Defrosting Control Methods Experimental Research Based On The Cascade Type Air-source Heat Pump Water Heater

The experimental study of frosting and defrosting features based on the cascadetype air-source heat pump water heater is not only in the easy frosting area(outdoortemperature is from-7℃to6℃), but also in the lower outdoor temperature(from-18℃to-7℃) in this paper. And the heat water can achieve the goal of the production of80℃hot water under-25℃ambient environment.First,meteorological parameters of three cities in northeast China have beenanalyzed to demonstrate the significance of the study, finding the climate is commonthat outdoor temperature is from-18℃to-7℃and outdoor relative humidity is from70%to90%.The frosting experiment based on the cascade type unit is carried out in theenthalpy-difference lab.And the water heater includes the low temperature level cycleand the high temperature level cycle. In the low temperature level cycle, R410A isused as the refrigerant, while in the high temperature level cycle, R134a is used as therefrigerant. In addition, the outdoor environmental conditions include that the outdoortemperature is from-18℃to6℃and outdoor relative humidity is from70%to90%in this experiment. According to the frosting experiment, some parameters would beobtained, including the coefficient of performance (COP), the amount of heat exchangeand the power of the heat pump unit, the evaporator coil temperature in the lowtemperature level cycle and the intake and exhaust gas temperature and pressure in thehigh or low temperature level cycle, etc.Based on the frosting experiment, the trial operation experiment of defrosting iscarried out and the Time-Temperature difference defrosting control method isoptimized. Finally, the optimized control method and the frosting factor (the ratio ofthe frosting amount and the frosting-time and the difference of the outdoor temperatureand the evaporator coil temperature in the low temperature level cycle) is also verifiedby the following defrosting experiments.Experimental results are described below. Firstly, the amount of frosting almostchanges linearly with the frosting time, and when the frosting amount reaches a certainlevel, the unit performance will be made great influences. Secondly, the frosting factorgeneralized in the frosting experiment which is8.58×10-4kg/(m2·℃·min) can predictthe frosting amount on the surface of evaporator coil and be used to the startingcondition of defrosting. At the same time, the reasonably and feasibility of the frosting factor are proved by the comparison of the theoretical and experimental values and thedifference of the theoretical and experimental values is not large. Thirdly, theTime-Temperature difference defrosting control method which is optimized by the trialoperation experiment of defrosting is imported into PLC of the unit. Then, the heatpump unit with the new control method is operated and the running results demonstratethat the phenomenon of mistaken defrosting or incomplete defrosting is not foundand the unit operating performance keep well,namely the difference of the averagecoefficient of performance after defrosting and the normal coefficient of performanceis not more than15%. In the end, the shortcoming of the defrosting method of hot gasbypass is found by the defrosting experiment that when outdoor temperature below acertain value (the value is located between-12℃and-9℃), the frost layer on thesurface of the evaporator coil in the low temperature level cycle can be cleanedincompletely by the defrosting method. To the later convenience of selecting theelectric heating defrosting method, the power of the electric heater is calculated in thepaper.