Exergy analysis of solid-oxide fuel-cell (SOFC) systems

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.