Comparison of R744 and R410A for Residential Heating and Cooling Applications

In the first part of this report, experimental results are presented comparing a prototype R744 system with a commercially available R410A system in heating mode. When the heating capacity of the two systems is matched at the ARI heating capacity rating condition, the heating efficiency of the two systems is comparable. When the cooling capacity of the two systems is matched at the ARI cooling capacity rating condition, the heating efficiency of R744 is lower than R410A. In the second part of this report, a theoretical comparison of R744 and R410A is made. The theoretical cycle performance of R410A and R744 is compared on the basis of the comfort of the conditioned space, which is determined by dehumidification in cooling mode and the supply air temperature in heating mode. Cycles are analyzed with both ideal and real compressors, and trade-offs between heat exchanger size and efficiency are presented. In cooling mode, the relative efficiency of the two refrigerants is primarily a function of the airflow rate over the outdoor coil, since the indoor coil performance is constrained by the dehumidification requirement. The advantage for R744 in terms of slightly lower compression ratios and slightly higher evaporating temperatures is far outweighed by the thermodynamic advantage of R410A in terms of lower heat rejection temperatures at extremely high airflow rates. To reach its ideal thermodynamic efficiency, however, requires the subcritical R410A cycle have an infinite airflow rate. At the ARI rating condition, reducing the R410A airflow rate over the outdoor coil to match the finite rate required by R744, reduces the relative advantage of R410A by nearly 70%. In heating mode, the R744 cycle is more efficient than R410A for supply air temperatures above 40oC, resulting primarily from the higher compression ratios required by R410A for elevated supply air temperatures. Additionally, for a compressors sized for equal capacity in cooling, the capacity of the R744 cycle is higher at lower outdoor temperatures, which has important practical benefits in terms of reduced dependence on lower efficiency supplementary heating