Coupling Rheological Stability and Thermal Transport in Expanded Graphite Reinforced Bio-Based Eutectic Phase Change Materials for Solar Thermal Energy Storage

Abstract Bio-based phase change materials (PCMs) are promising candidates for sustainable thermal energy storage (TES). However, their practical application is limited by low thermal conductivity, leakage during phase transition, and limited understanding of the relationship between microstructure and flow behavior. In this study, a bio-based eutectic PCM composed of candelilla wax (CW) and stearic acid (SA) was reinforced with expanded graphite (EG) to simultaneously improve thermal transport and rheological stability. The optimized eutectic PCM (CS28) exhibited a melting temperature of 57.34 °C and a latent heat of 172.64 J g–1, making it suitable for medium-temperature TES. Incorporation of 2 wt % EG produced a porous, layered graphitic morphology, with the presence and structural characteristics of EG supported by FESEM, Raman spectroscopy, and XPS, which enhanced both heat transfer and molten-state structural stability. Rheological analysis revealed pronounced shear-thinning behavior and an increased consistency index, indicating the formation of a graphite-mediated internal network that restricted PCM mobility while maintaining flowability. Consequently, the best-performing composite PCM within the investigated EG loading range (CS28G4) achieved a thermal conductivity of 0.119 W m–1 K–1 (45.1% enhancement) while retaining 80.1% of the latent heat storage capacity (138.34 J g–1). The composite also demonstrated excellent thermal reliability after 200 thermal cycles, with negligible changes in phase transition temperatures and latent heat, together with improved leakage resistance. Practical applicability was demonstrated by integrating the composite PCM into an indirect solar dryer, resulting in prolonged heat release and improved thermal regulation during agricultural drying. Unlike previous studies that primarily target thermal conductivity enhancement in EG-reinforced PCMs, this work establishes a structure–rheology–thermal transport relationship in EG-reinforced bio-based eutectic PCMs, providing a mechanistic framework for relating EG-induced structural and rheological changes to thermal transport and leakage resistance in composite PCMs.