The optimal defrosting condition of an air source heat pump based on entropy analysis

Abstract

ABSTRACT In winter, the heat transfer characteristics and energy efficiency of air source heat pumps (ASHP) will deteriorate due to the frost forming on the surface of the evaporator. In order to recover the thermal performance of the heat exchanger, defrosting is necessary. The optimal defrosting condition is one of the crucial factors to achieve efficient operation of ASHP. 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 between air and refrigerant. These methods cannot achieve the high efficiency of system energy utilization. Thus, this study investigated the feasibility of using the entropy analysis control method to seek the optimal defrost conditions. Firstly, the trend of the temperature difference and the frictional pressure drop (PTP) as the key parameters of entropy generation analysis is developed. Secondly, based on the minimum principle of entropy generation and entropy resistance, the optimal defrost conditions of the evaporator under frosting conditions are studied. Finally, experiments are carried out to verify the optimal defrost parameters for the entropy analysis control method using the proportion of the temperature difference and the drop (PTP. 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 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. It can be determined that the proportion of the temperature difference and the PTP can be used as irrational for defrosting control parameters. When the proportion of the temperature difference and the PTP is approximately 0.8, the system entropy generation and entropy resistance are the smallest. 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 ASHP system.