Role of Polymers in Developing Phase Change Materials for Energy Storage: A Review on Encapsulation Methods and Their Applications

With serious energy crisis in recent years, phase change materials (PCMs) are among the best choice for thermal energy storage. These materials have attracted special attention in energy saving. PCMs absorb energy during the heating process accompanied by their phase transitions. This energy can be transferred into the surrounding environment during the cooling process. In other words, these compounds have the ability to store and release high amount of heat through phase transitions with a slight temperature variation. PCMs are classified into four groups of organic, inorganic, eutectic and polymeric materials. However, the major limitation of PCMs is their mixing problem with other materials. Encapsulation might be the best way for effective use of PCMs to increase heat transfer rate, preventing their leakage, reducing their interaction with the external media and limit volume changes during phase change. Encapsulation is a coating process of PCMs with a suitable material to protect solids, liquids or gases in a solid shell. Encapsulated materials are categorized into macrocapsules, microcapsules, and nanocapsules, based on the capsule size. Among these, microencapsulation is a successful method to develop PCMs application in construction, pharmaceutics, agriculture, and textile industries. Different types of microcapsules can be made with a wide range of shell materials by various physical, physico-chemical and chemical methods. Depending on the chemical nature, three types of shell materials are used for microcapsulation, including organic, inorganic, and organic/inorganic hybrids. The conventional polymeric compounds in the shell include polypropylene, polyethylene, polyurethane, polystyrene, polyamide, urea-formaldehyde, melamine-formaldehyde and acrylic resins. Due to the importance of encapsulation methods and versatile applications of encapsulated PCMs in various industries, the most important methods for encapsulation of PCMs are reviewed in this paper in preparation of stable PCMs by consideration of the role of polymers.