Study on sodium acetate trihydrate-expand graphite-carbon nanotubes composite phase change materials with enhanced thermal conductivity for waste heat recovery

Abstract

Developing phase change materials (PCMs) with excellent thermal properties and low cost is of great significance for accelerating the applications of the latent heat storage technology in waste heat recovery. In this work, a novel composite PCM based on sodium acetate trihydrate (SAT), expanded graphite (EG) and carbon nanotubes (CNTs) was prepared and optimized for efficient heat storage and release effect. To confirm the phase change effect of composite PCM was acted above 50 °C, urea was introduced to improve the melt uniformity of SAT and its proportion was optimized in the range of 0–10 wt% based on the DSC results. The nucleating promotion of disodium hydrogen phosphate dodecahydrate (DHPD) on the SAT-Urea mixture crystals was characterized by a metallographic microscope. EG was used as the porous carrier to eliminate the liquid mobility of melted PCM. CNTs were introduced to further enhance the thermal conductivity of the composite PCM. It is shown that 4 wt% urea can effectively inhibit the phase segregation of SAT and keep the phase change temperature of mixture above 55 °C. The melting enthalpy of SAT-Urea mixture is as high as 250.5 kJ/kg. It is observed from the microscope images that 4 wt% DHPD can promote the rapid formation of small crystals of SAT-Urea mixture, thereby providing more nuclei and accelerating the crystal growth rate. And the supercooling degree can be reduced to 0.6 °C. The porous network provided by EG and the addition of CNTs can significantly enhance the thermal conductivity of SAT-Urea mixture. As a result, the SAT-Urea/EG/CNTs composite PCM exhibits a high enthalpy of 180.1 kJ/kg, a high thermal conductivity of 6.904 W/(m·K), and excellent thermal reliability after 300 cycles of melt-solidification experiments.