A Defrosting Control Method Considering Heating Property And Defrosting Time For Air Source Heat Pumps

When an air source heat pump (ASHP) operates for space heating in the winter, the frost formed on the surface of its outdoor fin-tube exchanger becomes problematic. The frost reduces the airflow rate through the exchanger, increases the thermal resistance between the air and the exchanger, then pulls the evaporating temperature down, leading to the operating efficiency of the unit degarding and even affecting the secure operation, so the frost must be removed in the right time. According to the misoperation of the defrosting and the ignorance of the efficiency which are common phenomenon for the existing defrost control, this paper put forward a defrost control method considering the heating property and defrost time of the unit, and realizing the best comprehensive performance of the unit.The defrost control method aimed at a biggest evaluation parameter COPc of the unit operating cycle performance to obtain the best initiating time of defrosting(BITOD) as the criterion of the defrost control. Thus the ASHP frosting/defrosting mathematical model and the ASHP experiment system were constructed, and the comparison and analysis of simulation and experiment on the frosting/defrosting performance of the unit in the standard frosting condition(Tdb 2 ℃, Twb 1 ℃) is studied, then the change rule of the system parameters such as compressor power, heating capacity, evaporating temperature/pressure, condensing temperature/pressure and frost growth is obtained. The results revealed the effect law of the frost time to the evaluation parameter COPC, and ultimately the BITOD aiming at the biggest COPC of the unit was obtained.Meanwhile, according to the influences of the different environment conditions on the BITOD, the paper adjusted the air temperature and relative-humidity of the fin-tube exchanger inlet to further research the BITOD in the every conditions, and the change law of the BITOD in the different environment temperature and humidity was acquired. The experimental results demonstrated that this defrosting control method can be applied in any conditions to obtain a better property.Based on the research on the BITOD and the ASHP frosting/defrosting model, a defrosting control strategy aimed at the best comprehensive performance of the unit was formed. The ASHP frost/defrost model computed the BITOD through to detecting the operating condition and state of the unit periodically. The defrosting control can accurately predict the state of the frost growth on the outdoor exchanger, not only avoiding the operating condition deteriorating, but also maintaining that the unit operates effectively.