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

Thermal energy storage has been recognized as one of the most perspective ways of improving the efficiency in energy utilization and energy recovery. Latent heat storage is widely used in thermal energy storage systems owing to its high energy storage density and nearly isothermal heat storage process. In this paper, the first chapter introduces the development of modern energy storage technology, the types of phase change material (PCM) and the preparation and characterization methods of composite phase change material. In the second chapter, the preparation and characteristics of composite PCM is presented via adsorption method. In chapter three, synthesis and characterization of microencapsulated PCM is presented through sol-gel process. The fourth chapter briefly describes the application of the composite phase change materials used in the building envelope.1. Composite PCM prepared by adsorption methodThe shape-stabilized lauric-stearic acid/carboxy methyl cellulose (LA-SA/CMC) composites as PCMs for thermal energy storage were prepared by adsorbing liquid PCM into the CMC. The LA-SA eutectics were used as the PCMs for thermal energy storage, and the CMC was used as the supporting material. The LA-SA eutectics were absorbed in the porous CMC fibers and exhibit uniform distribution in the composites as a result of the capillary and surface tensions. The leakage of the melted eutectics was prevented. The mass ratio of the LA-SA eutectics in this study can reach 70.4%, and the shape-stabilized LA-SA/CMC composites melted at 32.2℃ with latent heat of 114.6 kJ/kg and solidified at 29.2℃ with latent heat of 106.8 kJ/kg. Moreover, the thermal properties of the shape-stabilized CPCM were well maintained after 100 thermal cycles. It-is concluded that the prepared shape-stabilized LA-SA/CMC composites have considerable potential for developing their roles in thermal energy storage in building materials due to the suitable phase change temperature and the natural compatibility with the wood board.2. Microencapsulated PCM prepared by sol-gel methodMicroencapsulated paraffin with titanium dioxide (TiO2) shells as shape-stabilized thermal energy storage materials were prepared through a sol-gel process. In the coresshell structure, the paraffin was used as the phase change material, and the TiO2 prepared from tetra-n-butyl titanate (TNBT) acted as the shell material. Fourier transformation infrared spectroscope (FT-IR), X-ray diffractometer (XRD) and scanning electronic microscope (SEM) were used to determine the chemical structure, crystalloid phase and microstructure of the microencapsulated paraffin with titanium dioxide shells. The thermal properties and thermal stability were investigated by a differential scanning calorimeter (DSC) and a thermogravimetric analyzer (TGA). The FT-IR and XRD results confirmed that characteristic peaks of both paraffin and TiO2 can be observed in the microencapsulated paraffin with the TiO2 shells. The DSC results indicated that the microcapsules exhibit similar phase change characteristics as those of bulk paraffin, and the typical ones melt at 58.8 t:with latent heat of 161.1 kJ/kg and solidify at 56.5℃ with latent heat of 144.6 kJ/kg when the microencapsulation ratio is 85.5%. The TGA results showed that the microencapsulated paraffin have good thermal stability, the TiO2 shells can improve the thermal stability of the microcapsules.Microencapsulated palmitic acid (PA) with titanium dioxide (TiO2) shell as shape-stabilized thermal energy storage material was synthesized through a sol-gel process. In the microcapsule, the palmitic acid was used as the phase change material, and the TiO2 prepared from tetra-n-butyl titanate (TNBT) acted as the shell material. Scanning electron microscope, Fourier transformation infrared spectroscope, X-ray diffractometer and X-ray photoelectron spectroscopy (XPS) were used to determine the morphology, chemical structure, crystalloid phase and chemical state of the microcapsules, respectively. The thermal properties and thermal stability were investigated by a differential scanning calorimeter and a thermogravimetric analyzer. The microcapsules have relatively spherical shape and average size of 200-400 nm. The FT-IR, XRD and XPS results showed that the PA was well encapsulated in the TiO2 shell. The DSC results indicated that the typical microcapsules melt at 61.7℃ with a latent heat of 63.3 kJ/kg and solidify at 56.7℃ with a latent heat of 47.1 kJ/kg. The TGA results confirmed that the microcapsules have good thermal stability, resulting from the TiO2 shells. Based on the results, it can be concluded that the prepared microcapsules have good energy storage potential due to their non-inflammability, nontoxicity and good thermal stability.3. The application of the composite phase change materials in the building envelopeThe application of composite phase change material in building envelope such as wallboards, the Trombe walls, ceiling boards, shutters and under-floor heating systems has been studied. The choice of the PCM and the incorporation method play an important role in the thermal performance regulation. Current researches have proven that application of thermal energy storage materials in buildings can not only reduce the volatility of the indoor temperature, improve the indoor thermal comfort but also enhance the energy efficiency.