Palmitic acid/polyvinyl butyral/expanded graphite composites as form-stable phase change materials for solar thermal energy storage

In order to improve the performances of phase change material (PCM) in applications, a novel form-stable phase change materials were fabricated via solution blending method in this work. Palmitic acid (PA) was used as phase change material to release and absorb large amounts of latent heat at operating temperature. Polyvinyl butyral (PVB) is the polymer that was used as supporting matrix to prevent the leakage of palmitic acid in melting state. Expanded graphite (EG) was used not only to enhance thermal conductivity of form-stable phase change materials, but also to help reduce leakage. A series of form-stable phase change materials were prepared, containing pure palmitic acid/polyvinyl butyral composites, and palmitic acid/polyvinyl butyral composites doped with expanded graphite of 3 wt%, 5 wt% and 7 wt%. Fourier transformation infrared spectroscope (FT-IR), X-ray diffractometer (XRD), scanning electronic microscope (SEM) and thermal gravimetric analyzer (TGA) were used to analyze the chemical structures, crystal structures, microstructure and thermal stability of the form-stable phase change materials. Thermal storage properties were determined by differential scanning calorimeter (DSC), the latent heat value of form-stable phase change material with the highest palmitic acid content (70 wt%) was 128.08 J/g, corresponding to melting point of 59.5 °C. The thermal conductivity of form-stable phase change materials which was measured by thermal conductivity meter (TCM) was greatly enhanced by expanded graphite, and the thermal conductivity of form-stable phase change materials can be increased 4.2 times by 7 wt% expanded graphite. Therefore, the novel form-stable phase change materials are promising in thermal energy storage systems, especially in low-temperature solar energy systems.