Effect of Fuel Cell Operation Pressure on the Optimization of a Hybrid System SOFC/Turbine for Cogeneration

A solid oxide fuel cell (SOFC) integrated in a hybrid system with a gas turbine can achieve lower heating value (LHV) power of efficiencies of about 70%. Given the high operating temperature of the SOFC, it produces high grade heat, and a hybrid system designed for cogeneration may achieve total LHV efficiencies of 78% of 80% without post combustion and 85%–88% with post combustion. The present paper illustrates the optimum integration of a tubular solid oxide fuel cell in a cogeneration cycle with a multiple pressure heat recovery steam generator (HRSG) and a back pressure steam turbine. We considered fuel cells of 7.5 MW, 9 MW, 15 MW, 15 MW, 18 MW, 22.5 MW and 27 MW by scaling up published data for a 1.2 MW tubular solid oxide fuel cell. The operating pressures were 3 and 9atm. We used GateCycle™ heat balance software by GE Enter Software, LLC, to design a 20–40 MW high efficiency cogeneration plant. We performed a calculation of the heat balance of the fuel cell stack in Microsoft® Excel and then we imported the results into GateCycle™. We developed curves showing LHV “electric” efficiency versus power for different ratios of “fuel cell-to-gas turbine size”. Pressurization has a positive impact on the fuel cell polarization curve leading to higher power output. The gain in electric power, however, is offset by the additional power requirement of the compressor at higher pressures. Our analysis shows that an optimum pressure of about 9 atmospheres results in an overall hybrid system power efficiency of about 70% and a LHV “cogeneration” efficiency of about 80%. In conclusion, high efficiencies are obtained by optimization of a hybrid system consisting of pressurized high temperature fuel cells with gas turbines and a steam turbine.