Thermodynamic optimization of Rankine cycle using CO₂-based binary zeotropic mixture for ocean thermal energy conversion

This work provides an exploration on improving the performance of a closed ocean thermal energy conversion (OTEC) system. In order to approach the Lorenz cycle and obtain better thermal matching, a Rankine cycle using CO₂-based binary zeotropic mixtures is considered. Six organic working fluids, including R134a, R152a, R161, R1234yf, R1234ze(E) and R32, are selected to be additives for binary mixtures, in addition, various concentrations of CO₂ are investigated in order to obtain varying temperature glide. Besides, pure working fluids, including NH₃ and CO₂, are also comparatively investigated with the mixtures. The specific net power output and thermal efficiency are used to evaluate OTEC thermodynamic performance, and the ratio of net power output to total heat transfer area is adopted for a preliminary economic analysis. Different effects on cycle performance are analyzed. Finally, an overall optimization to maximize the system thermal efficiency and specific work are carried out, respectively. The simulation is based on a designed Matlab program. The results indicate that CO₂-based binary zeotropic mixtures could improve thermodynamic coupling of cycle and external seawater, achieving a deeper heat utilization of warm/cold seawater than that of pure working fluid. The performance of Rankine cycle is affected by the mixture composition, and composition at which mixture has evaporating temperature glide of 7–8 °C is recommended. The binary mixtures produce larger specific power output than pure working fluids, and CO₂/R32 (0.76/0.24 wt%) produces the maximum value of 0.696 kJ/kg, nearly 38% higher than that of pure NH₃. Although the mixtures are inferior to NH₃ according to preliminary economic analysis. The thermodynamic findings still prove that Rankine cycle with CO₂-based binary mixture is a promising alternative for OTEC system.