Supercooling of phase-change materials and the techniques used to mitigate the phenomenon

In the quest for alternatives for fossil fuels, phase change materials (PCMs) have attracted considerable attention due to their ability to store renewable thermal energy. Compared to other storage systems, PCM systems are of low cost and capable of the storage of a high density of energy. However, few drawbacks hinder their practical application at an industrial scale. Among the drawbacks, supercooling problem affecting all types of PCMs is crucial. Supercooling as a shortcoming in PCM applications limits their practical applications. However, a comprehensive discussion or review articles have not been published. A PCM can exists in the liquid form below the phase change temperature or its freezing point, without fully freezing, due to supercooling. Thus, practical applications are limited by major problems such as the temperature variations and the increase of energy consumption. In this paper, most of the reported supercooling mitigation techniques for various types of PCMs and nanofluids are reviewed. These techniques are based mainly on adding nucleating agents (such as carbon nanotubes, fine salt particles, and nanoaditives), thickeners (such as carbon, methyl cellulose), and macroporous structures. The mitigation of phase separation and thermal cycling effects on supercooling are also discussed. The mitigation of supercooling in encapsulated organic PCMs, which is an important issue that is not very well understood, too is briefly addressed. Recommendations and future challenges to enhance the application of PCMs are discussed.