Investigation of trans/supercritical CO2-based mixed working fluids cycle performance in marine environment

The marine environment has an abundant natural cold energy, using CO 2 closed cycle combined for development is considered one of the best options. However, seawater temperature fluctuates greatly with depth and can reach 4-6°C at a depth of 1000m, the optimal cycle configuration and parameter matching is still a mystery. Therefore, this paper established thermodynamic model to investigate the performance of CO 2 -based mixed fluid cycle under variable low-temperature seawater conditions. The results indicated that transcritical gas CO 2 -based mixed fluid Brayton cycle (TGBC) and supercritical CO 2 -based mixed fluid Brayton cycle (SBC) are more suitable for low-power level systems on seawater surface, while transcritical liquid CO 2 -based mixed fluid Brayton cycle (TLBC) and transcritical CO 2 -based mixed fluid Rankine cycle (TRC) are more suitable for deep-sea high-power level systems. The low critical parameter mixed working fluid cycle can further expand cycle temperature range and pressure ratio to improve cycle thermodynamic performance. Compared with S-CO 2 Brayton cycle, the thermal efficiency of CO 2 /Xe (0.5/0.5) transcritical Rankine cycle, CO 2 /SF 6 (0.9/0.1) transcritical liquid Brayton cycle and CO 2 /SF 6 (0.5/0.5) transcritical Rankine cycle can be increased by up to 3.79%. This work provides insights into the application of CO 2 -based mixed fluid cycle in marine environments and subsequent research priorities.