Fabrication and properties of nanoencapsulated stearic acid phase change materials with Ag shell for solar energy storage

Phase change micro/nanocapsules with a core-shell structure have great potential in solar water heating systems. To enhance the thermal conductivity of the shell, we synthesised stearic acid (SA)/Ag nanocapsules via a chemical reduction method using a Pickering emulsifier. The characterisation results confirmed the successful preparation of the SA/Ag nanocapsules. The nanocapsules had a mean diameter of 167-252 nm and a thermal storage capacity of 25.34-118.38 J/g corresponding to the volume encapsulation ratio (R-V) of 69.30-96.90%. The bulk thermal conductivity of the nanocapsules was determined using the laser flash method. They showed a remarkably high effective thermal conductivity of 0.974-6.020 W/m.K. In particular, the suspensibility of the as-prepared SA/Ag nanocapsules was studied in detail, considering that the Ag shell increased the density of the nanocapsules. A single-particle model was established, and the theoretical suspension critical diameter (TSCD) was defined to comprehensively consider the effects of the terminal velocity and Brownian diffusion of the nanocapsules in the fluid. The size of the fabricated SA/Ag nanocapsules was much smaller than the calculated TSCD, indicating that the nanocapsules had good suspensibility. In addition, the thermal reliability of the nanocapsules was evaluated by subjecting them to 2000 thermal cycles, after which the latent heat of the nanocapsules reduced slightly by 0.55%.