A fuel cell free piston gas turbine hybrid architecture for high-efficiency, load-flexible power generation

Abstract

Hybrid systems that incorporate fuel cells, such as fuel cell-gas turbine (FC-GT) and fuel cell-internal combustion engine (FC-ICE) systems, are increasing in popularity as a means to provide high-efficiency, load-flexible power generation. A free-piston gas turbine (FPGT) engine is a synergetic compromise between GTs and ICEs. This work is the first to propose an FC-FPGT hybrid architecture to provide high-efficiency, load-flexible power generation. To study the FC-FPGT system, a MATLAB thermodynamic system-level model was developed. Using this model, the effect of the subsystem and total compression ratios of the FPGT on the overall hybrid system efficiency was studied. When the FC electrical efficiency was 65%, the highest overall FC-FPGT efficiency was found to be 59.4% without the use of an exhaust heat recovery system.The FC-FPGT is also compared directly with two different FC-GT architectures: one in which only the anode tailgas is combusted before the turbine and a second in which additional fuel is input after the FC to be combusted before the turbine. The FC-GT architecture using no additional fuel after the FC has an overall system efficiency 6.1 percentage points higher than the FC-FPGT with half the power density. The FC-GT architecture using an equivalent amount of additional fuel has an efficiency that is 10 percentage points lower than the FC-FPGT, with a power density slightly lower than the FC-FPGT. Overall, the FC-FPGT is a better option than the FC-GT when power density, load flexibility, and system cost are more important than overall system efficiency.