| This project is sponsored by national natural science foundation (Project No.: 50606007). Air source heat pump (ASHP) is a high efficiency energy saving environmental protection type heat supply unit, whose low position heat source is air and no pollutant discharging, which is in accordance with the ideal ecological heating mode. Now ASHP units are applied more and more extensive in China and playing important roles in architecture energy saving, coal-fired heating substitution etc. projects. But ASHP's outdoor heat exchanger will frost in low temperature high humidity conditions, which decreases its performance and need periodical defrosting to ensure its normal running. So the stability of defrosting is the key to ASHP reliable running in all regions and all meteorological condition.In this paper ASHP conventional defrosting characteristics are analyzed comprehensively, pointing out that the energy lack of low position heat source is the basic reason of poor defrosting synthesis performance. Based on time and space reasonable energy utilize theory, a new phase change energy storage defrosting system is proposed. The new system adopt CaCl2·6H2O as phase change material(PCM), and stored energy through residual heat or normal heating. The heat accumulator served as the main low position heat source when defrosting, which thoroughly solved the problem of lacking defrosting energy.This is a absolutely new way of doing with ASHP defrosting problem. To investigate the conventional and the new defrosting system's characteristics, an experimental system was set up. A small heat pump type air condition is modified and the phase change energy storage was added into the heat pump system. A serial of conditional experiments were done in the two defrosting mode.In experiments the temperature, pressure, power, frost mass, course time, etc. defrosting parameters were carefully measured and recorded. Through analyzing the large number of experimental data, the system components'characteristics were found out in both conventional and the energy storage defrosting mode. The effect of conventional defrosting is satisfied when frost is moderate, but the low level suction pressure is its main shortcoming. The compressor discharge pressure and power are relatively low and indoor heat exchanger surface temperature will decrease greatly and adopt much heat from heating space. On the other hand, outdoor heat exchanger surface temperature increases slowly, defrosting lasts 8 minutes. In heavy frost conditions, the conventional defrosting system has poor performance, which means it has no ability to do with heavy frost.Energy storage defrosting has the feature of short time, cleanly defrosting, less affect to heat space. Heat accumulator working mode and its heat storage or release experiments show that the PCM phase change thoroughly and can storage much of heat. It releases heat rapidly and sufficiently. The accumulator worked well with heat pump system. The new defrosting way can evidently increase suction pressure and refrigerant mass flow rate, meanwhile, indoor heat exchanger absorbs little heat from heat space and outdoor heat exchanger fin-tube temperature increases rapidly and total compressor power consumption reduces comparing with the conventional system.Defrosting energy consumption were calculated from heat sauce and heat sink and their values were contrasted each other. Results showed that the conventional defrosting system energy mainly comes from compressor and indoor heat exchanger nature convection, whereas energy storage system may optimize the components of heat source.A dynamic model of outdoor heat exchanger surface water evaporation was set up, which was based on experimental parameters. Through solving the model, characteristics of surface water vaporizing, fin-tube dry heating can be acquired. The paper also investigated the problem of fault defrosting, analyzed its mechanism, feature, detriment and discussed the cause of it and proposed the method to avoid it.
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