Effects of refrigerant charge amount on the performance of a transcritical CO₂ heat pump

A typical transcritical CO₂ system shows lower performance than conventional air conditioners in cooling mode operation. In addition, the CO₂ system shows a large variation of the performance according to refrigerant charge whereas the conventional systems do not show large variation. In this study, the performance of the CO₂ heat pump was measured and analyzed by varying the refrigerant charge amount at standard cooling condition. In addition, the performance sensitivity of the CO₂ system as a function of refrigerant charge was compared to those for the R22, R410A, and R407C systems. The cooling COP of the CO₂ system was reduced more significantly at undercharged conditions than at overcharged conditions as the deviation from the optimal charge increased. The expansion loss was the dominant factor affecting system performance at undercharged conditions, while the gascooler loss became the major parameter at overcharged conditions. Among the systems investigated and compared in this study, the CO₂ system showed the most reduction in performance at undercharged conditions.