Development of oil-in-oil phase change emulsions with enhanced heat storage properties

Abstract

Addressing today's sustainability and energy transition challenges requires a move towards innovative technologies. Phase Change Material Emulsions offer a promising solution for the efficient storage and transfer of thermal energy. By integrating phase change materials into heat transfer fluids, the energy storage capacity of these fluids is enhanced. The purpose of this thesis is to formulate non-aqueous emulsions of phase change materials using two oily phases. This represents a significant advance by extending their service temperature range and improving their long-term chemical and thermal stability. This research suggests new possibilities for innovative applications, such as high-efficiency solar collectors, industrial heat recovery, intermediate-high temperature systems, refrigeration, or cold energy storage systems. During the PhD thesis, two groups of stable non-aqueous phase change materials were developed. The first group consists of emulsions formulated with polyethylene glycol of molecular weight approximately 4000 g/mol, with a melting point of 60 °C, dispersed in silicone oil. These emulsions are considered pioneering in the scientific literature, opening a new field of study. The development of these systems overcame several challenges, including the absence of data on appropriate emulsifiers for oil-in-oil dispersions and the requirement for the system to remain stable after the solid-liquid transition of the dispersed phase. The findings suggested that silicone surfactants are a viable option for stabilization. They also revealed not only the difficulty of their stabilisation but also the effect of composition (surfactant and dispersed phase concentrations) and processing conditions (agitation, temperature, time, etc.) on the final morphological, rheological and thermo-physical properties of phase change material emulsions. The properties mentioned were significantly influenced by changes in structure, particularly by the crystallinity, crystallisation mechanisms, or their modification, of the dispersed phase (phase change material) of the emulsion, as well as its partial compatibility with the other components. On the other hand, a second group of stable stearic acid emulsions dispersed in silicone oil have been developed. These emulsions have a melting point of 70 °C and maintain their crystallinity after processing. They also improve the thermal storage properties compared to polyethylene glycol emulsions. The emulsions showed high physicochemical and thermal stability under mechanical-thermal cycles, withstanding more than 100 heating-cooling cycles. This proves their stability during in-service operations. Therefore, they have demonstrated significant potential as novel heat transfer fluids in thermal energy storage systems, owing to their high phase change enthalpy of 22.32 J/g in emulsions containing 10%wt. of phase change material. Additionally, they retained their stability and storage capacity almost entirely for a minimum of one month. Furthermore, the thesis explored the potential use of phase change emulsions as lubricants and found a significant reduction in the friction factor across the entire temperature range when compared to silicone oil. Therefore, the study proposes the use of these emulsions as a new intelligent lubricant due to its thermal storage capacity, which enables it to buffer temperature changes. Finally, the thesis confirms the innovation of the research and the successful transfer and application of the results in industry through the application for a patent. It describes emulsions of non-aqueous phase change materials suitable for use in a temperature range of 100 to 800 °C. In summary, the main challenges encountered in the development of these emulsions are specified in this thesis, and the importance of their thermal stability and reliability in practical applications is highlighted.