Parametric Thermodynamic Analysis of a Solid Oxide Fuel Cell Gas Turbine System Design Space

Abstract

A parametric study of Solid Oxide Fuel Cell – Gas Turbine (SOFC-GT) hybrid system design is conducted with the intention of determining the thermodynamically-based design space constrained by modern material and operating limits. The analysis is performed using a thermodynamic model of a generalized SOFC-GT system where the sizing of all components, except the fuel cell, is allowed to vary. Effects of parameters such as pressure ratio, fuel utilization, oxygen utilization, and current density are examined. Operational limits are discussed in terms of maximum combustor exit temperature, maximum heat exchanger effectiveness, limiting current density, maximum hydrogen utilization, and fuel cell temperature rise. It was found that the maximum hydrogen utilization and combustor exit temperature were the most significant constraints on the system design space. The design space includes the use of cathode flow recycle and air preheat via a recuperator (heat exchanger). The effect on system efficiency of exhaust gas recirculation using an ejector versus the use of a blower is discussed, while both are compared to the base case of using a heat exchanger only. It was found that use of an ejector for exhaust gas recirculation caused the highest efficiency loss, and the base case was found to exhibit the highest overall system efficiency. The use of a cathode recycle blower allowed the largest downsizing of the heat exchanger, although avoiding cathode recycling achieved the highest efficiency. Efficiencies in the range of 50 – 75% were found for variations in pressure ratio, fuel utilization, oxygen utilization, and current density. The best performing systems that fell within all design constraints were those that used a heat exchanger only to preheat air, moderate pressure ratios, low oxygen utilizations and high fuel utilizations.