Investigation On Air Source Heat Pump Water Heater Combined With Microchannel Condensing And Thermal Storage Defrosting

ASHPWH (Air Source Heat Pump Water Heater) gets more and more attention due to itsenvironmental protection, energy-saving, safe and low-carbon. Nowadays ASHPWH has beenincluded in the national Torch Program project. Especially, the establishment of―Heat PumpIndustry Association‖promotes the development of industry, expecting3to5years marketshare of ASHPWH to10%~20%, and sales of ASHPWH to reach40billion RMB.Due to submerged coil condenser is very easy to generate scales, corrosion andbiological slime in the application process of ASHPWH, Europe and other countries haverequired ASHPWH adopting outer-coil heating instead of submerged coil. Outer-coilcondenser is in contact with inner tank shell plate by the arc surface, little contact square,requirement for high coiling density, long condenser pipe, great resistance of flowing and lowheat transfer efficiency.Enhancing heat transfer technology of micro channel is applied to outer-coil condenser,direct at the problem of little contact square and low heat transfer efficiency which exist intraditional outer-coil condense. Comparing the performance metrics of these two types heatexchanger through experimental researches, and analyzing the pressure drop, heating capacity,temperature distribution and exergy loss.During the heating in winter, the problem of frosting in evaporator is not well settled atall times. There were some defects in hot-gas bypass defrosting and reversing defrosting,manifested in follow: During the hot-gas bypass defrosting process, the defrosting loss basiccomes from compressor, suction pressure and discharge pressure change dramatically, that’s apretty big hit on the compressor. During the reversing defrosting process, the compressor iseasy to shut down coupling with pressure decay, long defrosting time and high defrost energyconsumption.A new defrosting method defined thermal storage defrosting is presented to overcomeunreliable and high energy consumption of conventional defrosting. Through experiment ofASHPWH, this article researches dynamic features of frosting&defrosting, restore heating response property and the effect of heat pump system during frosting and defrosting. Obtainthe change law of main component of heat pump system during hot-gas bypass defrosting andthermal storage defrosting, then analyze the energy consumption and heating capacity in twodifferent defrost modes, exploring internal disciplines of thermal storage heat exchanger andrefrigerant migration rules.Firstly, this article discusses the system design and optimizing operation of thermalstorage defrosting ASHPWH combined acted outer-coil microchannel tubes condenser.Theoretical analysis and experiments have been done for this system, and the calculatedmodels of ASHPWH are presented. Secondly, this paper also provides the exergy calculatedmodels,define the integral part and degree of energy loss, provides theoretical basis for furtherimprovements of energy efficiency about ASHPWH. Lastly, coupling analysis ofmicrochannel tubes condensing combined with thermal storage defrosting, investigate therelations of interaction between these two heat exchangers during heat-stored andheat-rejected process.According to the performance experiment and defrosting experiment of ASHPWH, thetheoretical analysis and experiments indicate that:(1) Compare with outer-coil copper pipes, outer-coil microchannel shows lowerpressure drop and higher heating capacity. The average pressure drop of outer-coilmicrochannel is only0.05MPa, and0.23MPa in outer-coil copper pipes. Under thesame condition of heat pump main engine, the heating capacity proportion ofouter-coil microchannel is higher than outer-coil copper pipes,11.2%above insummer conditions,8.2%above in normal conditions and16.4%above inMinimum conditions.(2) The system’s results of thermal storage defrosting are much better than hot-gasbypass defrosting. refrigerant flow, evaporating pressure, heating capacity，coolingcapacity and COP during the thermal storage defrosting are higher than thoseduring hot-gas bypass defrosting. The temperature of the surface of fin-tubeexchanger at the end of thermal storage defrosting is higher than the process of hot-gas bypass defrosting, effective prolong the defrosting cycle,reduce the defrostfrequency, improve the stability of the system.(3) Thermal storage heat exchanger can improve the degree of supercooling and theevaporating temperature of system during thermal storage defrosting, outer-coilmicrochannel is instrumental thermal storage of phase change regenerator.Promoting5~8℃addition effect of supercooling in ASHPWH system. Couplinganalysis of microchannel tubes condensing combined with phase changeregenerator，indicated shorter thermal storage time, higher thermal storage ability,greater supercooling degree and energy efficiency.