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Title: Development of encapsulation methods for organic-based phase change materials in water
Authors: Tan, Suqing
Degree: Ph.D.
Issue Date: 2019
Abstract: Thermal energy storage becomes increasingly important as fossil fuel reserves diminish and renewable energy is still catching up. Phase change materials (PCM) can store and release substantial amounts of thermal energy (as high as 250 J/g) in response to temperature changes in its environment. In comparison to popular inorganic PCMs, organic PCMs possess several advantages such as negligible loss of latent heat over time and low supercooling. However, organic PCMs suffer from low thermal conductivity which increases the response time to temperature changes in the environment. Furthermore, high flammability of organic PCMs is also a concern which limits their applications as thermal storage materials. To address the above-mentioned drawbacks of organic PCMs, encapsulated-PCM (EPCM) in water has been developed by various research groups. However, the current methods still suffer some drawbacks such as PCM leaching, loss of heat storage over time and low PCM loading. The goal of this work is to develop an innovative approach to fabricate encapsulated-PCM (EPCM) in the aqueous system. It was anticipated that the EPCM with particle sizes in a nanoscale range may increase the heat transfer rate because of the increase of heat transfer area. At the same time, the dispersed EPCM in water would reduce flammability and enhance thermal conductivity. The thesis begins by introducing the importance of thermal energy storage and the distinct types of PCM materials that have a solid-liquid phase transition taking place at temperatures in the range of ±15 °C of human skin temperature. The current approaches used to synthesize EPCMs and their limitations are presented. The applications of low-temperature EPCMs are also highlighted.
In this work, two approaches to fabricate the EPCMs have been developed: 1) Fabrication of EPCMs in water through a Pickering emulsion template using poly(butyl acrylate)/chitosan nanoparticles as the shell material: In this approach Pickering emulsion was utilized as a template to prepare EPCMs in water. The shell material used was amphiphilic poly(butyl acrylate)/chitosan (PBA/CTS) nanoparticles consisting of hydrophobic poly(butyl acrylate) cores and water-soluble chitosan shells synthesized with core-shell particle platform technology developed by Li's group. The resultant PBA/CTS particles were characterized by dynamic light scattering, ξ-potential measurements at different pHs, Fourier-transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FE-SEM). Encapsulation involved formation of a Pickering emulsion with PBA/CTS nanoparticles and an organic PCM, hexadecane, which has a melting point ranging from 16-18 °C, followed by crosslinking of functional groups on the PBA/CTS nanoparticle form a continuous polymer shell enclosing the PCM droplet. The effects of different monomer to chitosan ratio and different types of initiator were investigated to find out the optimal particle composition for stabilization of the dispersed PCM. To further optimize the emulsion stability, parameters such as solution pH, electrolyte concentration, and different types of surfactant were also examined. Furthermore, different crosslinking methods to form continuous shells of PBA/CTS nanoparticles were investigated. Phase change performance and thermal properties of the EPCMs were also studied to evaluate stability of EPCMs after multiple phase change cycles. Results suggested that the encapsulation efficiency was as high as 91%. However, there was ~5% latent heat loss after 20 heat-freeze cycles due to the PCM leakage from the encapsulated PCM capsules. SEM images revealed that the capsules had diameters in the range of 3-5 µm but showed ruptures upon drying. In conclusion, the EPCMs in water have been successfully fabricated with high encapsulation efficiency. However, heat-freeze cycling capability was still unsatisfactory. 2) Synthesis of EPCMs via a coacervation using chitosan-co-poly(methacrylic acid) as a shell material: The coacervation approach is based on an aggregation process of a dissolved polymer chain to form a coating around the hydrophobic droplet by decreasing polymer solubility. The amphoteric water-soluble polymer, chitosan-co-poly(methacrylic acid) (CTS-co-PMAA) copolymer was synthesized as the shell material. The synthesis involved a two-step reaction: 1) The chitosan was first modified with methacrylic acid (MAA) through an amidation reaction between the amine group of the chitosan and double bond of the MAA to form MAA@CTS; 2) Free radical polymerization of remaining MAA to generate CTS-co-PMAA copolymer. The copolymer was characterized by dynamic light scattering, surface charge measurement, FTIR and FE-SEM. The fabrication of encapsulated PCMs through coacervation of CTS-co-PMAA was optimized by varying various factors which include: pH titration profiles, CTS to PMAA ratio in the copolymer, the shell rigidity and addition of the nucleating agent to reduce supercooling. The resultant EPCMs could be applied in three forms: 1) EPCMs dispersing in water. The water-dispersible EPCMs showed up to 90% encapsulation efficiency, with little reduction in latent heat or leakage after 100 heat-freeze cycles. 2) Dried EPCM: The EPCMs were crosslinked with glutaraldehyde to form rigid shells, thus retaining their structure in the dried state. Results indicated that there was 5% PCM leakage after drying. SEM images of the dried EPCMs showed film structure encasing nodules of PCM which diameters ranging from 174 to 312 nm and latent heat of melting of 165 J/g. 3) Freeze-dried EPCM: The EPCMs dispersed in water were freeze-dried to remove water, giving a sponge-like structure with low density of 0.03 g/mL and latent heat of melting of 102 J/g. Finally, supercooling phenomena was reduced by the use of the nucleating agent, eicosane, which increased the onset freezing temperature (Tf) of the EPCMs from 5 to 10 °C. Overall, the coacervation approach presents a simple method to fabricate encapsulated nanosized PCM with high thermal energy storage and good phase change cycling that can be dispersed in water or used to form PCM composites which could potentially be used for thermoregulation.
Subjects: Hong Kong Polytechnic University -- Dissertations
Heat storage
Phase transformations (Statistical physics)
Pages: xx, 227 pages : color illustrations
Appears in Collections:Thesis

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