficiency of this system could reach a level of approximately 56%.Regarding entire conversion of char in gasifier and acceptable electrical efficiency above 45%, operating condition in this study is suggested to be in the range of moisture content less than 0.2, equivalence ratio more than 0.46 and mass flow rate of biomass less than 20  kg h⁻¹.     

A study of Ni + 8YSZ/8YSZ/La0.6Sr0.4CoO3−δ ITSOFC fabricated by atmospheric plasma spraying

An intermediate temperature solid oxide fuel cell (ITSOFC) based on 8YSZ electrolyte, La_0.6Sr_0.4CoO_3−δ (LSCo) cathode, and Ni − 8YSZ anode coatings were consecutively deposited onto a porous Ni-plate substrate by atmospheric plasma spraying (APS). The spray parameters including current, argon and hydrogen flow rate, and powder feed rate were investigated by an orthogonal experiment to fabricate a thin gas-tight 8YSZ electrolyte coating (80 μm). By proper selection of the spray parameters to decrease the particles velocity and temperature, the sprayed NiO + 8YSZ coating after reducing with hydrogen shows a good electrocatalytic activity for H₂ oxidation. With the same treatment, 100–170 μm dimensions LSCo particle could keep phase structure after spraying. And the deposited LSCo cathode shows a good cathode performance and chemical compatibility with 8YSZ electrolyte after operating at 800 °C for 50 h. Output power density of the sprayed cell achieved 410 mW cm⁻² at 850 °C and 260 mW cm⁻² at 800 °C. Electrochemical characterization indicated that IR drop of 8YSZ electrolyte, cathodic polarization, and the contact resistance at LSCo/8YSZ interface were the main factors restricting the cell performance. The results suggested that the use of APS cell allowed the reduction of the operating temperature of the SOFC to below 850 °C with lower production costs.     

Investigation of methane steam reforming in planar porous support of solid oxide fuel cell

Adopting the porous support in integrated-planar solid oxide fuel cell (IP-SOFC) can reduce the operating temperature by reducing thickness of electrolyte layer, and also, provide internal reforming environment for hydrogen-rich fuel gas. The distributions of reactant and product components, and temperature of methane steam reforming for IP-SOFC were investigated by the developed physical and mathematical model with thermodynamic analysis, in which eleven possible reaction mechanisms were considered by the source terms and Arrhenius relationship. Numerical simulation of the model revealed that the progress of reforming reaction and the distribution of the product, H₂, were influenced by the operating conditions, included that of temperature, ratio of H₂O and CH₄, as well as by the porosity of the supporting material. The simulating results indicate that the methane conversion rate can reach its maximum value under the operating temperature of 800 °C and porosity of ε = 0.4, which rather approximate to the practical operating conditions of IP-SOFC. In addition, characteristics of carbon deposition on surface of catalyst were discussed under various operating conditions and configuration parameters of the porous support. The present works provided some theoretical explanations to the numerous experimental observations and engineered practices.     

Studies of biomass fuelled MCFC systems

In the present work, the methods, techniques and results obtained during the studies of biomass fuelled molten carbonate fuel cell (MCFC) systems within the Swedish national fuel cell program are presented. The power plants are 60 MW class, utilising biomass (i.e. wood chips) as the primary fuel. The biomass is converted via pressurised gasification into a gaseous form that, after subsequent cleaning, can be used in the fuel cells. An investigation of the effects of gasification pressure, temperature and the influence of internal reforming on the overall system performance is presented.All studies were carried out using the Aspen Plus™ with Model Manager™ simulation package.     

Development of electrocatalysts for solid alkaline fuel cell (SAFC)

It is now widely accepted that the proton exchange membrane fuel cell (PEMFC) concepts can be applied for a rather large range of power production at low temperature, including ambient conditions for some applications. For small power units, the concept of fuel cell with a solid electrolytic membrane can be extended to the alkaline medium. The main condition is to develop an anionic membrane with a sufficient stability and a good electric conductivity. The aim of this paper is to discuss on the development of electrocatalysts (both anodic and cathodic) suitable with their use in a solid alkaline fuel cell working at room temperature. The fuel cell is conceived around a new kind of anionic membrane developed by an industrial partner.The catalysts need to be adapted to this new concepts and working conditions. For oxygen reduction, catalysts containing silver have been prepared and gave encouraging results. For the anodic side, fuels such as methanol or ethylene-glycol have been considered. Platinum-based catalysts have been developed and tested in addition with platinum-free electrocatalysts. The electrocatalytic activity of Pt–Pd catalysts was first evaluated in semi-cell in order to optimize the composition of the electrodes. Then, tests in fuel cells working at room temperature were carried out, and confirm the validity of the solid alkaline fuel cell (SAFC). A power density of 18–20 mW cm⁻² was observed with methanol or ethylene-glycol at 20 °C.     

Single solid oxide fuel cell modeling and optimization

In this paper a dynamic model of a single solid oxide fuel cell (SOFC) is developed using a volume element methodology. It consists of a set of algebraic and ordinary differential equations derived from physical laws (e.g., the first law of thermodynamics, Fick's law, and Fourier's law), which allow for the prediction of the temperature and pressure spatial distribution inside the single SOFC, as functions of geometric and operating parameters. The thermodynamic model is coupled with an electrochemical model that is capable of determining the voltage, current, and power output. Based on the simulation results, the internal configuration (structure of the positive electrode-electrolyte-negative electrode assembly) and the operating conditions (air stoichiometric ratio and fuel utilization factor), as well as their impact on the performance of the single SOFC are discussed. Optimal geometric and operating parameters are obtained so that electrical power of the single SOFC at the nominal operating point is maximized. The method used is general and the fundamental optimization results are sharp, showing up to a 357% single SOFC performance variation within the studied parameters’ range, therefore these findings show the potential to use the model as a tool for future SOFC design, simulation and optimization.