Abstract 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.

中文翻译：

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用于热能储存的高度稳定的 PCM-水纳米乳液的关键乳化因素的评估和操作

摘要 PCM 乳液代表了热能储存 (TES) 系统中常见的流体介质类型。然而，它们应用的一个主要挑战是保持稳定的均质流体。减小液滴尺寸是提高稳定性的最有效方法之一，例如纳米乳液的制备。本工作旨在通过控制关键的乳化因素，特别是乳化剂组合和工艺条件，开发出稳定的 PCM-水纳米乳液，由正十六烷制备。以Brij L4为乳化剂，采用相转化温度（PIT）和乳液转化点（EIP）两种低能乳化方法制备纳米乳液。评估了乳液的几个重要特性，包括液滴尺寸分布、电导率、流变学特性以及乳液在长时间和多次冻融循环中的稳定性。此外，还检查了它们在 TES 系统中潜在应用的热性能。最终，通过 PIT 和 EIP 方法以合适的乳化剂浓度获得具有小而均匀液滴的纳米乳液。通过 PIT 方法使用 11% 乳化剂和 30% PCM 获得最小液滴尺寸 (~60 nm)，使用 15% 乳化剂和 30% PCM 获得最稳定的乳液。PCM 纳米乳液表现为牛顿液体，具有良好的流动性和在长期储存和冷冻循环中的优越稳定性。添加纳米SiO2作为成核剂降低了过冷度。