Preparation And Properties Of Composite Phase Change Materials For Thermal Energy Storage

With the rapid development of economy, energy shortage is becoming more and more serious. The technology of thermal energy storage has been developed rapidly for its efficient ways to use energy. Phase change energy storage technology which uses phase change materials (PCMs) to store energy has attracted great attention because it has high energy storage density. In this paper, the first chapter introduces the background and development of the technology of thermal energy storage, the development condition of phase change energy storage technology, the types and characteristics of phase change materials and the preparation and testing methods of composite phase change materials (CPCMs). In the second chapter, a CPCM is prepared by adsorption method and the properties of the CPCM are also presented. In the third chapter, a CPCM with enhanced thermal conductivity is prepared by adsorption method and its performance in application is tested. In the fourth chapter, a microencapsulated PCM (MPCM) is prepared by sol-gel method and the characteristics of the MPCM are examined.1. CPCM prepared by adsorption methodStearic acid/titanium dioxide composites as shape-stabilized phase change materials for thermal energy storage were prepared. In the composites, the stearic acid acted as phase change material for thermal energy storage, and the titanium dioxide was used as supporting material. A series of samples with different mass ratio of stearic acid in the composites were prepared (stearic acid:titanium dioxide,1:5,1:4, 1:3,1:2,1:1). Fourier transformation infrared spectroscope (FT-IR), scanning electronic microscope (SEM) and X-ray diffractometer (XRD) were used to investigate chemical structure, micro structure and crystal structure of the composites. Besides, the thermal characteristics of the composites were tested by differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA). Stearic acid was uniformly dispersed in titanium dioxide due to the high wetting ability of the stearic acid. So the titanium dioxide used as supporting material prevented the leakage of melted stearic acid. The satisfactory stearic acid/titanium dioxide composite melted at 53.84℃ with a latent heat of 47.82 kJ/kg and solidified at 53.31℃ with a latent heat of 46.60 kJ/kg. The composites could be used as shape-stabilized phase change materials for thermal energy storage for its non-inflammability and nontoxicity.2. CPCM prepared by adsorption method with enhanced thermal conductivityPalmitic acid/capric acid eutectics and diatomite composites were prepared as shape-stabilized phase change materials for thermal energy storage by absorbing method. In the composites, the eutectic mixtures were used as phase change materials for thermal energy storage, and the diatomite acted as supporting material. A series of the Palmitic acid/capric acid mixtures were prepared in order to get the accurate mass ratio of the two components (palmitic acid:capric acid,12:88) in the eutectic. Then three CPCMs with different mass ratio of the eutectics and diatomite were prepared (eutectics:diatomite,1:2,1:1,2:1). Fourier transformation infrared spectroscope (FT-IR), scanning electronic microscope (SEM) and X-ray diffractometer (XRD) were used to investigate chemical structure, microstructure and crystal structure of the composites. Besides, the thermal characteristics of the composites were tested by differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA). The diatomite prevented the melt eutectics from leakage due to its porous structure. Different mass ratio of expanded graphite (3% and 5%) was added to enhance thermal conductivity of the CPCMs. The satisfactory CPCM (eutectics:diatomite,2:1) melted at 26.69℃ with a latent heat of 98.26 kJ/kg and solidified at 21.85℃ with a latent heat of 90.03 kJ/kg. The thermal conductivity of the CPCM increased by 25.2% in the solidifying state (20℃) and 53.7% in the melting state (35℃) after 5% expanded graphite was added. The CPCM had good thermal stability and relatively high thermal conductivity, so it could be used as shape-stabilized phase change materials for thermal energy storage.3. Microencapsulated PCM prepared by sol-gel methodMicroencapsulated octadecane microcapsules with silicon dioxide (SiO2) shells as thermal energy storage materials were prepared by using sol-gel method. In the microcapsules, octadecane acted as phase change material for thermal energy storage, and SiO2 prepared from methyl triethoxysilane (MTES) was uesd as the shell material. Fourier transformation infrared spectroscope (FT-IR), scanning electronic microscope (SEM) and X-ray diffractometer (XRD) were used to investigate chemical structure, microstructure and crystal structure of the microcapsules. Besides, the thermal characteristics of the microcapsules were tested by differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA). The FT-IR results indicated that there was no chemical interaction between octadecane and SiO2. Curves of the XRD pattern signified the crystal structure of octadecane and SiO2 remained unchanged. The SEM photographs showed that the octadecane was well encapsulated in the SiO2 shells, and the particle size of a microcapsule was 500nm to 2μm. The satisfactory octadecane/SiO2 microcapsules melted at 28.32℃ with a latent heat of 227.66 kJ/kg and solidified at 26.22℃ with a latent heat of 226.26 kJ/kg. The microcapsules had good thermal stability and could be used as shape-stabilized phase change materials for thermal energy storage.