Evaluation and manipulation of the key emulsification factors toward highly stable PCM-water nano-emulsions for thermal energy storage

PCM emulsions represent a common type of fluid media for thermal energy storage (TES) systems. However, a major challenge for their application is to maintain a stable homogeneous fluid. Reduction of the droplet size is one of the most effective approaches for improving stability, such as the preparation of nano-emulsions. This work aims to develop stable PCM-water nano-emulsions prepared with n-hexadecane by manipulating the key emulsification factors, particularly the emulsifier combinations and process conditions. Two low-energy emulsification methods, phase inversion temperature (PIT) and emulsion inversion point (EIP), were applied to prepare the nano-emulsions, using Brij L4 as the emulsifier. Several important properties of the emulsions were evaluated including droplet size distribution, conductivity, and rheological characteristics and the stability of emulsions over extended periods and multiple freeze-thaw cycles. Moreover, the thermal performance for their potential application in TES systems were examined. Eventually, nano-emulsions with small and uniform droplets were obtained by both PIT and EIP methods with suitable emulsifier concentrations. The smallest droplet size (~60 nm) was attained with 11% emulsifier and 30% PCM by the PIT method, and the most stable emulsion attained with 15% of emulsifier and 30% of PCM. The PCM nano-emulsions behaved as a Newtonian liquid with a good fluidity and a superior stability over long-time storage and freezing-cycles. The degree of supercooling was reduced with the addtion of nano SiO₂ as a nucleating agent. The findings from the study are useful for better understanding of the controlling factors and further development of stable and effective nano-emulsions.