Mitigating Defrosting in a Hybrid Air Conditioner Using a Ground Heat Exchanger Placed Downstream from an Outdoor Unit: Numerical Simulation and a Preliminary Experimental Demonstration

Air-source heat pumps suffer from the formation of frost on their outdoor coils while operating under cold and humid conditions, and the resulting periodic defrost cycles interrupt the heat supply and degrade both thermal comfort and efficiency. This study extends the hybrid air conditioner previously proposed by the authors, in which a ground heat exchanger is placed downstream of the outdoor heat exchanger, employing a shallow-buried water tank as a supplementary ground heat source to mitigate defrosting. A quasi-steady R32 vapor-compression cycle model was developed, coupled with lumped-capacitance thermal models of the room, the water tank, and the ground, including a frosting/defrosting model. Considering 2.2 kW heating at an outdoor temperature of +3.5 °C (with a relative humidity of 80%), the simulation predicted that the cumulative defrost time over 6 h would be reduced from 35 min (five events) to 0, the time required to reach the 20 °C set point would be shortened from 120 to 72 min, and the integrated coefficient of performance (COP) over 6 h would improve from 5.4 to 7.1. A preliminary one-hour winter heating experiment on a minimally modified commercial unit—one run per operating mode—showed that no defrosting occurred during hybrid operation, despite a higher outdoor relative humidity (74% vs. 46%), whereas two defrost events (inferred from the indoor outlet air velocity) totaling 12% of the run occurred in conventional operation, and the average COP increased from 1.8 to 3.0. An additional simulation with the as-built parameters of the prototype (300 L tank, tank-side conductance of about 60 W/K) reproduced this qualitative pattern under the hybrid-run weather, although with a small, nonzero predicted frost accumulation that remained below the defrost threshold. Because the two runs were conducted under different ambient conditions and the model was not calibrated against the tested system, the experiment provides qualitative rather than quantitative support for the simulated behavior; nevertheless, the simulation and the experiment consistently indicated the same relative advantage of the hybrid system, namely, the absence of defrosting and a higher COP. Future work will address repeated and extended experiments under matched conditions, model calibration, the optimization of the tank’s geometry and capacity, and the necessity of water agitation inside the tank.