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.     

Thermodynamic Analysis of an Integrated SOFC,Solar ORC and Absorption Chiller for Tri‐generation Applications

Thermodynamic performance assessment of an integrated tri‐generation energy system for power, heating and cooling production is conducted through energy and exergy analyses. Sustainability assessment is performed and some parametric studies are undertaken to analyze the impact of system parameters and environmental conditions on the system performance. The tri–generation system consists of (a) an internal reforming tubular type solid oxide fuel cell (IR‐SOFC), which works at ambient pressure and fueled with syngas, (b) a combustor and a air heat exchanger, (c) a heat recovery and steam generation unit (HRSG), (d) a two‐ stage Organic Rankine cycle (ORC) driven by exhaust gases of SOFC, (e) parabolic trough solar collectors (PTSC), and (f) a lithium‐bromide absorption chiller (AC) cycle driven by exhaust gases from SOFC unit. The largest irreversibility occurs at the SOFC unit due to high temperature requirement for reactions. Fuel utilization factor, recirculation ratio, dead state conditions, and solar unit parameters have influential effects on the system efficiencies. Energy and exergy efficiencies of tri‐generation unit become 85.1% and 32.62%, respectively, for optimum SOFC stack and environmental conditions. The overall system energy and exergy efficiencies are 56.25% and 15.44% higher than that of conventional SOFC systems, respectively.     

Effect of Creep of Ferritic Interconnect on Long‐Term Performance of Solid Oxide Fuel Cell Stacks

High‐temperature ferritic alloys are potential candidates as interconnect (IC) materials and spacers due to their low cost and coefficient of thermal expansion (CTE) compatibility with other components for most of the solid oxide fuel cells (SOFCs). However, creep deformation becomes relevant for a material when the operating temperature exceeds or even is less than half of its melting temperature (in degrees of Kelvin). The operating temperatures for most of the SOFCs under development are around 1,073 K. With around 1,800 K of the melting temperature for most stainless steel (SS), possible creep deformation of ferritic IC under the typical cell operating temperature should not be neglected. In this paper, the effects of IC creep behaviour on stack geometry change and the stress redistribution of different cell components are predicted and summarised. The goal of the study is to investigate the performance of the fuel cell stack by obtaining the changes in fuel‐ and air‐channel geometry due to creep of the ferritic SS IC, therefore indicating possible changes in SOFC performance under long‐term operations. The ferritic IC creep model was incorporated into software SOFC‐MP and Mentat‐FC, and finite element analyses (FEAs) were performed to quantify the deformed configuration of the SOFC stack under the long‐term steady‐state operating temperature. It was found that the creep behaviour of the ferritic SS IC contributes to narrowing of both the fuel‐ and the air‐flow channels. In addition, stress re‐distribution of the cell components suggests the need for a compliant sealing material that also relaxes at operating temperature.     

Alternative concept for SOFC with direct internal reforming operation: Benefits from inserting catalyst rod

Mathematical models of direct internal reforming solid oxide fuel cell (DIR‐SOFC) fueled by methane are developed using COMSOL® software. The benefits of inserting Ni‐catalyst rod in the middle of tubular‐SOFC are simulated and compared to conventional DIR‐SOFC. It reveals that DIR‐SOFC with inserted catalyst provides smoother temperature gradient along the system and gains higher power density and electrochemical efficiency with less carbon deposition. Sensitivity analyses are performed. By increasing inlet fuel flow rate, the temperature gradient and power density improve, but less electrical efficiency with higher carbon deposition is predicted. The feed with low inlet steam/carbon ratio enhances good system performances but also results in high potential for carbon formation; this gains great benefit of DIR‐SOFC with inserted catalyst because the rate of carbon deposition is remarkably low. Compared between counter‐ and co‐flow patterns, the latter provides smoother temperature distribution with higher efficiency; thus, it is the better option for practical applications. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Theoretical Model for the Optimal Design of Air Cooling Systems of Polymer Electrolyte Fuel Cells. Application to a High‐Temperature PEMFC

The cooling system of a high‐temperature PEM fuel cell with a nominal electric power of 1.5 kW for a combined heat and power unit (CHP) has been designed using a thermochemical model. The 1D model has been developed as a simple, predictive, and useful tool to evaluate, design, and optimize cooling systems of PEM fuel cells. As proved, it can also be used to analyze the influence of different operational and design parameters, such as the number and geometry of the channels, or the air flow rate, on the overall performance of the stack. To validate the model, predicted results have been compared with experimental measurements performed in a commercial 2 kW air‐forced open‐cathode stack. The model has then been applied to calculate the air flow required by the designed prototype stack as a function of the power output, as well as to analyze the influence of the cooling channels configuration (cross‐section geometry and number) on the heat management. Results have been used to select the optimum air‐fan cooling system, which is based on compact axial fans.     

Enhancing the Efficiency of SOFC‐Based Auxiliary Power Units by Intermediate Methanation

Fuel‐cell‐based auxiliary power units benefit from the high power density and fuel flexibility of solid oxide fuel cells (SOFCs), facilitating straightforward onboard fuel processing of diesel or jet fuel. The preferred method of producing the fuel gas is autothermal reforming, which to date has shown the best practical applicability. However, the resulting reformate is poor in methane, so that cell cooling is not supported by internal methane steam reforming. Accordingly, large flow rates of excess air are required to cool the stack. Hence, the power demand of the cathode air blower significantly limits the net electrical power output of the system and large cathode flow channels are required. The present work examines attempts to further increase the system efficiency in middle‐distillate‐fueled SOFC systems by decreasing the cathode air flow rates. The proposed concept is generally based on inducing endothermic methane steam reforming (MSR) inside the cells by augmenting the methane content in an upstream methanation step. Methanation, however, can only yield significant methane production rates if the reaction temperature is limited. Therefore, four process layouts are presented that include different cooling measures. Based on these setups, the general feasibility and the benefit of intermediate methanation are demonstrated.     

Thermodynamic Analysis of Solid Oxide Fuel Cell Gas Turbine Systems Operating with Various Biofuels

Solid oxide fuel cell–gas turbine (SOFC‐GT) systems provide a thermodynamically high efficiency alternative for power generation from biofuels. In this study biofuels namely methane, ethanol, methanol, hydrogen, and ammonia are evaluated exergetically with respect to their performance at system level and in system components like heat exchangers, fuel cell, gas turbine, combustor, compressor, and the stack. Further, the fuel cell losses are investigated in detail with respect to their dependence on operating parameters such as fuel utilization, Nernst voltage, etc. as well as fuel specific parameters like heat effects. It is found that the heat effects play a major role in setting up the flows in the system and hence, power levels attained in individual components. The per pass fuel utilization dictates the efficiency of the fuel cell itself, but the system efficiency is not entirely dependent on fuel cell efficiency alone, but depends on the split between the fuel cell and gas turbine powers which in turn depends highly on the nature of the fuel and its chemistry. Counter intuitively it is found that with recycle, the fuel cell efficiency of methane is less than that of hydrogen but the system efficiency of methane is higher.     

Analysis of the Ripple Current in a 5 kW Polymer Electrolyte Membrane Fuel Cell Stack

Polymer electrolyte fuel cell systems are increasingly being used in applications requiring an inverter to convert the direct current (DC) output of the stack to an alternating current (AC). These inverters, and other time‐varying inputs to the stack such as the anode feed pressure, cause deviations from the average stack current, or ripple currents, which are undesirable for reasons of performance and durability. A dynamic fuel cell model has been developed and validated against experimental data for a 5 kW fuel cell stack, examining in detail the ripple current behaviour. It was shown that the ripple currents exceed the 2% maximum recommended value, and may lead to long‐term degradation of the fuel cell stack.     

Emulator Rig for SOFC Hybrid Systems: Temperature and Power Control with a Real‐Time Software

This work is based on the hybrid system emulator plant developed by the Thermochemical Power Group (TPG) of the University of Genoa. This rig is composed of a 100 kW microturbine coupled with high temperature fuel cell emulation devices. A real‐time model is used for components not physically present in the laboratory (solid oxide fuel cell (SOFC), reformer, anodic circuit, off‐gas burner, cathode blower). It is necessary to evaluate thermodynamic and electrochemical performance related to SOFC systems. Using an User Datagram Protocol (UDP) based connection with the control/acquisition software, it generates a hardware‐in‐the‐loop (HIL) facility for hybrid system emulation. Temperature, pressure, and mass flow rate at the recuperator outlet and machine rotational speed are measured in the plant and used as inputs for the model. The turbine outlet temperature (TOT) calculated by the model is fed into the machine control system and the turbine electric load is changed to match the model TOT values (effective plant/model coupling to avoid modifications on microturbine controller). Different tests were carried out to analyze hybrid system technology through the interaction between an experimental plant and a real‐time model. Double step and double ramp tests of current and fuel provided the system dynamic response.     

SOFC 5 kWe CHP Field Unit: Effect of the Methane Dilution

TurboCare and Politecnico di Torino (Italy) have installed an SOFC laboratory in order to analyse the operation of two SOFC generators (Project EOS‐100 kW and EBE‐5 kW) built by Siemens Power Corporation (SPC). In the EBE project the installation of the SFC5 SOFC generator (3.5 kW_e and 3 kW_th) was carried out. To date, it has operated in the workshop canteen for more than 15,984 h with very high reliability. The real stack is a complex system not installed in a laboratory environment, and has several effects of non‐homogeneity in terms of electrochemical response to fuel or air management modifications. Moreover, many of the parameters of the stack are not directly measurable, and have to be inferred by indirect measurements. In this paper, the analysis of the non‐homogeneous behaviour of the different segments of the complete stack is performed, through an experimental session using a non‐conventional fuel. The obtained data have been analysed using the ANOVA for every dependent variable and a non‐linear regression model for the voltage. Those models were used to evaluate the effect of the fuel modification on the local fuel utilisation in different sectors of the stack.     

Control Loop Design and Control Performance Study on Direct Internal Reforming Solid Oxide Fuel Cell

A solid oxide fuel cell (SOFC) stack is a complicated nonlinear power system. Its system model includes a set of partial differential equations that describe species, mass, momentum and energy conservation, as well as the electrochemical reaction models. The validation and verification of the control system by experiment is very expensive and difficult. Based on the distributed and lumped model of a one‐dimensional SOFC, the dynamic performance with different control loops for SOFC is investigated. The simulation result proves that the control system is appropriate and feasible, and can effectively satisfy the requirement of variable load power demand. This simulation model not only can prevent some latent dangers of the fuel cell system but also predict the distributed parameters' characteristics inside the SOFC system.     

Numerical Investigation into Thermofluidic and Electrochemical Characteristics of Tubular‐type SOFC

Three‐dimensional simulations based on a multi‐physics model are performed to examine the thermofluidic and electrochemical characteristics of a tubular, anode‐supported solid oxide fuel cell (SOFC). The simulations focus on the local transport characteristics of the cathode and anode gases and the distribution of the temperature field within the fuel cell. In addition, the electrochemical properties of the SOFC are systematically examined for a representative range of inlet gas temperatures and pressures. The validity of the numerical model is confirmed by comparing the results obtained for the correlation between the power density and the current density with the experimental results presented in the literature. Overall, the present results show that the performance of the tubular SOFC is significantly improved under pressurised conditions and a higher operating temperature.     

PEM – Fuel Cell System for Residential Applications

Viessmann is developing a PEM fuel cell system for residential applications. The uncharged PEM fuel cell system has a 2 kW electrical and 3 kW thermal power output. The Viessmann Fuel Processor is characterized by a steam‐reformer/burner combination in which the burner supplies the required heat to the steam reformer unit and the burner exhaust gas is used to heat water. Natural gas is used as fuel, which is fed into the reforming reactor after passing an integrated desulphurisation unit. The low temperature (600 °C) fuel processor is designed on the basis of steam reforming technology. For carbon monoxide removal, a single shift reactor and selective methanisation is used with noble metal catalysts on monoliths. In the shift reactor, carbon monoxide is converted into hydrogen by the water gas shift reaction. The low level of carbon monoxide at the outlet of the shift reactor is further reduced, to approximately 20 ppm, downstream in the methanisation reactor, to meet PEM fuel cell requirements. Since both catalysts work at the same temperature (240 °C), there is no requirement for an additional heat exchanger in the fuel processor. Start up time is less than 30 min. In addition, Viessmann has developed a 2 kW class PEFC stack, without humidification. Reformate and dry air are fed straight to the stack. Due to the dry operation, water produced by the cell reaction rapidly diffuses through the electrolyte membrane. This was achieved by optimising the MEA, the gas flow pattern and the operating conditions. The cathode is operated by an air blower.     

Investigation on Flow Distribution in an External Manifold SOFC Stack by Computational Fluid Dynamics Technique

In this study, the flow distribution in a planar solid oxide fuel cell (SOFC) stack with external manifolds is investigated by computational fluid dynamics (CFD) technique. Three dimensional external manifold models are constructed for a SOFC stack composed of 24 cells. CFD simulations with air as operating gas are implemented for two types of stacks with different inlet manifolds, including the manifold with three tube inlets (T‐manifold) and the manifold with a gas chamber on top (C‐manifold). The influences of different parameters such as channel resistance and gas feeding rate on flow distribution are studied. Modeling results indicate that the increase of channel resistance and a lower gas feeding rate can respectively improve the uniformity factor of T‐manifold and C‐manifold from 0.963 to 0.995 and 0.989 to 0.998. For a given channel resistance, the pressure distribution in the inlet manifold plays a dominant role in the flow distribution. In addition, flow distribution in the stack with C‐manifold is generally more uniform than the stack with T‐manifold. Furthermore, flow characteristics of the two type inlet manifolds are investigated by measuring velocity distribution of the gas at manifold outlets using a hot‐wire anemometer.     

A Control‐oriented Dynamic Model Adapted to Variant Steam‐to‐carbon Ratios for an SOFC with Exhaust Fuel Recirculation

The steam‐to‐carbon ratio (S/C) is a typical disturbance parameter in the operation of solid oxide fuel cell (SOFC) power generation system. A planar SOFC with a pre‐reformer and exhaust fuel recirculation system is investigated in this work. A lumped, nonlinear dynamic model is developed for the SOFC with consideration both of the spatial effect and the variant S/Cs. The dynamic model is deduced based on a fitting function so‐called Exponential Association Function, which is employed to describe the spatial distribution of state variables in SOFC. Three parameters of the fitting function are identified to integrate the spatial effect and S/C effect in the model. The parameters of Exponential Association Function are determined by function fitting on three‐dimensional numerical data at the sample operation points. Carbon formation activity is analysed using the simulation results and thermodynamic data. Dynamic simulation is implemented with the help of software MATLAB/SIMULINK. The results show that the developed model has good performance in predicting the responses of the state variables and catching the changes of S/C.     

Simulation and Experiment of a Cogeneration System Based on Proton Exchange Membrane Fuel Cell

This project designs and simulates a cogeneration system of proton exchange membrane fuel cell using Matlab/Simulink software and Thermolib heat module components. The system not only satisfies the need for electric power, but also provides heat recovery for future uses, thus increasing energy transfer efficiency. PEM fuel cell‐based cogeneration system is introduced, including the hydrogen supply subsystem, air supply subsystem, load control subsystem, real‐time monitoring block, and heat recovery subsystem. The complete fuel cell‐based cogeneration system is constructed by assembling the fuel cell stack, fuel, coolant flow rate control system, and all the subsystems. In addition to the fuel cell experiment, influencing factors on the fuel cell‐based system, such as the fuel inhale rate, coolant flow rate, system temperature, fuel humidification, thermal efficiency, electrical efficiency, and combined heat and power (CHP) system efficiency, are analyzed and charted regarding different loads. In this system, with the power at 3 kW, the CHP efficiency reaches 64%. The CHP efficiency is 76.6% with the load power at 4 kW. When the power is at 5 kW, the thermal efficiency reaches 36.9% and the CHP efficiency reaches 82.9%.     

SOFC Thermal Transients: Modeling by Application of Experimental System Identification Techniques

Solid oxide fuel cell (SOFC) is one of the most promising technologies for future power generation. In order to make this technology marketable, many issues as cost reduction, durability, and operational management have to be overcome. Therefore, the understanding of thermodynamic and electrochemical mechanisms, that govern the SOFC behavior in steady‐state and in transient operation, becomes fundamental. In this context, the modeling of fuel cell (FC) thermal transient is of great interest because it can predict the temperature time variation, useful to the dimensioning of auxiliary devices and to avoid unwanted operational states affecting cell durability. In the present study, a 0‐D model of SOFC thermal transients was developed by applying system identification techniques, starting from experimental tests carried out on a stack made up of four single cells. Moreover, it was successfully validated in reference to further experimental data. The model allows to evaluate, in term of dynamic response, the effect of the main operating parameters on FC temperature. As further result, some control/regulation considerations useful to limit thermal stresses were proposed.     

Start‐Up Characteristics of Segmented‐In‐Series Tubular SOFC Power Modules Improved by Catalytic Combustion at Cathodes

For large‐scale SOFC power generation systems, a shorter start‐up time of SOFC cell stacks with relatively large heat capacity is one of the most important technological issues to determine the flexibility in SOFC system operation. In this study, start‐up procedures have been examined to shorten the start‐up time period. The conventional heating procedure using pre‐heated hot air and self‐heating by SOFC operation at low temperatures had a difficulty to shorten the start‐up time period because of the limitation in power generation at lower temperatures. In this study, as an alternative start‐up procedure, catalytic combustion at the SOFC cathodes is, for the first time, demonstrated to be useful on the system level. The applicability of the catalytic combustion to shorten the start‐up time period has been verified numerically as well as experimentally by using a large‐scale cell stack cartridge. This unique start‐up procedure enables to operate SOFC‐based large‐scale power generation systems.     

Dynamic modeling of a finite volume of solid oxide fuel cell: The effect of transport dynamics

A dynamic model for a finite volume of cell based on physical principles is built in the form of a nonlinear state-space model to investigate dynamic behaviors of tubular solid oxide fuel cell (SOFC) and develop a control relevant model for further control studies. Dynamic effects induced by diffusions, intrinsic impedance, fluid dynamics, heat exchange and direct internal reforming/shifting (DIR) reactions are all considered. Cell temperature, ingredient mole fractions, etc. are the state variables and their dynamics are investigated. Dynamic responses of each variable when the external load changes are simulated. Simulation results show that fuel flow, inlet pressure and temperature have significant effects on the dynamic performance of SOFC. Further it is shown that, compared to other inlet flow properties, cathode side air inlet temperature has the most significant effect on SOFC solid phase temperature and performance. Compared with inlet pressures and temperatures, the effect of flow velocity is not significant. Simulation also indicates that the transient response of SOFC is controlled mainly by the dynamics of cell temperature owing to its large heat capacity.     

Optimization of a High Temperature PEMFC micro‐CHP System by Formulation and Application of a Process Integration Methodology

A 1 kWe micro combined heat and power (CHP) system based on high temperature proton exchange membrane fuel cell (PEMFC) technology is modeled and optimized by formulation and application of a process integration methodology. The system can provide heat and electricity for a single‐family household. It consists of a fuel cell stack, a fuel processing subsystem, heat exchangers, and balance‐of‐plant components. The optimization methodology involves system optimization attempting to maximize the net electrical efficiency, and then by use of a mixed integer nonlinear programming (MINLP) problem formulation, the heat exchange network (HEN) annual cost is minimized. The results show the high potential of the proposed model since high efficiencies are accomplished. The net electrical efficiency and total system efficiency, based on lower heating value (LHV), are 35.2% and 91.1%, respectively. The minimized total annual cost of the HEN is $8,147 year^–1.