Effects of expanded graphite on NaNO3/semi-coke ash shape-stable phase change composites for thermal energy storage

Abstract

As an alkaline industrial solid waste with a porous structure, semi-coke ash can have adverse effects on the environment if accumulated or buried in large quantities. The research group has previously used this material in combination with NaNO3 to prepare a shape-stable phase change composite. Building upon the previous research team, this work manufactured the shape-stable phase change composite by cold-compression hot-sintering method using NaNO3 as the phase change material, semi-coke ash as the skeleton material and an innovative addition of expanded graphite as the thermal conductivity enhancer to enhance thermal conductivity of phase change material. Then, key thermal performance, mechanical strength, chemical compatibility, microscopic morphology, economic efficiency and emission reduction capacity were investigated. Results demonstrated that the sample SS2 with 0.5 wt% expanded graphite performed best, which also presented a mechanical strength of 113.82 MPa, a thermal conductivity of 1.844 W/(m·K), and an excellent thermal energy storage density of 424.91 J/g between 100–400 °C; Both the thermal performance and mechanical strength of sample SS2 decreased to some extent after the heating/cooling cycles; The semi-coke ash, expanded graphite, and NaNO3 showed excellent chemical compatibility while both the thermal energy storage cost (0.022 ¥/kJ) and the raw material carbon emissions (1041 kg) of producing per ton sample SS2 in this work were far below traditional shape-stable phase change materials.