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 c_p 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.