A Critical Look at R-744 and R-134a Mobile Air Conditioning Systems

There is intense research activity to appraise the merits of the carbon dioxide (R-744) mobile air conditioning system due to its perceived amelioratory effect on the total global warming impact which comprises two components: direct global warming due to refrigerant leakage into the atmosphere and indirect global warming due to power consumption by the system. While the direct global warming impact of R-744 is negligible compared to that of R-134a, the indirect global warming impact of the R-744 system is intrinsically higher than that of the R-134a system. In order to quantify the indirect global warming impact of the R-744 system, an accurate assessment of its coefficient of performance (COP) vis-a-vis COP of the present baseline R-134a system is necessary. Presented in this paper is the critical assessment of the R-744 system, including its COP under realistic operating conditions, heat exchanger and compressor characteristics, total global warming impact, and historical evidence that suggests that the proposal to replace R-134a by R-744 is untenable in view of lower energy efficiency and 5 to 12 times higher operating pressure for the R-744 system.