Role of CO2-based mixtures in the organic Rankine cycle using LNG cold energy

Organic Rankine cycle (ORC) is an attractive method for recovery of liquefied natural gas (LNG). Zeotropic mixtures are optimal working fluids for improving the ORC power generation performance. In this study, eight CO2-based binary mixtures were selected as alternative working fluids for the utilization of medium-temperature LNG cold energy for ORC. The effect of CO2-based binary mixture on thermodynamic performance of ORC was presented and investigated. The genetic algorithm was applied to optimize the thermodynamic performance of ORC so as to determine the optimal working fluid among CO2-based binary mixtures. The results indicate that CO2-based binary mixtures significantly improve the thermodynamic performance of ORC in the recovery of LNG cold energy when compared with pure-CO2 ORC. There is a specific CO2 mass fraction that leads to the highest net power output of the ORC, and the mass fraction of CO2 increases with higher condensation temperatures (For CO2/Propane, the condensation temperatures are −55 °C, −40 °C, and −30 °C, and this CO2 mass fraction is 0.35, 0.65, and 0.8, respectively). Furthermore, the ORC using CO2/R134a exhibits the highest net power output at 13.50 kW among the eight alternative CO2-based binary mixtures.