Microencapsulation of phase change materials (PCMs) for cryogenic energy storage

Thermal energy storage (TES) technologies have been copiously garnering a proliferation in attention over the past decade and have been employed to increase renewable energy utilisation and to mitigate the climate change crisis. Phase change materials (PCMs) are a class of organic or inorganic compounds, which have the capability to store and release vast amounts of energy due to their latent heat energy storage (LHES) capabilities, and various melting point ranges that are suitable for an array of applications. A challenge with these PCMs, especially organic PCMs, is their toxicity, flammability, and their tendency to leak during the phase change process. Therefore, the PCMs can be amalgamated within an inert shell, in a process known as ‘microencapsulation’. In this study, volatile cryo-PCMs were encapsulated via the one-step in situ polymerization technique, in which the mechanical and barrier properties were characterized to observe their feasibility to be applied in real world applications. It has been observed that with the one-step in situ polymerization method, the choice of emulsifier used is an imperative consideration, and such emulsifiers greatly affect the microencapsulated PCM (MPCM) properties, such as their mechanical strength, morphology, long term core material retention, payload, yield, and encapsulation efficiency. A range of emulsifiers were employed in this process, which were categorised into two main categories: natural and synthetic emulsifiers. A significant finding in this work is that the functional groups of the emulsifiers predominantly affect the reaction rate, and the order of reactivity was seen to be as follows: carboxyl > amino/amide > hydroxyl. Additionally, the process optimization of the polymerization process was carried out, in which the Taguchi and ANOVA methods were employed. Various parameters such as reaction time, pH, reaction temperature and homogenization speed were studied. The ANOVA analysis conveyed that the parameters that affect the microcapsule formulation in terms of magnitude are as follows: temperature, reaction time, pH, homogenization speed. PCMs when amalgamated in a shell material exhibited high levels of supercooling due to the limited crystallization process, and various efforts were made to reduce this phenomenon. Such efforts included increasing the MPCM size, employing nucleating agents, and embedding copper nanoparticles on the MPCM shell to increase the thermal conductivity. Furthermore, di(propylene glycol) methyl ether was used as a carrier fluid for the MPCMs to study the viscosity changes with various MPCM compositions