A review on encapsulation techniques for inorganic phase change materials and the influence on their thermophysical properties

Phase change materials (PCM) are characterized for storing a large amount of thermal energy while changing from one phase to another (generally solid-liquid states) at a specific temperature and presenting a high specific heat of phase change process. PCMs can be classified as organic, inorganic and eutectic. Such materials present some limitations, including subcooling, phase segregation, flammability, low thermal conductivity and thermal instability, among others. In order to overcome these problems, encapsulation of PCMs is being successfully developed, providing decreased subcooling, large heat transfer area, and controlling the volume change of the storage materials when the phase transition occurs. A considerable amount of studies has been published in the field of encapsulation methods for organic PCMs. Nevertheless, the information available on inorganic PCMs is scattered. Furthermore, the influence of the encapsulation techniques on thermophysical properties of PCMs is not reported in these reviews most of the time. Hence, the aim of this review is to summarize the encapsulation and characterization techniques for inorganic PCMs and to provide the analysis about the influence of synthesis parameters on thermophysical properties of encapsulated PCMs. Two principal types of encapsulated inorganic PCMs were found: core-shell PCMs (core-shell EPCMs) and shape stabilized PCMs (SS-PCMs). Classification of encapsulation methods of core-shell EPCMs and SS-PCMs are reported in this work. Among all the microencapsulation methods, inverse Pickering emulsion, electroplating, solvent evaporation-precipitation method and mechanical packaging are the most common methods described in the literature for the production of core-shell EPCM. On the other hand, for SS-PCMs, mainly sol-gel process, infiltration and impregnation encapsulation methods were found. Scientific works report a reduction in the heat of phase change for core-shell EPCMs. This is mostly because of the low content of salt in the final material. Moreover, an improvement of thermal conductivity was procured for SS-PCMs. Finally, PCM percentage, particle size, stirring rate, type of crosslinking agent and solvent properties were established as principal factors influencing the final properties of the encapsulated materials. For the best of our knowledge, this is the first profound review of encapsulation techniques for inorganic PCM.