Alkali-activated fly ash composite NaNO3 thermal energy storage materials: Low-temperature preparation and high-temperature stability

Abstract

The development of high-performance medium- to high-temperature phase change thermal storage materials through simple preparation methods is essential for advancing thermal energy storage (TES) technologies. However, the conventional cold-pressing sintering techniques face limitations due to excessively high sintering temperatures and poor skeleton stability at elevated temperatures. In this study, an innovative approach was employed to fabricate shape-stabilized phase change materials (SSPCMs) by encapsulating NaNO₃ using alkali-activated fly ash (AAF) as the skeleton support materials (SSMs) under low-temperature. The results demonstrated that SSPCMs maintained stable morphology under thermal cycling without molten salt leakage and exhibited excellent chemical compatibility. The maximum encapsulation capacity of molten salt reached 70 %, with a melting enthalpy of 114.82 J/g, and retained a high value of 105.08 J/g after 100 thermal cycles. The SSPCMs exhibited compressive strength up to 5.50 MPa at 350 °C, meeting practical application requirements. SEM and MIP analyses revealed that SSMs formed by AAF provided a robust foundation for encapsulation. During thermal cycling, NaNO₃ migrated into micro-pores of SSMs driven by capillary forces, further enhancing the effective encapsulation of NaNO₃. This study presents a novel low-temperature preparation method for SSPCMs using AAF, significantly improving the high-temperature stability of medium- to high-temperature phase change thermal storage materials. The approach offers a cost-effective and scalable solution for advancing TES technologies.