Comparative analysis and multi-objective optimization of organic Rankine cycle (ORC) using pure working fluids and their zeotropic mixtures for diesel engine waste heat recovery

In this paper, the thermodynamic and thermoeconomic performance of an ORC system with pure working fluids and their zeotropic mixtures each having different critical temperatures are compared. Two high critical temperature (toluene and decane) and two low critical temperature (R245fa and R123) working fluids as well as their zeotropic mixtures are considered. The effects of four parameters including evaporation temperature, condensation temperature, superheat degree and mass fraction of the zeotropic mixtures on the thermodynamic and thermoeconomic performances of the ORC system are investigated. The multi-objective optimization based on genetic algorithm is conducted to reveal the optimal performances of the ORC system. The results show that high critical temperature working fluids and their zeotropic mixtures possess better thermodynamic and thermoeconomic performances, however, the zeotropic mixtures do not improve the thermodynamic and thermoeconomic performances obviously. Furthermore, the zeotropic mixtures at the mass fraction of 0.9 toluene/0.1 decane have the best thermoeconomic performance. The zeotropic mixtures can significantly reduce the heat transfer area of the heat exchangers under high evaporation temperature condition. In addition, the multi-objective optimization indicates that the optimal evaporation temperature approaches its upper bound while the optimal condensation temperature and superheat degree almost close to their lower bounds.