Thermodynamic modeling and analysis of a novel PEMFC-ORC combined power system

In this study, a novel proton exchange membrane fuel cell (PEMFC) system is proposed, which uses organic working fluid to cool the fuel cell stack directly and recovers the waste heat by combining with an Organic Rankine Cycle (ORC) system. A thermodynamic model of each component and subsystem is established for the combined PEMFC-ORC system. The influence of the PEMFC stack inlet temperature and current density, the ORC working fluid, superheat temperature and saturation pressure on the system performance is studied. The flow and distribution of energy and exergy in the whole PEMFC-ORC system are analyzed comprehensively. It is found that the system performance indicators reach the optimum when the stack inlet temperature is about 343.15 K. Lower current density improves the system efficiency, but reduces the system power density. R245fa shows the best performance among the five organic working fluids investigated for the designed ORC system. Higher superheat temperature and saturation pressure of the organic working fluid in the cycle improves the ORC efficiency. The PEMFC stack has the largest exergy loss in the system, and the cathode side heater and air compressor also contribute much to the large power and exergy loss. The optimization of these components should be the focus of system performance improvement.