Tailored phase change behavior of Na2SO4·10H2O/expanded graphite composite for thermal energy storage

Abstract

Sodium sulfate decahydrate (SSD, Na2SO4·10H2O) is a suitable candidate for thermal storage applications as its high thermal latent heat and desirable phase change temperature, but it suffers from the supercooling, phase separation, and recalescence. In this paper, the SSD-CBO synthesized by SSD, carboxymethyl cellulose (CMC), borax decahydrate, and OP-10 successfully suppressed supercooling, and phase separation. Expanded graphite (EG) was used to stabilize and prevent the recalescence of SSD-CBO as well as enhance the thermal conductivity of pure SSD. The microstructure and chemical structure of composites were further explored, indicating the obtained composites possess uniform distribution and good chemical compatibility. The DSC analysis shows that the composite with 7 wt% EG has latent heat value of 114.0 J g−1 for melting and 105.5 J g−1 for freezing, respectively. The supercooling was reduced by more than 19 °C. The thermal conductivity of SSD-CBO/EG7 was as high as 3.6 times that of SSD-CBO and reached to 1.96 W m−1 K−1. The heating and cooling curves confirmed that the heating and cooling times of SSD-CBO/EG7 were reduced by 36.2% and 44.5% compared to those of the pure SSD, further investigating the effect on heat transfer rate by adding EG into SSD-CBO. The excellent transient temperature response of SSD-CBO/EG7 was recorded by thermal infrared images. The thermal storage and release performance evaluation suggested the SSD-CBO/EG composites especially the SSD-CBO/EG7 processes the good potential for thermal energy storage in the floor heating application. Furthermore, the mechanism of the tailored phase change behavior of SSD-CBO/EG composites was investigated.