| Air source heat pumps have attracted attention through the advantages of clean,environmental protection and energy saving and high efficiency,and it plays an important role in replacing the heating of small and medium-sized coal-fired boilers.Under the condition of low temperature and high humidity,its frosting/defrosting is one of the key points and difficulties of the study.The formation of the frost layer will reduce the air circulation area between the fins,increase the air flow resistance and the heat transfer heat resistance of the fins,make the evaporator heat exchange effect worse,resulting in a decrease in the heating performance of the heat pump system,so periodic defrosting is required to maintain the stability of the system.For large and medium-sized ASHP,conventional reverse cycle defrosting will make the compressor frequently cause high pressure protection and low pressure protection,and the temperature fluctuates violently,affecting indoor comfort.In view of the above-mentioned problem of frost formation of ASHP,in order to achieve heat pump defrosting and uninterrupted heating,in this thesis,an outdoor heat exchanger rotation defrost air source heat pump circulation type is proposed,and carries out theoretical analysis and experimental research.The main research contents and conclusions are as follows:Construct an air source heat pump system model,establish an evaporator frosting-defrosting model and a mathematical model of other parts of the system;And put forward the evaluation of the system operating cycle performance coefficient COP_C,to establish the best defrosting start time;Simulations were made to study the formation of the frost layer and the change of system performance under different subcooling degrees and condensation temperature conditions of the supercooler defrosting type;The two types of supercooled liquid defrosting and hot gas bypass defrosting were compared and analyzed.The simulation results show that when the condensation temperature is 45°C,the ambient temperature is-5°C,and the relative humidity of the air is 80%,the frost layer of the system runs for 80min completely covers the fins,and the system stops running;When the supercooled liquid defrosts,the supercooling degree has less impact on the heating performance of the system,but can effectively reduce the defrosting time;The increase in condensation temperature will shorten the defrosting time,but the decrease in heating COP is more obvious;The heating and heating COP of supercool liquid defrost are larger than that of hot gas bypass defrosting,and the more hot gas bypass,the smaller the heating performance of defrosting.Construction of an outdoor heat exchangers rotating defrost ASHP system test bench based on 4 outdoor heat exchangers,monitoring the formation and melting process of frost layer on the fins during the frosting-defrosting process.Under different ambient temperatures and relative humidity of the air,for the supercooled liquid defrost system and the hot gas bypass defrost system,through experiments study the influence of different defrosting start time on the system suction and exhaust temperature,pressure,heating and heating COP,and get the best defrost start moment and the corresponding maximum average heating and heating COP_C.The experimental results show that the lower the ambient temperature and the greater the relative humidity of the air,the smaller the heat and heating COP of the system during the frosting process,the faster the decline rate,the longer the time required for defrosting,the earlier the best defrost start moment,and the corresponding maximum average heating and heating COP_C was smaller.When the ambient temperature is-10°C～0°C,the relative humidity of the air is 70%～90%,the system can operate normally and defrost thoroughly under both defrosting types.The average comprehensive heating and heating COP_C of supercooled liquid defrosting are above8.64k W and 3.36,and the average comprehensive heating and heating COP_C of hot gas bypass defrosting are above 7.96k W and 3.03. |
