Thermodynamic and dynamic analysis of a hybrid PEMFC-ORC combined heat and power (CHP) system

Abstract

To improve the electrical efficiency of proton exchange membrane fuel cell (PEMFC) combined heat and power (CHP) systems, a novel hybrid CHP system combining PEMFC with organic Rankine cycle (ORC) has been proposed in this study. The proposed system recovers waste heat from the PEMFC stack, electrochemical reaction products, and air compressor, and then further generates electricity through the ORC subsystem. The ORC adopts seawater as the coolant to absorb the low-grade heat generated by the novel system. Based on a 60-kW typical PEMFC CHP system in a hydrogen-electric coupling demonstration project, a thermodynamic model of the novel system is established, and the steady-state, dynamic and economic analysis are performed. At rated stack current, the novel system shows an energy efficiency of 75% and an exergy efficiency of 51%. The electrical efficiency and the electricity exergy efficiency are improved by 4.3% and 5.1% respectively compared to the typical system. As the stack current increases, the electrical efficiency of the novel system declines more slowly. Moreover, the temperature sensitivity analysis shows that the lower the seawater temperature, the higher the system exergy efficiency. During 24 h dynamic operation, the novel system has higher electrical efficiency and exergy efficiency, the average response time of the cycle pump is reduced by 27.5%, and 4.56 kg hydrogen is saved compared to the typical system. In the economic analysis, the levelized cost of energy is 0.34 $/kWh for the novel system at rated operating condition, which is slightly better than the typical system. If the cost of PEMFC stack and hydrogen price decrease by 50%, the levelized cost of energy for the novel system will drop by 47.4% and reach 0.179 $/kWh.