Experimental Study On Optimal Defrosting Strategy Of Evaporator Based On Entropy Analysis

The energy efficiency of the air source heat pump system is often weakened due to the frost layer condensing in the outdoor side heat exchanger during the winter.Defrosting is one of the widely used measures to restore the performance of air source heat pumps.However,defrosting not only affects the heating stability,but also increases the energy consumption of the system.Therefore,a reasonable defrosting start time is essential to reduce defrosting energy consumption.However,most previous control strategies are only devoted to the change of experimental parameters after frosting and influence on the heat transfer performance of the heat exchanger,and pay no attention to the influence of the frosting process on the energy transfer process between air and refrigerant.This paper aims to study on thermodynamic property base on the second law of thermodynamics during the frosting process of the evaporator to reduce the defrosting energy consumption.The defrosting strategy of evaporator is proposed based on entropy analysis.The change of the energy transfer process between air and refrigerant in the frosting process are studied to analyze the index of irreversibility-entropy and seek the optimal defrost-start time so as to reduce the defrost energy consumption,and then improve the system energy efficiency.Results indicate that due to the formation of frost,the entropy flow and entropy production caused by the temperature difference and pressure drop show a negative correlation trend,which makes the entropy generation and the entropy resistance show decreasing trend initially and then increasing.Simultaneously the irreversibility distribution ratio is approximately 1 and the minimum of the entropy generation and entropy resistance was achieved.Hence,the minimum irreversible degree and the peak of the energy efficiency can be obtained.When the PTP is approximate 0.8 K/P.It can be determined that PTP can be used as rational for defrosting control parameters.The defrosting strategy of evaporator based on entropy analysis was compared with traditional time-based defrost methods,through experiment under frost conditions.It is shown that the defrosting strategy of evaporator based on entropy analysis is more in line with the frost formation law,and the possibility of misoperation is smaller.From the start of the air source operation to the end of the frost,as the amount of frost per m~2 on the surface of the evaporator is smaller,it is concluded that the thermal energy from indoor,the electricity inputs to compressor and fan,the energy consumed on melting frost,and the energy consumed on evaporating water are also smaller.It is verified that the defrosting strategy of evaporator based on entropy analysis can realize the optimal heat transfer performance,the highest heat transfer efficiency and the high-efficiency operation in the low temperature environment of the air source heat pump system.