Thermodynamic and Exergy Analysis of a Novel PEMFC-ORC-MH Combined Integrated Energy System

Nowadays, Proton Exchange Membrane Fuel Cell (PEMFC) generates a large amount of heat in operation, resulting in energy waste. To improve energy utilization through waste heat recovery, a novel integrated energy system coupling with PEMFC, Organic Rankine Cycle (ORC) system and Metal Hydride (MH) hydrogen storage system was unprecedentedly presented and investigated. The ORC system utilizes the heat generated by the stack and further generates electric energy, while the MH system absorbs the heat in the exhaust steam of the ORC working medium and then desorbs hydrogen. Based on the above design, this study established the mathematical model of the system, and then analyzed the variation trends of system performance versus the stack current variation. Following that, this study investigated the effect of crucial parameters, including the inlet temperature for the stack cooling water, the superheat temperature, the saturation pressure, the condensation temperature, the pressure in MH tank and the hydrogen weight fraction, on the system performance. Results demonstrated that, when compared with that of the single PEMFC, the overall performance of the system significantly improves, and at the maximum power point of the system, the output power, electrical efficiency, energy efficiency and exergy efficiency rise by 15.0%, 4.3%, 14.1% and 4.9%, respectively. Besides, the simultaneous increase of the superheat temperature and saturation pressure can improve the electrical efficiency of the system without reducing the hydrogen desorption rate. Moreover, the higher hydrogen weight fraction and lower pressure in MH tank could improve the hydrogen desorption rate. Additionally, the system's electrical efficiency and the hydrogen desorption rate are highly sensitive to the condensation temperature, and it is recommended that the condensation temperature be controlled within a reasonable range of 300 K-310 K. Present works perform a theoretical basis for the construction and performance optimization of this proposed system.