Enhancement of Thermo-Physical Properties of Expanded Vermiculite-Based Organic Composite Phase Change Materials for Improving the Thermal Energy Storage Efficiency

In this work, expanded vermiculite (EVM) was modified by acid leaching with different concentrations (0.01, 0.05, and 0.1 mol/L) of HCl solution to obtain three kinds of acid-modified EVM (AEVM-1, AEVM-2, and AEVM-3, respectively). In the composite, polyethylene glycol (PEG) was served as a phase change material (PCM), while EVM and AEVM were served as supporting matrixes. Then, graphite was served as an additive to enhance thermal conductivity, and a series of shape-stabilized composite PCMs (PEG/EVM, PEG/AEVM-1, PEG/AEVM-2, PEG/AEVM-3, and PEG-C/AEVM-3 ss-CPCMs) were prepared by physical impregnation. The latent heats of PEG/AEVM-3 and PEG-C/AEVM-3 in the melting process were 154.8 and 144.7 J/g, respectively, which increased by 22.7 and 14.7%, respectively, compared with that of PEG/EVM, indicating that acid modification effectively enhanced the heat storage capacity. The thermal conductivity of PEG-C/AEVM-3 was 0.43 W/mK, which was 65.4 and 48.3% higher than that of PEG and PEG/EVM, respectively. The results of Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, and the thermal cycle test indicated that PEG-C/AEVM-3 reflected favorable chemical stability, thermal stability, and thermal reliability. Therefore, the prepared PEG-C/AEVM-3 with high latent heat and acceptable thermal conductivity was a promising composite PCM in the field of building energy storage.