Developing thermal energy storage using PCM/water emulsion : media preparation

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Developing thermal energy storage using PCM/water emulsion : media preparation


Author: Zhang, Xiyao
Title: Developing thermal energy storage using PCM/water emulsion : media preparation
Degree: Ph.D.
Year: 2017
Subject: Hong Kong Polytechnic University -- Dissertations
Heat storage
Heat storage devices -- Design and construction
Solar thermal energy
Department: Dept. of Building Services Engineering
Dept. of Building Services Engineering
Pages: xxxviii, 218 pages : color illustrations
Language: English
Abstract: The building sector consumes nearly one-third of the total global energy consumption, of which most power is used to achieve favorable indoor temperature conditions. The use of thermal energy storage (TES) systems is a potential way of energy conservation and reducing CO2 emissions associated with space and water heating, cooling, and air conditioning (AC) as an efficient energy utilization pattern. The energy storage (ES) material applied in a TES system is a critical factor. The phase change materials (PCMs) are capable of storing or releasing a large amount of latent heat during phase transitions, and these materials are expected to help reduce the system volume and narrow the working temperature range of TES devices. This study aims to develop stable PCM-water emulsions with low viscosity for cooling storage and solar thermal applications. Paraffin materials were selected as the PCMs in this study, and they were engineered in PCM-water emulsion forms to store thermal energy in latent heat at selected working temperatures. The phase change temperature of AC applications is around 18 °C by use of n-hexadecane (C16H34), while the working temperature of solar heating applications is around 60 °C by use of n-octacosane (C28H58) as the PCMs. Such fluid PCM technique improves the heat transfer between the PCM and the ambient by increasing the surface-to-volume ratio of the PCM. A PCM emulsion is a surfactant-oil-water (SOW) system that consists of two phases by mixing water as the continuous phase and a PCM as the dispersed phase with the presence of surfactants or emulsifiers. The emulsifier determines many of the emulsion properties, and its selection is thus crucial for emulsification. The application of PCM emulsion is also limited by supercooling, which shifts the controlling temperature range and increases the energy consumption of the TES system. Several types of nanoparticles have been evaluated in the PCM products as nucleating agents to reduce supercooling. Instability is also a concern when a PCM emulsion is used as a working fluid. The mechanisms of instability are complex and related to the combination of factors. Thus, selecting appropriate processing conditions and product formulations is essential for forming PCM emulsions with the desired physicochemical properties. The effects of different emulsifier systems on the droplet diameter distribution, apparent viscosity, and stability of the emulsions were evaluated in this study. Compound emulsifiers were found to stabilize the products effectively. An effective range of the emulsifier concentration and the dispersed phase PCM content was identified for maintaining the stability and fluidity of the emulsions.
The thermophysical properties of PCM emulsions were investigated by differential scanning calorimetry (DSC). The DSC results indicate that the supercooling degree of emulsion increased with the decline in droplet size, and that dispersed nanoparticles (modified multi-walled carbon nanotubes and hydrophobic SiO2 particles) were effective as the nucleating agent for reducing supercooling. Optimized concentrations of the nucleating agent were also found. A minimum effective concentration was determined for reducing supercooling, and multiple phase transitions were observed when the nucleating agent was insufficient. The rheological behavior of PCM emulsions was also investigated. All emulsions showed the shear thinning property of a pseudoplastic fluid. The rheological characteristics of emulsion were quantitatively described by the power law fluid model, and the flow behavior index m and consistency coefficient K within the non-Newtonian regions at different temperatures were calculated. Emulsion stability was examined by observing the difference in droplet size and phase separation of the samples after the storage period and repeated thermal cycles. The mechanisms of instability were also comprehensively examined. Experimental results indicate that the effective approaches for improving the stability of PCM emulsion were as follows: (1) avoiding the aggregation of droplets, (2) reducing the droplet size, and (3) narrowing the droplet size distribution. The emulsification process played an important role in controlling the size distribution of the PCM droplets in the emulsions. The high-speed homogenization and phase inversion emulsification resulted in small-sized droplets with a uniform distribution. Consequently, the creaming speed decreased and the gravitationally induced aggregation occurred such that the PCM-water emulsion stabilized. In particular, the PCM emulsions with sufficiently small droplets of a diameter of less than 200 nm became nano-emulsion and exhibited super stability. The developed PCM-water emulsion can help (1) expand the usage of renewable heating and cooling sources, (2) promote the energy efficiency of TES systems through compact storage and quick charging, and (3) extend the service life cycle of energy storage media by use of the nano-emulsification technology.

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