**2. Phase change materials (PCMs)**

PCMs due to their higher latent heat values can store and release a large amount of heat energy during melting and solidifying processes [14]. These materials have been thought to act as a storage medium with numerous applications such as cooling of food products, buildings, textiles, solar systems, spacecraft thermal systems and waste heat recovery systems [15]. On the basis of phase conversion PCMs are categorized into solid–liquid, solid–solid, solid–gas and liquid–gas [1]. Among these categories, solid–liquid PCMs due to their high density, favorable phase equilibrium, minor volume changes and low vapor pressure at the operating temperature during phase transition are more suitable for thermal energy storage systems. Furthermore, solid–liquid PCMs show little or no subcooling during freezing, melting/freezing at the same temperature and phase separation, and sufficient crystallization rate.

PCMs possess high chemical stability, nontoxic, nonexplosive and noncorrosive nature, do not undergo degradation after long-term thermal cycles, and have good chemical properties capable of completing reversible freezing/melting cycle [6]. Solid–liquid PCMs can be divided into three major types: (a) organic PCMs (b) inorganic PCMs and (c) eutectic PCMs [16]. Organic PCMs include paraffin and nonparaffin (alcohols, fatty acids and glycols) materials [2]. Inorganic PCMs generally include salt hydrates, metallic compounds and metal alloys with the advantages of a broader range of transition temperature, high thermal conductivity and high latent heat storage capacity, low cost and nonflammable nature. In contrast, lack of thermal

stability, phase segregation, supercooling, corrosion and decomposition, are problems that dominate their benefits [17]. Eutectic PCMs are the combination of two or more low melting components, each of which freezes and melts congruently to make a mixture of the components' crystals upon crystallization [6]. Eutectic PCMs can be prepared for a specific application by mixing organic–organic, inorganic–inorganic, or a combination of the two PCMs at a given ratio. These PCMs have high thermal conductivity and density without segregation and supercooling, while their specific heat capacity and latent heat are much lower than those of paraffin/salt hydrates [18].
