Exergy analysis of solid-oxide fuel-cell (SOFC) systems
ABSTRACT The exergy concept has been used to analyze two methane-fueled SOFC systems. The systems include preheating of fuel and air, reforming of methane to hydrogen, and combustion of the remaining fuel in an afterburner. An iterative computer program using a sequential-modular approach was developed and used for the analyses. Simulation of an SOFC system with external reforming yielded first-law and second-law efficiencies of 58 and 56%, respectively, with 600% theoretical air. Heat released from the afterburner was used to reform methane, vaporize water, and preheat air and fuel. When these heat requirements were satisfied, the exhaust-gas temperature was so low that it could only be used for heating rooms or water. Because of heat requirements in the system, fuel utilization (FU) in the FC was limited to 75%. The remaining fuel was used for preheating and reforming. Reduced excess air led to reduced heat requirements and the possibility of a higher FU in the FC. Irreversibilities were also reduced and efficiencies increased. Recycling fuel and water vapor from the FC resulted in first-law and second-law efficiencies of 75.5 and 73%, respectively, with 600% theoretical air, vaporization of water was avoided and the FU was greater.
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ABSTRACT: In this review paper, a comprehensive literature survey on macro-level modeling of solid oxide fuel cells (SOFCs) is presented. First, the current status of the SOFC modeling is assessed. Second, modeling techniques are discussed in detail. These include the thermodynamics, electrochemistry and heat transfer aspects of the modeling. Thermodynamic relations for pure hydrogen as the fuel and then gas mixture as the fuel are given. Additionally, exergy destructed due to polarizations is shown. Then, modeling equations for ohmic, activation, and concentration polarizations are given. Handling the carbon deposition problem in the modeling is discussed. The inclusion of the convection and radiation heat transfer processes to the modeling is explained. Finally, the models in literature are compared in terms of the methodology used and suggestions for increasing the accuracy of the future models are given. Copyright © 2007 John Wiley & Sons, Ltd.International Journal of Energy Research 03/2008; 32(4):336 - 355. · 2.74 Impact Factor
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ABSTRACT: Autothermal biomass gasification produces a gaseous fuel that, after gas cleaning and conditioning, can be used in solid oxide fuel cells (SOFC). Conceptually, the integrated system can be near atmospheric or at elevated pressures, allowing combination with a micro gas turbine (MGT) expander. This work comparatively investigates three small scale combined heat and power (CHP) configurations that integrate these technologies: (a) gasification at 4 bar and MGT, (b) gasification at 1.4 bar and SOFC and (c) gasification at 4 bar and SOFC–MGT. Aspenplus™ process simulation software was used for modelling each major and peripheral component of the CHP. Interestingly, the MGT system proved more efficient than the atmospheric SOFC, both of which were surpassed by SOFC–MGT performance that reached an exergetic electrical efficiency of 35.6% using an SOFC of 100 m2 active surface area and nominal biomass throughput of 200 kg/h. An exergy analysis allowed optimisation of the SOFC fuel utilisation factor (Uf) and efficiency impact of system capacity and level of product gas moistening prior to the cell.Energy Conversion and Management. 01/2008;
- Journal of Energy Resources Technology-transactions of The Asme - J ENERG RESOUR TECHNOL. 01/2009; 131(3).