Defrosting Performance Of Heat Pump System With Partial Defrost

Due to that the air source heat pump has the advantages of excellent energy saving,safety and durability,it is widely used in various fields.However,the operation of the air source heat pump in low outdoor temperature and high humidity will cause frost on the outdoor heat exchange to seriously affect the performance of the heat pump.Therefore,the frost problem of the air source heat pump has been a popular research issue.The problem also created many different methods of defrosting.In this paper,a heat pump system with partial defrosting is proposed based on the hot gas bypass defrosting method,and its system performance and defrosting performance are simulated and tested.The commonly used defrosting methods of the air source heat pump system in the project include manual defrosting,reverse cycle defrosting,hot gas bypass defrosting,and electric heating defrosting,but the above defrosting methods all have different problems.Manual defrosting will increase manpower consumption.Reverse cycle defrosting cannot provide heat and defrosting in the meantime and will reduce the indoor ambient temperature.The structure of the hot gas bypass defrosting system is more complicated and generates more waste of heat.Electric heating defrosting exists potential safety hazards and additional power consumption.For the shortcomings in existing defrosting methods,this paper proposes a partial defrosting heat pump system with one throttling and incomplete cooling.The system can be applied to the lower outdoor environment temperature by adopting the quasi two-stage compression cycle with incomplete cooling in the middle of one throttling.The partial defrosting mode is obtained by improving the hot gas bypass defrosting mode.The outdoor heat exchange is divided into three groups of heat exchange finned tubes with independent inlet and outlet to realize the partial control,which avoids the heat absorption from indoor environment and the melting of air source heat pump when defrosting Frost produces excess energy consumption.After setting up the experimental platform,it is going to explore the effect of the defrosting rate and performance of the system under different conditions of evaporation temperature and operating frequency of outdoor fans under certain conditions.The experiment uses R22 as the refrigerant and uses 12 shut-off valves to control the defrosting operation of the system.Two manually adjustable throttle valves are used to control the intermediate temperature and the evaporation temperature.The frequency converter is used to change the operating frequency of the outdoor fan.Using FORTRAN to establish differential equations,the defrosting process was simulated and theoretically analyzed.When the condensation temperature is 55℃,the outdoor environment temperatureis-20 ℃,the air supply in the middle is 30%,the relative humidity is 80%,the frost mass is2.5kg,and the frost thickness is 3mm,the heating cop of the system will increase at the early stage of the defrosting,which will be higher than that of the system in normal operation.When the defrosting cycle lasts for a period of time,the heating cop of the system will decrease and finally tend to be stable.The cop of the system under normal operation and defrosting operation decreased with the decrease of evaporation temperature.However,the average COP of the system under defrosting operation is smaller than that under normal operation.Under the condition that the fan operation frequency and outdoor environment temperature are fixed,the defrosting rate decreases with the decrease of evaporation temperature;under the condition that the evaporation temperature and outdoor environment temperature are fixed,the defrosting rate decreases with the increase of the fan operation frequency;under the condition that the outdoor environment temperature is fixed,the fan operation frequency is in the range of 0Hz~35Hz,and the fan operation frequency has an effect on the defrosting time The influence of evaporation temperature on defrosting time is less than that of evaporation temperature.When the operating frequency of fan is in the range of 35Hz~50Hz,the operating frequency of fan has more influence on defrosting time.Through the experiment,it is found that when the thickness of frosting on the surface of outdoor heat exchange is 3mm,the decrease of evaporation temperature will lead to the decrease of frosting time on the cold surface of outdoor heat exchange,which is approximately linear.Under the same evaporation temperature,the higher the operation frequency of the fan,the more frosting time.When the outdoor environment temperature is-20℃,the condenser inlet water temperature is 45℃,the outlet water temperature is 50℃and the flow rate is kept constant,when the heat pump system starts defrosting cycle,the condenser outlet water temperature will decrease with the decrease of the evaporation temperature,but the decrease range is gradually gentle.Under the same evaporation temperature,the fan operation frequency has little effect on the condensate outlet water temperature.When adjusting the evaporation temperature in the range of-25℃ ~-35℃,the defrosting time of outdoor heat exchange increases with the decrease of evaporation temperature,which basically shows a positive proportion function change.When the fan is in the off state,the defrosting time increases by 42.37% when the evaporation temperature is-35℃ compared with the defrosting time when the evaporation temperature is-25℃,and when the fan is in the 50 Hz operation frequency state,the defrosting time increases by42.37% When the fan is closed,the defrosting time increases by 53.38% on average.It is found that the defrosting time is decreased first and then increased with the change of fan operating frequency.When the evaporation temperature changes in the range of-33℃~-35℃,the fan operating frequency has the minimum defrosting time in the range of20Hz~30Hz.When the evaporation temperature changes in the range of-25℃~-33℃,there is a minimum defrosting time in the range of 15Hz~25Hz.When the fan operating frequency changes in the range of 0Hz~25Hz,the change of evaporation temperature has a greater impact on defrosting time.When the fan operating frequency changes in the range of 25Hz~50Hz,the change of fan operating frequency has a greater impact on defrosting time.