Thermodynamic analysis of a novel transcritical-subcritical parallel organic Rankine cycle system for engine waste heat recovery

Abstract

In this study, a novel transcritical-subcritical parallel organic Rankine cycle system is proposed to recover the engine exhaust gas and coolant waste heat. Firstly, the proposed system is compared with the dual-loop organic Rankine cycle and parallel organic Rankine cycle systems. According to the results, the proposed system shows more net power output, higher thermal and exergy efficiency, and lower heat transfer requirement. Then the thermodynamic analysis including energy and exergy analysis are carried out, and the effects of design parameters including high-pressure turbine inlet temperature and pressure, low-pressure evaporation temperature on the performance of proposed system based on R601a are investigated. The results indicate that for a given high-pressure turbine inlet temperature, the net power output of system firstly increases and then decreases as high-pressure turbine inlet pressure increases and reaches to the maximum at turbine inlet temperature and pressure of 520 K and 8 MPa, and the net power output also shows a trend of firstly increasing and then decreasing with low-pressure evaporation temperature and reaches to the maximum at 330 K. Furthermore, five low global warming potential fluids are selected as working fluids of proposed system, and the results demonstrate that the R1233zd is the best candidate working fluid for proposed system. The maximum net power output of proposed system based on R1233zd reaches to 119.72 kW at the turbine inlet temperature and pressure of 530 K and 10 MPa, low-pressure evaporation temperature of 330 K, which achieves net power output increment of 12.02% for the engine system. Finally, an exergy destruction analysis demonstrates that the heat transfer processes have less exergy destruction for proposed system based on R1233zd compared with other working fluids.