Performance analysis and optimization of engine waste heat recovery with an improved transcritical-subcritical parallel organic Rankine cycle based on zeotropic mixtures

Effective recovery of engine waste heat is a key strategy to alleviate energy and environmental issues. In this study, an improved transcritical-subcritical parallel organic Rankine cycle based on zeotropic mixtures is proposed to recover engine waste heat. Firstly, the proposed system is compared with other systems and shows the best performance. Then energy and exergy analyses are conducted, and three design parameters are analyzed. According to the results, for each turbine inlet temperature of high-pressure branch, the system performance initially increases then decreases with turbine inlet pressure of high-pressure branch, and the net power output firstly increases then decreases with evaporation temperature of low-pressure branch. Furthermore, the effect of composition of zeotropic mixtures on system performance is investigated, and zeotropic mixtures R600a/R601a and R600/R601 are compared. The results indicate that adopting zeotropic mixture can improve system performance significantly, and system based on R600/R601 performs better than based on R600a/R601a. Finally, performance optimization of system is conducted. The results show that system based on R600/R601 (0.5/0.5) achieves the maximum net power output of 114.47 kW which is higher 10.68% and 6.42% than based on pure R600 and R601 respectively, and the engine power output is improved by 11.49%.