The Case for Natural Gas Fueled Solid Oxide Fuel Cell Power Systems for Distributed Generation

Natural‐gas‐fueled solid oxide fuel cell (NGSOFC) power systems yield electrical conversion efficiencies exceeding 55% and may become a viable alternative for distributed generation (DG) if stack life and manufacturing economies of scale can be realized. Currently, stacks last approximately 2 years and few systems are produced each year because of the relatively high cost of electricity from the systems. PNNL has performed cost modeling for production of 270 kW (DC) NGSOFC power systems, sized for light industry or large box stores. If mass manufacturing (10.000 units per year) and a stack life of 15 years can be reached, the cost of electricity from an NGSOFC system is estimated to be about 8,2 ¢/kWh, well within the range of commercial and residential retail prices at the national level (9,9–10 ¢/kWh and 11–12 ¢/kWh, respectively). With 5 ¢/kWh in estimated additional benefits from DG, NGSOFC could be well positioned to replace the forecasted 59–77 gigawatts of capacity loss resulting from coal plant closures due to stricter emissions regulations and low natural gas prices.     

The Development and Application of a Novel Optimisation Strategy for Solid Oxide Fuel Cell‐Gas Turbine Hybrid Cycles

A combined power system with solid oxide fuel cell (SOFC) and gas turbine (GT) is modelled and analysed thermodynamically in this paper. A novel optimisation strategy including the design of optimal parameters is proposed and applied to the hybrid system. Different sources of irreversible losses are specified, and entropy analyses are used to indicate the multi‐irreversibilities existing, and to assess the work potentials of the system. Expressions of the power output and efficiency for both the subsystems and the SOFC‐GT hybrid system are derived. The optimal performance characteristics are presented and discussed in detail through a parametric analysis. The developed model is expected to provide not only a convenient tool to determine the optimal system performance and component irreversibility, but also an appropriate basis to design similar complex hybrid power plants. This new approach can be further extended to other energy conversion settings and electrochemical systems. Decision makers should therefore find the methodology contained in this paper useful in the comparison and selection of advanced heat recovery systems.     

Gas Recirculation at the Hydrogen Electrode of Solid Oxide Fuel Cell and Solid Oxide Electrolysis Cell Systems

In this study it is theoretically analyzed how flue gas recirculation at the hydrogen electrode of solid oxide cells (SOC) systems effects fuel utilization and carbon formation. Interdepence between cell fuel utilization, system fuel utilization and gas recirculation is investigated numerically. Tendency towards carbon deposits is evaluated via thermodynamic equilibrium calculations. It is quantified which gas recirculation rates are necessary to achieve high values of system fuel utilization even if the cell fuel utilization is kept at a moderate level. Furthermore, tendency towards carbon deposition strongly depends on temperature, pressure and feed gas composition and can be reduced by adequate recirculation rates. The presented results can be used for the configuration of gas recirculation in SOC systems.     

固体氧化物燃料电池的研究进展

固体氧化物燃料电池(SOFC)是先进陶瓷材料的一种重要应用,可以通过电化学反应将燃料的化学能直接转换为电能。SOFC具有效率高、性能稳定、便携、低污染等优点,可以实现能源的有效清洁利用。本文结合国内外SOFC的研究情况,分析讨论了SOFC的技术优势、地方产业优势及产业难题,并结合国家政策方面对我国SOFC产业的未来发展做了展望。     

Effect of Anode off‐gas Recycling on Reforming of Natural Gas for Solid Oxide Fuel Cell Systems

The effect of anode off‐gas recycling (AOGR) on the characteristic performance of a natural gas reformer equipped with a precious metal catalyst is investigated experimentally. The reformer is operated both with synthetic AOGR gas and in steam reforming (SR) conditions. The characteristic performance in SR and AOGR mode are compared with equilibrium, and it is found that equilibrium is more readily achieved in AOGR mode. The reformer is used for extended periods of time (100–1,000 h) in conditions where carbon formation is thermodynamically possible to measure any changes in characteristic performance. No significant change in the performance is observed due to carbon formation or catalyst deactivation. The reformer could be successfully implemented in a 10 kW SOFC system with an anode off‐gas recycling loop.     

Analysis of Leakages in a Solid Oxide Fuel Cell Stack in a System Environment

A solid oxide fuel cell (SOFC) stack can exhibit both anodic and cathodic leakages, i.e. a fuel leak from the anode side and an air leak from the cathode side of the stack, respectively. This study describes the results of an in‐situ leakage analysis conducted for a planar SOFC stack during 2000 hours of operation in an actual system environment. The leakages are quantified experimentally at nominal system operating conditions by conducting composition analysis and flow metering of gases for both fuel and air subsystems. Based on the calculated atomic hydrogen‐to‐carbon ratio of the fuel and air gases, it is found that the fuel leakages are mostly selective by nature: the leaking fuel gas does not have the same composition as the fuel system gas. A simple diffusive leakage model, based on the leakage being driven by concentration differences weighted by diffusion coefficients, is applied to quantify the amount of leakages. The leakage model provides a good correspondence with the experimental results of the gas analysis.     

System Study on Hydrothermal Gasification Combined With a Hybrid Solid Oxide Fuel Cell Gas Turbine

The application of wet biomass in energy conversion systems is challenging, since in most conventional systems the biomass has to be dried. Drying can be very energy intensive especially when the biomass has a moisture content above 50 wt.% on a wet basis. The combination of hydrothermal biomass gasification and a solid oxide fuel cell (SOFC) gas turbine (GT) hybrid system could be an efficient way to convert very wet biomass into electricity. Therefore, thermodynamic evaluation of combined systems with hydrothermal gasification units and SOFC–GT hybrid units has been performed. Three hydrothermal gasification cases have been evaluated; one producing mainly methane, a second one producing a mixture of hydrogen and methane and the last one producing mainly hydrogen. These three gasification systems have been coupled to the same SOFC–GT hybrid system. All the integrated systems have electrical exergy efficiencies around 50%, therefore, the combination of supercritical water gasification and SOFC–GT hybrid systems seems promising. The overall system performance depends for a large part on the liquid gas separation. Further research is required for finding out the optimal separation conditions.     

Electrochemical Impedance Modeling of a Solid Oxide Fuel Cell Anode

A simulation package for the impedance response of SOFC anodes is presented here. The model couples the gas transport in gas channels and within a porous electrode with the electrochemical kinetics. The gas phase mass transport is modeled using mass conservation equations. A transmission line model (TLM), which is suitably modified to account for the electrode microstructural details, is used for modeling the impedance arising from the electrochemical reactions. In order to solve the system of nonlinear equations, an in‐house code based on the finite difference method was developed. Some of the model constants have been calibrated against experimental data. It is demonstrated that the simulation tool is capable of predicting the impedance response of an experimental data set obtained on symmetrical cells with Ni/ScYSZ SOFC anodes. A parametric study is also carried out using the developed simulation tool and the results are further discussed.     

Modelling and Performance Evaluation of Solid Oxide Fuel Cell for Building Integrated Co‐ and Polygeneration

Models of fuel cell based combined heat and power systems, used in building energy performance simulation codes, are often based on simple black or grey box models. To model a specific device, input data from experiments are often required for calibration. This paper presents an approach for the theoretical derivation of such data. A generic solid oxide fuel cell (SOFC) system model is described that is specifically developed for the evaluation of building integrated co‐ or polygeneration. First, a detailed computational cell model is developed for a planar SOFC and validated with available numerical and experimental data for intermediate and high temperature SOFCs with internal reforming (IT‐DIR and HT‐DIR). Results of sensitivity analyses on fuel utilisation and air excess ratio are given. Second, the cell model is extended to the stack model, considering stack pressure losses and the radiative heat transfer effect from the stack to the air flow. Third, two system designs based on the IT‐DIR and HT‐DIR SOFCs are modelled. Electric and CHP efficiencies are given for the two systems, as well as performance characteristics, to be used in simulations of building integrated co‐ and polygeneration systems.     

Long‐Term Stability Studies of Anode‐Supported Microtubular Solid Oxide Fuel Cells

A long‐term stability study of an anode‐supported NiO/YSZ‐YSZ‐LSM/YSZ microtubular cell was performed, under low fuel utilization conditions, using pure humidified hydrogen as fuel at the anode side and air at the cathode side. A first galvanometric test was performed at 766 °C and 200 mA cm^–2, measuring a power output at 0.5 V of ∼250 mW cm^–2. During the test, some electrical contact breakdowns at the anode current collector caused sudden current shutdowns and start‐up events. In spite of this, the cell performance remains unchanged. After a period of 325 h, the cell temperature and the current density was raised to 873°C and 500 mA cm^–2, and the cell power output at 0.5 V was ∼600 mW cm^–2. Several partial reoxidation events due to disturbance in fuel supply occurred, but no apparent degradation was observed. On the contrary, a small increase in the cell output power of about 4%/1,000 h after 654 h under current load was obtained. The excellent cell aging behavior is discussed in connection to cell configuration. Finally, the experiment concluded when the cell suffered irreversible damage due to an accidental interruption of fuel supply, causing a full reoxidation of the anode support and cracking of the thin YSZ electrolyte.     

Development of Single Chamber Solid Oxide Fuel Cells (SCFC)

Single Chamber Solid Oxide Fuel Cells (SCFC) have been prepared using an electrolyte as support (Ce_0.9Gd_0.1O_1.95 named GDC). Anode (Ni‐GDC) and different cathodes (Sm_0.5Sr_0.5CoO₃ (SSC), Ba_0.5Sr_0.5Co_0.2Fe_0.8O₃ (BSCF) and La_0.8Sr_0.2MnO₃ (LSM)) were placed on the same side of the electrolyte. All the electrodes were deposited using screen‐printing technology. A gold collector was also deposited on the cathode to decrease the over‐potential. The different materials and fuel cell devices were tested under propane/air mixture, after a preliminary treatment under hydrogen to reduce the as‐deposited nickel oxide anode. The results show that SSC and BSCF cathodes are not stable in these conditions, leading to a very low open circuit voltage (OCV) of 150 mV. Although LSM material is not the more adequate cathode regarding its high catalytic activity towards hydrocarbon conversion, it has a better chemical stability than SSC and BSCF. Ni‐GDC‐LSM SCFC devices were elaborated and tested; an OCV of nearly 750 mV could be obtained with maximum power densities around 20 mW cm^–2 at 620 °C, under air–propane mixture with C₃H₈/O₂ ratio equal to 0.53.     

High Temperature Micro‐Fan for Solid Oxide Fuel Cell Applications

The possibility of using a μ‐fan in tubular solid oxide fuel cells module (SOFC‐M) is shown. The μ‐fan is placed instead of the ejector and fulfills its role. The main advantages of this solution are: lower power demanded by the fuel compressor (blower), more stable working characteristics, and the possibility of more accurate control of the recycled part of the anode gas during part load operation. A comparison of two SOFC‐Ms, with and without the ejector, is also shown and commented.     

A Novel Method for GDC Recovery During Solid Oxide Fuel Cell Manufacturing

There have been a tremendous research affects in recent years for alternative routes of electricity generation using some maximum yield technologies, increased reliability, and minimum pollution. From this point of view, the solid oxide fuel cells (SOFCs) are considered as the cleanest technologies for obtaining electrical energy generation. However, an important fraction of production is wasted during the manufacturing steps. From both economical and environmental point of view, recovery of waste GDC (Gd_0.1Ce_0.9O₃) materials is deemed important. Hence, it is the main purpose of the present study to develop a novel method to recover waste GDC materials and afterwards to produce a new SOFC (solid oxide fuel cell) from the recovered materials. The results showed that recovered GDC cell worked as efficient as the fresh materials, revealing the success of the recovery process proposed.     

Enhanced Performance of Solid Oxide Fuel Cell by Manipulating the Orientation of Cylindrical Pores in Anode Substrate

Effect of the orientation of cylindrical pores within an anode has been studied on the performance of anode‐supported solid oxide fuel cell (SOFC). Paper‐fibers are used as pore‐former and highly oriented cylindrical pores are formed within the anode prepared by uniaxial compaction. A thick anode brick is fabricated followed by cutting in different directions to obtain anode substrates with desirable orientation of pores. When the orientation of cylindrical pores is perpendicular to the anode surface, the gas transport is significantly improved so that the reduction rate of the NiO/YSZ anode is considerably accelerated and the cell concentration polarization is minimized. The corresponding single cell exhibits a maximum power density as high as 1.54 W cm^–2 in hydrogen and 0.90 W cm^–2 in nitrogen diluted methane at 800 °C. The result indicates that the output performance of anode‐supported cells could be significantly improved by manipulating the orientation of pores.     

Evaluation of a Non‐sealed Solid Oxide Fuel Cell Stack with Cells Embedded in Plane Configuration

A non‐sealed solid oxide fuel cell stack with cells embedded in plane configuration was fabricated and operated successfully in a box‐like stainless‐steel chamber. For a two‐cell stack, it demonstrated an open circuit voltage (OCV) of 2.13 V and a maximum power output of 569 mW at the flow rate of 67 sccm CH₄ and 33 sccm O₂. A fuel utilization of 4.16% was obtained. The cell performance was dominated by two different mechanisms, the polarization of the cathode at low current and the concentration polarization of the anode at high current. Finally, a scaled‐up stack with six cells in series generated an OCV of 6.4 V and a maximum power output of 8.18 W.     

A Degradation Model for Solid Oxide Fuel Cell Anodes due to Impurities in Coal Syngas: Part I Theory and Validation

A new phenomenological one‐dimensional model is formulated to simulate the typical degradation patterns observed in solid oxide fuel cell (SOFC) anodes due to coal syngas contaminants such as arsenic (As) and phosphorous (P). The model includes gas phase diffusion and surface diffusion within the anode and the adsorption reactions on the surface of the Ni‐YSZ‐based anode. Model parameters such as reaction rate constants for the adsorption reactions are obtained through indirect calibration to match the degradation rates reported in the literature for arsine (AsH₃), phosphine (PH₃), hydrogen sulfide (H₂S), and hydrogen selenide (H₂Se) under accelerated testing conditions. Results from the model demonstrate that the deposition of the impurity on the Ni catalyst starts near the fuel channel/anode interface and slowly moves toward the active anode/electrolyte interface as observed in the experiments. Parametric studies performed at different impurity concentrations and operating temperatures show that the coverage rate increases with increasing temperature and impurity concentration, as expected. The calibrated model was then used for prediction of the performance curves at different impurity concentrations and operating temperatures. Good agreement is obtained between the predicted results and the experimental data reported in the literature.     

Phenomenological Theory of Solid Oxide Fuel Cell Anode

We develop a phenomenological theory of oxygen‐ion‐conducting porous cermet anode for solid oxide fuel cells utilizing hydrogen, based on a simple picture of macro‐ and microkinetics of charge and gas transport in the cermet. Its basic equations account for the transport of hydrogen molecules and oxygen anions to the reaction spots, the hydrogen oxidation reaction (whose various mechanisms, including different adsorption stages, are considered) and the water‐product removal. Simple analytical results are obtained for a linear current‐voltage regime, which demonstrate the interplay of these three processes. The nonlinear behavior is analyzed and classified. Various mechanisms of reaction kinetics are considered, subject to three possible mechanisms of water adsorption, in order to specify the law of conversion of ionic current into electronic one. Revealed is the nature of the intermediate quasi‐Tafel regime, in which the anode is usually employed, and of two possible large current regimes: the saturation regime and the blocking regime (due to oxidation of the anode). The study rationalizes principles of anode functioning and builds a basis for a systematic analysis of the effects due to composite structure, that enter through the basic parameters of the theory.     

Modelling of a Pressure Element for Solid Oxide Fuel Cell Stack

The paper deals with the structural analysis of a corrugated pressure element for the SOFC stack. The aim of the element application is assurance of the fuel cell tightness and sufficient electrical contact between components by means of its uniform pressure. In this paper, a method of the pressure element modelling is proposed and results of its behaviour analysis for different assembly conditions (free ends, fixed ends and ends subjected to friction force) are presented. Vertical and horizontal stiffness of the corrugated element as a function of its geometrical parameters has been determined which facilitates computer simulation and engineering design of the element.     

Degradation Mechanism of Anode‐Supported Solid Oxide Fuel Cell In Planar‐Cell Channel‐Type Setup

The degradation mechanism of anode‐supported planar solid oxide fuel cells is investigated in the present work. We fabricate a large‐area (10 cm × 10 cm) cell and carry out a long‐term test with the assembly components. A constant current of ∼0.4 A cm^–2 is applied to the cell for ∼3,100 h, and the furnace temperature is controlled in the sequence 750–800–750 °C to investigate the effect of operating temperature and thermal cycling on the degradation rate. Impedance spectra and current–voltage characteristics are measured during the operation in order to trace any increase in Ohmic and non‐Ohmic resistance as a function of time. The degradation rate is rapid during the operation at the higher temperature of ∼800 °C compared to that during the operation at ∼750 °C. Even after cooling down to ∼750 °C, that rate is still accelerated. The main contribution to the cell degradation is from an increase in the Ohmic resistance. Postmaterial analyses indicate that the cathode is delaminated at the electrolyte/cathode interface, which is attributed to the difference in thermal expansion coefficient (TEC). Thus, the present results emphasize the importance of matching the TEC between cell layers, especially under severe operating conditions such as long duration and complex thermal cycling.     

Anode Current Collecting Efficiency of Tubular Anode‐supported Solid Oxide Fuel Cells

Anode current collection points (ACCPs) were fabricated on the outside surface of a tubular anode‐supported solid oxide fuel cell (SOFC). The ACCPs were distributed axially along the SOFC tube with the distance between every adjacent two ACCPs the same. The effect of collecting current with different number of ACCPs on the performance of the SOFC was studied. It was found that with the same effective area, using more ACCPs to collect the current leads to better performance, while with a SOFC with a determined total surface area, there is an optimum number of ACCPs to be made and used considering the area occupied by the ACCPs themselves.