Studies on a transcritical R744 based summer air-conditioning unit: Impact of refrigerant charge on system performance

The present study is carried out to investigate the performance of a transcritical R744 based system for air conditioning. In this study, a detailed numerical model of the R744 based system is developed. An experimental test-rig of a R744 based air conditioning unit is designed and developed as well to perform experimental studies and to validate the theoretical model. Using the validated model, simulations are carried out to investigate the effects of refrigerant charge, ambient temperature, and gas cooler face velocity on the performance of the system. The optimum charge for which the system delivers maximum COP is predicted. It is observed that variation in COP is marginal for a charge variation of  ± 18% from the optimum value. However, COP decreases significantly if the charge varies more than  ± 18% from the optimum value. Results show that for each 10 K increase in the ambient temperature, the system COP decreases by about 24%. It is observed that COP drops gradually up to 17% for a 60% reduction in gas cooler face velocity. However, on reducing the face velocity beyond 60%, COP drops significantly with rapid increase in discharge pressure.