Evaluation of carbon dioxide as R-22 substitute for residential air-conditioning

This paper compares the performance of CO₂ and R-22 in residential air-conditioning applications using semi-theoretical vapor compression and transcritical cycle models. The simulated R-22 system had a conventional component configuration, while the CO₂ system also included a liquid-line/suction-line heat exchanger. The CO₂ evaporator and gas cooler were microchannel heat exchangers originally designed for CO₂. The R-22 heat exchangers employed the same microchannel heat exchangers as CO₂ with the difference that we modified the refrigerant passages to obtain reasonable pressure drops. The study covers several heat exchanger sizes. The R-22 system had a significantly better coefficient of performance (COP) than the CO₂ system when equivalent heat exchangers were used in the CO₂ and R-22 systems, which indicates that the better transport properties and compressor isentropic efficiency of CO₂ did not compensate for the thermodynamic disadvantage of the transcritical cycle in comfort cooling applications. An entropy generation analysis showed that the CO₂ evaporator operated with fewer irreversibilities than did the R-22 evaporator. However, the CO₂ gas cooler and expansion device generated more entropy than their R-22 counterparts and were mainly responsible for the low COP of the CO₂ system.