| Author: | Liu, Liu |
| Title: | Development of high-performance nano-emulsions of phase change materials for thermal energy storage |
| Advisors: | Wu, Jian-yong (FSN) Niu, Jian-lei (BSE) Wong, Wai-yeung (ABCT) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Heat storage Phase-change materials Nanotechnology Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Food Science and Nutrition |
| Pages: | xxi, 195 pages : color illustrations |
| Language: | English |
| Abstract: | The development and implementation of effective energy-saving measures and the utilization of renewable energy sources are important for achieving carbon neutrality and environment conservation. Energy storage technology plays a crucial role in preserving energy for utilization where and when it is needed. Thermal energy involved in heating and cooling constitutes a significant portion of energy use globally. Therefore, the development of thermal energy storage (TES) techniques has attracted considerable research and commercial interest. Latent heat storage with phase change materials (PCMs) is more efficient due to their higher energy capacity compared to sensible heat storage. In TES systems, PCMs are typically confined within tanks built with heat exchangers through which a heat transfer fluid circulates for charging and discharging of thermal energy. PCM emulsions can fulfil both energy storage and heat exchange by circulating within the TES systems, offering multiple advantages such as enhanced heat transfer and simplified tank configurations. The efficient application of PCM emulsions currently faces two major challenges, poor stability and high supercooling. The aim of this thesis is to develop high-performance PCM emulsions with high stability and low supercooling for efficient thermal energy storage applications. As one of the most preferred organic PCM agents, paraffin-in-water emulsions were developed and investigated throughout this thesis research. Three major studies have been performed as follows. In the first study, the low-energy phase inversion temperature (PIT) method was explored, which relies on the spontaneous change in surfactant curvature induced by temperature variation. Nonionic polyethoxylated surfactants were employed, and key processing factors were identified, including the cooling rate and the category and concentration of surfactants. In general, increasing the surfactant concentration or decreasing the hydrophilic-lipophilic balance value of surfactants reduced the PIT point. It was essential for the PIT point to be higher than the meting point of PCMs, with a suggested difference of at least 20 ℃ for optimal emulsion stability. Therefore, the PIT method was restricted to high melting paraffins. By increasing the surfactant concentration, the droplet size was reduced and could be controlled below 100 nm at a surfactant-to-oil (S/O) ratio higher than 0.5, resulting in nano-emulsions. Smaller droplet sizes remarkably improved emulsion stability, achieving year-long stability periods. However, it was challenging to fabricate nano-emulsions with PCM mass fractions larger than 25%, due to a drastic viscosity increase and the occurrence of sol-gel transition. Additionally, the dominant crystallization mechanism in small PCM droplets was homogeneous nucleation, which resulted in lower freezing points and increased degrees of supercooling, owing to a volume-dependent rate. Incorporating nucleating agents faced occasional ineffectiveness and low entrapment efficiency due to small droplet size and low energy input during emulsification. It turns out that supercooling in nano-emulsions with droplets under 100 nm remains an obstacle. Secondly, the high-energy ultrasonic emulsification (UE) was employed to manipulate the key properties of PCM emulsions. The mechanism behind this way is primally attributed to acoustic transient cavitation, which involves the formation, growth, and collapse of vapor bubbles. These collapsing bubbles create spots with high temperature and pressure, facilitating the emulsification of paraffin into water. Response surface methodology was utilized to assess the influence of key processing factors, including amplitude, treatment duration, and surfactant concentration. Among these factors, surfactant content emerged as the most significant factor determining the emulsion properties. Specifically, at a S/O ratio of 0.32 and a PCM mass fraction of 25%, the droplet size and viscosity were measured as about 124 nm and 7.3 mPa·s, respectively, using a 13-mm probe-type ultrasonic processor with a middle-range amplitude for a duration of nine minutes. PCM emulsions produced via the UE method exhibited better stability than the emulsions prepared using the PIT method at low S/O ratios. This enhanced stability can be attributed to the smaller droplet size achieved through the UE, although it was less effective than the PIT in reducing droplet size less than 100 nm by increasing surfactant concentration. The relatively larger droplets generated by the UE, coupled with the higher energy input, make it a more promising approach than the PIT for solving supercooling in the nano-emulsions. Furthermore, the low viscosity observed in the 25% nano-emulsion suggested that higher PCM mass fractions can be feasibly formulated using the UE method without triggering sol-gel transition. In the third study, the engineered PCM nano-emulsions were employed in two demonstrative systems to assess their practical performances in comparison to water. One promising application of low-melting nano-emulsions, ranging from 10 ℃ to 20 ℃, is room air conditioning. Therefore, a pilot-scale TES system was designed, circulating 55 L nano-emulsion as the coolant for cold energy storage and release. The nano-emulsion exhibited significant potential for long-term use, supported by stable performance over a period of nearly three months and capability to be regenerated to the initial state. The volumetric thermal storage capacity of the nano-emulsion was about 1.5 times higher than that of water. The TES system achieved fast charging and discharging rates, along with a high round-trip efficiency. Later, the nano-emulsions with slightly higher meting points, ranging from 30 ℃ to 40 ℃, were utilized in a lab-scale photovoltaic/thermal (PV/T) system to enhance the electrical efficiency by cooling PV panels and to recovery the wasted low-grade thermal energy. It found out that the emulsion-cooled PV/T system attained a higher overall energy efficiency in both electrical and thermal aspects compared to the water-cooled system. Through this thesis research, the fundamental principles and key parameters for developing PCM nano-emulsions through the major fabrication methods have been elucidated, and their potential applications have been successfully demonstrated in self-designed and constructed laboratory and pilot units. These findings are valuable to the formulation of PCM nano-emulsions for thermal energy storage and thermal management applications. With these, this thesis can made valuable contributions to the development of latent functional thermal fluids and their efficient applications. |
| Rights: | All rights reserved |
| Access: | open access |
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