Study On Operation Characteristics And Experimental Research Of Air Source Heat Pump With Multiple Evaporators In Parallel For Sub-cooling Defrosting In Turn

Air source heat pump is the mainstream alternative to promote clean heating in winter,reduce coal pollution and improve air quality in northern China.Household small-scale air source heat pump generally adopts reverse circulation defrosting mode.with the four-way reversing valve,defrosting and defrosting can be realized at the cost of stopping heating and sacrificing some useful energy.However,for large and medium-sized air source heat pumps,the flexibility of heating capacity adjustment with load is not high,the heat loss of cold and hot fluid mixing cannot be ignored,and the system is too hot inert.The traditional four-way reversing valve refrigerant reverse cycle defrosting is no longer suitable for large-scale air source heat pump cycle.In this paper,an air source heat pump cycle with multiple outdoor units rotating in parallel for sub-cooling defrosting was proposed.Single or multiple compressors were connected in parallel to adjust the total heating capacity,and multiple outdoor heat exchangers were connected in parallel.Under the condition of no-stopping heating,one of the evaporators was converted into a condenser through the switching of valves,so as to realize the sub-cooling of the main circuit and achieve the purpose of defrosting at the same time.The operation mechanism and design method of the system were studied by establishing simulation model and testing verification.The main contents and conclusions are as follows:1)Using the finite time steady-state thermodynamics method,the frosting and defrosting process model of finned tube heat exchanger was established,and the relationship between frosting time and thickness with working conditions was studied,as well as the operation performance of the heat pump system after frosting.The theoretical model of multiple outdoor heat exchangers rotation defrosting cycle was established,the number and capacity matching characteristics of multiple outdoor unit rotation defrosting structure were obtained,and the influence rules of defrosting time and defrosting cycle on the thermal performance of the system under different outdoor environment temperature,relative humidity were studied.Based on the comprehensive economy and system performance,the number of outdoor units was optimized for this study.2)The influence of sub-cooling degree on the heating performance of the system was studied.The results show that when the outdoor temperature was not too low and the condensation temperature was not too high,the use of natural cold source subcooling had little effect on the heating performance of air source heat pump.When the outdoor temperature was low and the condensing temperature was high,the sub-cooling degree would make the heating capacity and COP of the system decrease slightly,while the exhaust temperature of the compressor increased obviously.The sub-cooling temperature of 20-25? was generated for the liquid in front of the expansion valve when the defrosting was run in turn.It is theoretically feasible to defrost with subcooled heat.3)Based on four outdoor units,an experimental platform was built.The effects of defrosting time,defrosting cycle and defrosting effect on the key operating parameters of the system,such as evaporation temperature,condensation temperature,exhaust temperature and compressor power were studied.The instantaneous and comprehensive heating capacity and COP of the system were investigated.The results showed that the system can operate normally and defrost thoroughly under the outdoor environment temperature of-20?0 ? and relative humidity of 80%.The cop of comprehensive heating could reach more than 2.0.4)For units with multiple outdoor units after parallel connection of multiple compressors,the method of multiple outdoor units' rotation defrosting when starting different compressors was studied,sums up the strategy of outdoor units' rotation defrosting when compressor capacity was adjusted,and its comprehensive thermal performance was tested.The purpose of this study is to provide a new circulation mode for large and medium-sized air source heat pump,and to provide theoretical and experimental data for promoting heat pump technology and developing clean heating technology.