Nanocomposite Phase Change Materials For Heat Storage

With the increasingly serious energy crisis, phase change material(PCM), a new type of environ mental friendly energy storage material, has become a research focus in the field of energy materials. Hydrous salt is one of the most flamboyant PCM for its high latent heat, high thermal conductivity and low phase transition temperature. CH3COONa·3H2O(sodium acetate trihydrate, SAT) latent heat as high as 264 k J/kg, is limited in its scope of application because of high melting point(58℃), seri ous super-cooling and phase separation phenomenon. Therefore, the focus of research in SAT at hom e and abroad is lowing the melting point, inhibiting super-cooling and phase separation. In this Mast er thesis, we focused on lowering the melting point, inhibiting the super-cooling and phase separation phenomenon, SAT/Urea/nano-material composites prepared. Research is divided into the following aspects:1.SAT/Urea/Nano-Si O2 mixed salt PCM was prepared through adding nano Si O2, as nucleating agent, to SAT/Urea at a low-temperature.The effect of nano Si O2 was investigated on the performance of mixed salt PCM, such as phase transition temperature, crystallization process, super-cooling and phase separation. The results show that the super-cooling degree of SAT/Urea/nano-Si O2 composite reduces from 22.8℃to 2.8℃,fell by 87.7%; no phase separation appears in the melting process; crystallization process can be shorted from72 h to 40 minutes; Composite’s latent heat can reach 186.4 k J/kg.2. SAT/Urea/Nano-MMT composite PCM was prepared by means of melting and interaction. In thi s composite, nano-MMT played a role of improving formability, super-cooling, phase separation and t hermal stability for its own formability. XRD, FT-IR, DSC, SEM results show that mixture of nanoMMT and mixed salt PCM is physical, nano-MMT lamellas evenly distributed in the composite PCM construct a kind of cavity structures to maintain the same shape before and after the phase transitio n; Nano-MMT acts as nucleation agent, reduced the super-cooling degree from 23.0℃ to 4.0℃; Twin peaks in the DSC curve demonstrate that excess nano-MMT in free state could restrain mixed salt PCM crystallize perfectly in some ways, resulting in the decrease of grain size. Besides, melting enth alpy of SAT/Urea/Nano-MMT reaches 132.0 k J/kg and after 20 times circulation of melt – freeze, th e composite PCM shows stable thermal performance.3. SAT/Urea mixed salt PCM was processed by ultrasonic to improve the super-cooling and phase separation. T-history curve and pictures of phase change show that ultrasonic wave can shorten melti ng time for 10 minutes and restrain phase separation; ultrasonic wave can also induce PCMs to cryst alize at 22.6℃, resulting in the reduce of super-cooling degree by 44.7% and process of crystallizatio n finished in 44 minutes. Moreover, longer the ultrasonic processing time, the crystallization will be more intense and finished faster.