A model for improvement of water heating heat exchanger designs for residential heat pump water heaters : a thesis presented in fulfilment of the requirement for the degree of Master of Engineering at Massey University, Palmerston North, New Zealand
Heat pump water heaters are a promising technology to reduce energy use and greenhouse gas emissions. A key component is the water heating heat exchanger. Two multi-zone models of the double-wall counter-current flow heat exchanger (condenser and gas cooler models) for residential air-source heat pump water heaters were developed. These models were validated against available data in the open literature. They predicted heat exchanger size within -0.8% for a HFC-134a (with oil) condenser and within -14% for a CO2 gas cooler. The multi-zone model was significantly more accurate than one and three zone models. The models for a R410A subcritical heat pump and a CO2 transcritical heat pump were used to investigate the effect of key design parameters by varying water or refrigerant flow channel size for three water heating heat exchanger configurations: circular tube-in-tube, flat tube-on-tube, and twisted tube-in-tube. For the circular tube-in-tube configuration, refrigerant flow in the annulus (case B) performed better than refrigerant flow in the inner tube. The optimal flow channels for the circular tube-in-tube configuration case B with 0.1 mm thick air gap in the double wall were found to be di (inside diameter of the 1st tube) of 8 mm and annulus [Di (inside diameter of the 3rd tube) -d2 outside diameter of the 2nd tube)] of 1.5 mm for R410A and di of 7 mm and Di −d2 of 1.0 mm for R744. The optimal flow channels for the flat tube-on-tube configuration with b1i (major length of the refrigerant flow channel) and b2i (major length of the water flow channel) both of 9 mm were found to be a1i (minor length of the refrigerant flow channel) and a2i (minor length of the water flow channel) of 1.5 mm for R410A and a1i of 1 mm and a2i of 1.5 mm for R744. The optimal flow channels for the twisted tube-in-tube configuration were found to be di of 7.94 mm and d1 (original inside diameter of twisted tube) of 12.7 mm for R410A and di of 6.35 mm and d1 of 9.525 mm for R744. At the optimal flow channel size in each configuration, heat exchanger weight of the flat tube-on-tube was lower than the circular tube-in-tube by about 34.4% for R410A and by about 66.6% for R744. This was mainly due to elimination of the air gap resistance with the tube-on-tube configuration. Heat exchanger length, weight, and pumping power of the twisted tube-in-tube with 94% contact were significantly lower than the flat tube-on-tube by about 85%, 62%, and 97% respectively for R410A and by about 65%, 35.7%, and 98% respectively for R744. Overall, the flat tube-on-tube and the twisted tube-in-tube configurations are most promising for the water heating heat exchanger in terms of the lowest investment and running costs respectively.