Novel air-coupled and hydronically coupled heat pumps using microchannel components were investigated in this study. The air-coupled system uses microchannel tube, multilouver fin heat exchangers as the evaporator and condenser. In the hydronically coupled heat pump, refrigerant in the evaporator as well as the condenser transfers heat to an intermediate fluid such as an ethyleneglycol solution. The glycol loops are connected to the indoor/outdoor air through liquid-air heat exchangers. Models to simulate cycle thermodynamics, and single- and two-phase heat transfer in the components were developed to design these systems for cooling and heating mode operation. The components were optimized to develop the most compact systems that would satisfy system performance requirements. These systems were also compared with a conventional round-tube, plate-fin heat pump, which was designed using a commercially available simulation tool.
Results from this study show that indoor and outdoor units of air-coupled microchannel systems can be packaged in only one-half and one-third the space required for a conventional system. Even more compact refrigerant heat exchangers are required in the hydronically coupled system, because of the high heat transfer coefficients for these liquid-coupled heat exchangers, and the counterflow orientation. The hydronic coupling offers flexibility in system location, and is well suited for integrated space-conditioning and water heating systems. Both air-coupled and hydronically coupled systems result in significant reductions in refrigerant inventories compared to round-tube systems. The refrigerant charge of the microchannel air-coupled system is 20% less than that of the round-tube heat pump. For the hydronically coupled system, the refrigerant charge is only 10% of the charge in the round-tube heat pump.