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Sustainability 2018, 10, 191 13 of 32 (1) Paraffin waxes consist of a mixture of mostly straight chain n-alkenes CH3 –(CH2 )–CH3 . The crystallization of the (CH3)– chain release a large amount of latent heat. Both the melting point and latent heat of fusion increase with chain length. Due to cost consideration, however, only technical grade paraffin’s may be used as PCMs in latent heat storage systems. Paraffin is safe, reliable, predictable, less expensive, non-corrosive, and available in a large temperature range (5–80 ◦C) [11]. (2) Non-paraffin organic PCMs are the most numerous of the PCMs with highly varied properties. A number of esters, fatty acids, alcohols, and glycols suitable for energy storage have been identified [41]. Main features of these organic materials include high heat of fusion, inflammability, low thermal conductivity, low flash points, and instability at high temperatures. Details of thermal properties, applications, and limitations of fatty acids are discussed in [42] and those of sugar alcohols/polyols are found in [43,44]. 4.1.2. Inorganic PCMs Inorganic PCMs are (mostly) used in high-temperature solar applications and one of the most reported challenges is their maintenance. At lower temperatures, they freeze; at high temperatures, they are difficult to handle. These PCMs do not super-cool appreciably and their melting enthalpies do not degrade with cycling. The two main types are as follows: (1) Salt hydrate. This is classified as a congruent, incongruent, and semi-congruent melting method [11]. They are alloys of inorganic salts (AB) and n kmol of water, forming a typical crystalline solid of general formula AB·nH2O, whose solid–liquid transition is actually a dehydration and hydration of the salt. A salt hydrate usually melts either to a salt hydrate with fewer moles of water, i.e., or to its anhydrous form, AB·nH2O → AB·mH2O+(n−m)H2O (17) AB · nH2O → AB + nH2O. (18) At the melting point, the hydrate crystals break-up into anhydrous salt and water, or into a lower hydrate and water. One problem with most salt hydrates is that of incongruent melting caused by the fact that the released water of crystallization is not sufficient to dissolve all the solid phase present. Due to density difference, the lower hydrate (or anhydrous salt) settles down at the bottom of the container. Salt hydrates have been extensively studied in heat storage applications because of their positive characteristics: high latent heat of fusion per unit volume, a relatively high thermal conductivity (almost double that of paraffin), low corrosiveness, and compatibility with plastics. As an example, the main characteristics of some salts hydrate of the Phase Change Material Product Limited (UK) are depicted in Table 6 [8]. Some disadvantages include incongruent melting and super-cooling, which can be tackled in different ways (by adding thickening agents, by mechanical stirring, by encapsulating the PCM to reduce separation, etc.). Other problems include the spontaneity of salt hydrates and lower number of water moles during the discharge process. Adding chemicals can prevent the nucleation of lower salt hydrates, which preferentially increases the solubility of lower salt hydrates over the original salt hydrates with a higher number of water moles. Several research studies have shown the suitability of salt hydrates for thermal energy storage [45]. These materials dissociate into anhydrous salts, release water vapor when subjected to heat source, and store energy supplied for dehydration upon heating. A numerical study was conducted to investigate the performance of three different salt hydrates, namely, magnesium sulfate (MgSO4·7H2O), cupric sulfate (CuSO4·5H2O), and gypsum (CaSO4·2H2O), in order to investigate their abilities to efficiently store thermo-chemical energy. It was shown that cupric sulfate had the highest efficiency and required the least heating time to initiate the chemical reaction.PDF Image | Comprehensive Review of Thermal Energy Storage
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