| When air source heat pumps (ASHPs) operate for space heating in the condition with a low ambient air temperature and high humidity in winter, frost formed on the tube surface of its outdoor coil becomes problematic, because it leads to the ASHP's performance degrading and poor reliabilty, even the shutdown of the unit. So periodic defrosting was needed to help the unit return to its rated performance. However, the existing and most widely used standard reverse-cycle hot gas defrosting method was poorly reliable because it may cause shutting down due to low-pressure cut-off and wet compression may take place, possibly damaging compressor. Study shows that insufficient heat available during defrosting is currently a fundamental problem for present reverse cycle defrosting method. In order to solve this problem, a novel PCM (phase change material) based reverse-cycle hot gas defrosting method for ASHPs was proposed. This paper studied the dynamic characteristics of this novel system and the results and contents are as following:As a key component of the PCM based ASHP defrosting system, PCM based double spiral coil heat exchanger was developed. Based on the PCM basd reverse-cycle hot gas defrosting experiment setup, experiments were performed to study three energy storage modes, three heat supply modes and four defrosting modes, respectively, and then selected the best mode among the feasible modes. Experiment results indicated that heat could be stored in PCM heat exchanger efficiently and the series-wound energy storage mode was the best operation mode with high reliability and stability. Series wound heating mode had the highest COP among three heat supply operating modes. Among the series wound defrosting, sole defrosting, parallel defrosting and traditional defrosting method, solo defrosting had the most fast defrosting speed and the compressor had highest suction pressure. Furthermore,when the system restarts for heat supply, the air temperature out of the indoor coil was the highest and the indoor environment was almost not affected. Mathematical model of the PCM-HE (phase change material heat exchanger), in both side of which phase change took place, was developed on the basis of simplification in geometry and heat transfer mechanism. When solving the irregular grid close to the pipe in numerical way, the equivalent heat transfer coefficient based on the law of energy was used to void meshing a large number of grids.The dynamic characteristics of the PCM-HE in heat storage and energy release process were studied and found that energy could be stored into or released efficiently from the PCM-HE and could satisfy the need of defrosting.A complete opration process for the PCM based ASHP was analysed. Based on the analyzation of the characteristics and heat transfer mechanism of every operation stage, their corresponding mathematical models of each component were developed, especially the models of outdoor coil defrosting and frosting; Based on the law of mass, the law of energy and law of momentum, the dynamic mathematical model for the PCM based ASHP defrosting system were developed by connecting all those developed components'models, mainly composed of the system's frosting model, system's defrosting model and system's energy storage and heat supply dynamic model. Then, the developed dynamic model of the system was verified by comparing some typical parameters of the simulation results and the experimental results of operating modes in defrositng stage and energy storage and heat supply stage and it could be confirmed that the mathematicl model of the sytem could reflect the physical process in every stage of the system.Based on the dynamic model verified, the PCM based ASHP defrosting system's dynamic characteristics, such as the variation of refirerant's temperature and dry degree along the pipe, temperature field on the outdoor coil and the transfer of refrigerant in the system, were studied. It was found that the condenser pressure and the outdoor coil temperature increased first and then decreased later. The mass of refrigerant in the low and high pressue side had the changing trend similar to sine wave and cosine wave.Based on the dynamic model verified, some influencing factors to the system, such as the length of the capillary tube, the heat transfer area of the PCM-HE, the volume of the gas and liquid separator and the air temperature, were studied and it could be confirmed that the shorter length of the capillary tube, the faster defrosting speed the system had. Enlarging the heat transfer area of the PCM-HE can speed up slightly the evaporation of the water on the out coil in the end of defrosting, but could lower the liquid level in the gas and liquid separator, decreasing the potential of wet compression. By adding the PCM-HE to the ASHP, the volume of the gas and liquid separator affected the defrosting speed no longer. The air temperature affected geeatly the heat loss caused by the natural convection in the end of defrosting.Based on the dynamic model verified, the heat consumption during defrosting was analysed. By analyzing the heat consumption and loss in defrosting process, it was found that the main heat loss was caused by natural convection between the outdoor coil and the outdoor air, and took place when the the dry area percentage was more than 70%, so how to reduce this part of heat loss by choosing effective control strategy should be studied further. PCM-HE provided a great percentage of energy for system defrosting and played a role of heat souce.The study of project will provide therocical foundation and thechnical storage for the ASHPs'wide use and highly efficient operation in hot-summer and cold-winter zone in our country, and also provide thechonoly support with important significance for our country's building energy saving.
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