The performance analysis of the transcritical Rankine cycle using carbon dioxide mixtures as the working fluids for waste heat recovery

The aim of this study is to investigate the economic performance improvement of a transcritical Rankine cycle system employing carbon dioxide mixtures for waste heat recovery. Five lower global-warming-potential working fluids, namely difluoromethane, fluoroethane, propane, tetrafluoroethane, and tetrafluoropropene, are selected to blend with carbon dioxide. In addition to thermodynamic analysis, the apparatus and maintenance costs of the waste heat recovery system are considered to evaluate the levelized energy cost for the economic performance study. The results indicate that the system cycled with carbon dioxide/fluoroethane exhibits the most effective economic performance, and is superior to that cycled with carbon dioxide/difluoromethane, carbon dioxide/tetrafluoropropene, carbon dioxide/tetrafluoroethane, carbon dioxide/propane, and pure carbon dioxide by 0.7, 13.89, 15.27, 20.88, and 21.52%, respectively. In the transcritical Rankine cycle operated with the carbon dioxide mixtures, not only are energy costs reduced, but the high pressure and temperature are also decreased. Furthermore, the optimal expander inlet pressures for the transcritical Rankine cycle with a minimal levelized energy cost are always smaller than those with maximal thermal efficiencies. Finally, economic performance correlations are proposed for the waste heat recovery system, utilizing various waste heat source temperatures.