Performance advantages of transcritical CO2 cycle in the marine environment

Low-temperature seawater is far away from CO2 critical temperature, which has significant impacts on the performance of CO2 closed cycle in the marine environment. The temperature adaptability of CO2 closed cycle to the marine environment and its performance remain open issues. In this paper, we compare performance advantages of transcritical/supercritical CO2 cycle based on thermodynamic and dynamic models. Compared with supercritical CO2 Brayton cycle, transcritical gas-phase CO2 Brayton cycle exhibits higher thermal efficiency and specific power at low cycle maximum pressure, and its reduced heat load and thermal inertia of regenerator facilitate cycle rapid response. However, transcritical liquid-phase CO2 Brayton cycle and transcritical CO2 Rankine cycle demonstrate higher thermal efficiency and specific power at high cycle maximum pressure. Lower compressor inlet temperature causes CO2 pseudo-critical point to migrate into regenerator, and the intersection point of cp curves of CO2 on both sides is located within regenerator. This can lead to pinch point and non-physical design within regenerator that inhibits cycle response. It can be avoided by adjusting cycle matching parameters so that the temperature corresponding to intersection point is lower than regenerator hot side outlet temperature. This study provides insights into performance advantages of transcritical CO2 cycles in marine environments.