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 SiO 2 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.