Simulation On Discharging Characteristics Of Supercooled Salt Hydrate Phase Change Materials

Based on the existing studies on supercooling mechanism of inorganic salt hydrate phase change materials (PCMs), Supercooling characteristics are preliminarily experimentally investigated. The results indicate that PCM encapsulated in the smooth and rounded container can be supercooled in a stable way. Volume ratio of NaCH3COO and H2O in NaCH3COO solution with 1:8 can be stably supercooled more efficiently. And mechanical vibration can promote nucleation of inorganic PCMs. With the increase of vibration power, the nucleation effect is more obvious.Stable supercooling and triggering discharging characteristics of PCM are simulated with CFD. The results show that PCM Na2S2O3·5H2O cooled to ambient temperature of 12℃ from superheat state is kept in the supercooled liquid sate to store thermal energy. Phase transformation occurs instantaneously in phase change container after triggered. When the temperature rises up to the melting temperature, the volume fraction of solid phase is 0.35. In the normal stage of solidification, it takes about 1.7 hours for PCM to be solidified completely with phase transition range between 44 ℃ and 48 ℃ and the maximum average heat transfer rate of 1300W.The effect of different operating conditions and physical parameters on discharging characteristics of PCM is then investigated. The results show that with the increase of supercooling degree, phase transition time is smaller but heat transfer rate is less affected; with the increase of surface volume ratio of phase change container, phase transition time is reduced and heat transfer rate is increased; with the increase of the heat transfer fluid temperature, phase transition time is increased and heat transfer rate is lower. The simulation results on different physical parameters of PCM are as follows:the higher the melting temperature is, the less phase transition time will be and the larger the heat transfer rate is; the greater the heat of fusion is, the longer the phase transition time can be and the heat transfer rate is less affected; the greater the thermal conductivity is, the shorter the phase transition time is and the heat transfer rate can be increased greatly. In addition, different PCMs, NaCH3COO·3H2O and Na2S2O3·5H2O, are studied in the paper. Discharging characteristics are different due to the different physical properties. In practical application, in order to better meet the heating demand, suitable working conditions and PCMs should be chosen according to the specific needs.