Investigation on the effects of the optimal medium-temperature on the system performance in a transcritical CO₂ system with a dedicated transcritical CO₂ subcooler

For solving the sharp deterioration of system performance in a transcritical CO₂ heat pump under high water-feed-temperature, a dedicated transcritical CO₂ subcooler was proposed to be installed at the exit of the main-cycle gas-cooler. As a main factor affecting the system performance, the medium-temperature of the proposed system, which is defined as the expansion-valve inlet temperature of the main-cycle, is deeply investigated in this study. With the assistance of a validated coupled simulation model, it could be observed that, with the medium-temperature increased, the suction-pressure rose while the compressor-displacement declined in the subcooler; the suction-pressure and mass-flow-rate increased gradually in the main-cycle. Besides, the heating capacity and power consumption reduced while the heating-COP increased in the subcooler; the heating capacity and power consumption went up while the heating COP went down in the main-cycle. Finally, the existence of the optimal-medium-temperature was observed from the system COP.