heat pump, outdoor heat exchanger, frosting, defrosting, water retention
This paper presents the performance in frosting and defrosting of an automotive carbon dioxide (CO2) heat pump system, which is composed of a microchannel indoor and outdoor coil, a reciprocating compressor, an electronic expansion valve, and an integrated accumulator and internal heat exchanger. The effects of the criterion for the start of defrosting on the heat pump system performance were investigated experimentally. The defrost-start criterion was determined based on the reduction in the performance of the heat pump system instead of only considering the performance of the outdoor coil. Results show the heat pump system can operate for 64 minutes at 0℃ and 90% RH with a defrost-start criterion of 10 times the initial air-side pressure drop. The accumulator and internal heat exchanger improve system efficiency during frosting conditions. The mass of frost accumulation increases faster in the first 40 minutes and slower in the next 24 minutes for 0℃ and 90% RH. The retained water on the outdoor coil gets saturated after three short frosting/ defrosting cycles or a 64-minute long run. Also, the frosting and defrosting processes are monitored by a web camera to investigate the frost distribution on the two-pass outdoor microchannel heat exchanger. The results are presented for evaluating the frosting distribution and the effects on the overall performance of the heat pump system.