Thermal conductivity enhancement of a sodium acetate trihydrate–potassium chloride–urea/expanded graphite composite phase–change material for latent heat thermal energy storage

To create an energy–efficient heat pump latent heat thermal energy storage (HPLHTES) system, a novel sodium acetate trihydrate (SAT)–potassium chloride (KCl)–urea/expanded graphite (EG) composite phase–change material (CPCM) was developed in this study. EG was introduced into the base composite salt to prepare the novel CPCM, SAT–KCl–urea/EG. The thermal properties, crystalline phase, and morphology of the resulting CPCM were experimentally characterized. It was found that the SAT–KCl–urea/EG CPCM comprising 9 wt% EG was optimal for affording improved thermal–energy storage performance without compromising phase–change properties. The obtained CPCM exhibits an appropriate melting point of 47.5 °C and a high phase–change enthalpy of 200.3 kJ/kg. The thermal conductivity of the SAT–KCl–urea/EG CPCM was 1.48 W/m·K, approximately 5.3 times that of the SAT–KCl–urea composite salt (0.28 W/m·K). The required phase–change duration of the SAT–KCl–urea/EG CPCM was approximately 30% less than that of the SAT–KCl–urea composite salt. After 100 thermal–cycle tests, the SAT–KCl–urea/9 wt% EG CPCM retained a desirable phase–change temperature of 45.9 °C, high latent heat of 190.8 kJ/kg, and an acceptable supercooling degree of 1.56 °C. These thermal feature results showed that the SAT–KCl–urea/9 wt% EG CPCM exhibited great potential for use in the HPLHTES systems.