Frost/Defrost Models for Air-Source Heat Pumps with Retained Water Refreezing Considered

Abstract

Cyclic frosting and defrosting operations constitute acommon characteristic of air-source heat pumps in coldclimates during winter. Simulation models that can capturesimultaneous heat and mass transfer phenomena associatedwith frost/defrost behaviors and their impact on theoverall heat pump system performance are of criticalimportance to improved controls of heat delivery and frostmitigation. This paper presents a novel frost formulationusing an enthalpy method to systematically capture allphase-change behaviors including frost formation andmelting, retained water refreezing and melting, and waterdrainage during cyclic frosting and defrosting operations.A Fuzzy modeling approach is proposed to smoothly switchsource terms when evaluating the dynamics of frost andwater mediums for numerical robustness. The proposedfrost/defrost model is incorporated into a flat-tubeoutdoor heat exchanger model of an automotive heat pumpsystem model to investigate system responses under cyclicoperations of frosting and reverse-cycle defrosting.