Novel Methods Of Retarding Frost And Defrosting For An Air Source Heat Pump

For a space heating air source heat pump (ASHP) unit, when its outdoor coilsurface temperature is below both the air dew point and the freezing point of water,frost will form on its outdoor coil surface. Frost affects its operational performanceand energy efficiency. Therefore, periodic defrosting is necessary. During a reversecycle defrosting operation, the indoor coil in an ASHP unit actually acts as anevaporator while the outdoor as an condenser. Therefore, there is no heatingprovided and hence indoor air temperature in a heated space can drop. At the sametime, the indoor fan is turned off during defrosting operation to avoid cold air fromindoor coil. And hence the defrosting energy is mainly from input power intocompressor and energy stored in indoor coil metal. As a result, there is insufficientenergy to melt and drain the frost on outdoor coil surface, which leads to lowsuction pressure, prolonged defrosting period and deteriorated indoor environment.To improve the system performances under frosting condition and increasedefrosting efficiency, some experimental and theoretical work about retarding frost,energy storage defrosting based on sub-cool energy of refrigerant and defrostingcontrol method were conducted in this paper.Firstly, three test-rigs were built in two artificial environment chambers. Firstof all, the experimental ASHP units and parameter measurement and data acquisitionequipment were chosen, and then control strategies of simulated indoor and outdoorenvironments were determined. Finally, test error was calculated, which resultsshowed that the built test-rigs were enough accurate for the following experiments.Secondly, a novel method of retarding frost on outdoor coil surface for anASHP unit was presented based on the study of frost mechanism and the effect ofoutdoor air distribution on frost. Using which, the air pressure of outdoor coilchanged from negative zone to positive by changing the operation direction ofoutdoor fan, which facilitated the frost remove from coil surface and finally retardedfrost accumulation. In this study, the novel retarding frost method by changingoutdoor fan direction and air flow rate was experimentally studied, and then themechanism was analyzed. The experimental results indicated that it couldeffectively retard frost accumulation on outdoor coil surface by using the providednovel method.Thirdly, the experimental study on defrosting heat supplies and energyconsumptions during a reverse cycle defrost operation for an ASHP unit wasconducted when the indoor fan was turned on. The sources of heat supplies andrespective amounts, the purpose of energy consumptions and respective amount were respectively measured. And then the defrosting efficiency was calculated.Finally, methods of providing enough defrosting energy, reducing energyconsumption and improve defrosting efficiency were suggested, which was in favorof perfecting the system performances during defrosting operation.Fourthly, the energy storage defrosting method using sub-cool energy ofrefrigerant for an ASHP unit was presented to provide enough defrosting energy. Inwhich the phase change material-heat exchanger (PCM-HE) was importantequipment. In this paper, the energy storage and release process in the PCM-HEwere simulated and analyzed. The fin efficiency in PCM-HE was calculated duringphase change process. And then the energy storage and release process under withand without fins were simulated, and the simulation results were analyzed.Fifthly, the experimental study on energy storage and defrosting using thenovel method was measured. During normal heating operation, the refrigerantleaving indoor coil flowed to PCM-HE first, before passing through the capillarytube. In this way, the sub-cool energy of refrigerant was absorbed by the PCM andstored in the PCM-HE. At the same time, the safe system operation could be ensuredbecause further sub-cooling of refrigerant reduced the likelihood of liquidrefrigerant flashing and suction refrigerant over-superheating. When defrost becamenecessary, the heat stored in the PCM-HE could be used so as to provide sufficientthermal energy to melt the frost off the outdoor coil surface. In this study, the novelsystem form was provided, and then the PCM-HE was designed and appropriatePCM was chosen and measured. Finally, system performance during heating anddefrosting were measured.Sixthly, a novel defrost control method based on degree of refrigerant superheatwas provided. A conventional experiment was carried out on an ASHP unit withcapillary as throttle device under simulated frosting and defrosting conditions basedon time control defrosting method, and its experimental result was analyzed,especially the degree of refrigerant superheat. And then the novel defrost controlmethod was provided. To validate the provided novel defrost control method,another experiment was conducted on ASHP unit with fixed opening electronicexpansion valve (EEV) as throttle device under simulated frosting and defrostingconditions, and its experimental results validated the correctness of the providedmethod. Further study on this novel control method could achieve defrostingoperation when it was needed and performance optimization for an ASHP unit.Accomplished work in this paper and further study was in favor of achievingretarding frost and defrosting when it was needed, so far as to no defrostingoperation during heating cycle.