Abstract:

Zeotropic mixtures are popular alternatives to chlorofluorocarbons (CFCs）and hydrochlorofluorocarbons (HCFCs）in the high temperature heat pump system.Zeotropic mixtures exhibit two major characteristics during phase change：temperature gliding and nonlinear relationship between temperature and enthalpy.In order to reveal the distribution of temperature difference between zeotropic mixture and heat transfer fluid in heat exchangers and get clear how temperature differences affect the performance of the system, experiments were carried out with the binary mixture Z1 in the high temperature water-water heat pump system, which was composed of 17% R290 and 83% R600 (mass ratio).The experimental results showed that as the water flow rate decreased the maximum temperature difference occurred in the condenser and its location moved to the cold end of the condenser, while the minimum temperature difference occurred in the evaporator and its location moved to the cold end of the evaporator.It was found that the occurrence of maximum temperature difference decreased the coefficient of performance of the system and increased the exergy loss caused by irreversible heat transfer in the condenser, while the occurrence of pinch point increased the coefficient of performance of the system and decreased the exergy loss caused by irreversible heat transfer in the evaporator.The results demonstrate the importance and the need for pinch point analysis in the evaluation of zeotropic refrigerant mixtures applied in air-conditioner or heat pump.In addition, the change of the temperature difference between the working fluid and heat transfer media found in this study is helpful to design heat exchangers in heat pump units.