Investigation on reverse cycle defrosting strategy of an outdoor heat exchanger in air conditioning heat pump system for electric vehicles

Abstract

Outdoor heat exchanger in air conditioning heat pump (ACHP) system would frost quickly under a low-temperature but high-humidity surroundings. System performance and efficiency would significantly decrease without a rapid and appropriate defrosting strategy. In this work, a quick response reverse cycle defrosting strategy for an ACHP system in electric vehicles is presented, and its effectiveness is validated experimentally. Frosting experiments were conducted to capture the typical heat transfer parameters and frosting behaviors, and then the frost coverage ratio was obtained with threshold division method. An uneven frosting process were observed on the surface of the microchannel outdoor heat exchanger due to uneven dryness distribution. Then a certain frost coverage ratio was obtained as the reverse start criteria, and a series of defrosting experiment were conducted in validation of the reserve defrosting strategy. Results show that the frost coverage ratio of 0.56, or 2 °C deterioration of indoor outlet air temperature can be chosen as the reverse start criterion value with the current reverse cycle defrosting strategy for vehicular ACHP system. Frost can be almost removed completely within 40s with the proposed reverse defrosting strategy under the condition of ambient temperature 0–5 °C and relative humidity 80–85%.