Preparation and Thermal Properties of a Novel Modified Ammonium Alum/Expanded Graphite Composite Phase Change Material

Thermal energy storage (TES) using phase change materials (PCMs) is a powerful solution to the improvement of energy efficiency. The application of Ammonium alum (A-alum, NH 4 Al(SO 4 ) 2 ·12H 2 O) in the latent thermal energy storage (LTES) systems is hampered due to its high supercooling and low thermal conductivity. In this work, modified A-alum (M-PCM) containing different nucleating agents was prepared and further adsorbed in expanded graphite (EG) to obtain composite phase change material (CPCM) to overcome the disadvantages of A-alum. Thermal properties, thermal cycle stability, microstructure and chemical compatibility of CPCM were characterized by differential scanning calorimetry, thermal constant analysis, scanning electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy. The cold rewarming phenomenon of CPCM was established and explained. Results showed that the latent heat and melting point of CPCM were 187.22 J/g and 91.54°C, respectively. The supercooling of CPCM decreased by 9.61°C, and thermal conductivity increased by 27 times compared with pure A-alum. Heat storage and release tests indicated that 2 wt% calcium chloride dihydrate (CCD, CaCl 2 ·2H 2 O) was the optimum nucleating agent for A-alum. The result of TG and 30 thermal cycles revealed that CPCM exhibited favorable thermal stability and reliability during the operating temperature. The prepared modified A-alum/EG CPCM has a promising application prospect for LTES.