Preparation and Thermal Characterization of Nitrates/Expanded Graphite Composite Phase-Change Material for Thermal Energy Storage

Molten nitrate is widely used as thermal storage medium in the solar thermal power plants for its appropriate phase-change temperature, high heat storage density and low cost, etc. But its low thermal conductivity, heat absorbing and releasing rate limited its application. Expanded graphite (EG) can compensate the low thermal conductivity of nitrate. In this study, binary nitrates at the weight ratio of 4:6 for LiNO3:KNO3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {LiNO}_{3}:\hbox {KNO}_{3}$$\end{document} were prepared using static mixed melting method. EG with the mass fraction of 5 %, 10 %, 15 %, 20 % and 30 % was used to enhance the thermal conductivity. The compound of nitrates/EG was prepared using the ultrasonic smashing method. The thermal conductivity of binary nitrates, EG and nitrates/EG composite was measured by the transient plane heat source technique (TPS). The thermal behaviors were analyzed with a differential scanning calorimeter (DSC). Results showed that the addition of EG significantly enhanced the thermal conductivity, e.g., the thermal conductivity of 10 wt% EG composite phase-change material (PCM) is 8.5 W(m-1K-1)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {W}(\hbox {m}{^{-1}} \hbox {K}{^{-1}})$$\end{document} to 9.5  W(m-1K-1)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {W}(\hbox {m}{^{-1}}\hbox {K}{^{-1}})$$\end{document}, which is about eight times larger than that of binary nitrates. To observe the combination morphology, pure EG, nitrates/EG composite PCM and binary nitrates were characterized using scanning electron microscope (SEM). The thermal reliability of the binary nitrates and the composite PCM was determined by DSC. Thermal cycling test showed that both binary nitrates and nitrates/EG composite material have good thermal reliability.