## 3 Latent Heat Storage (LHS) Method
### Latent Heat Storage Materials (LHSMs or PCMs)  
Since PCM is the central ingredient (_component_) of an LHS system, it is expected to meet certain requirements to improve energy storage density and ensure thermal stability under long and/or short term loops. Its sound selection is based on its main desired thermophysical, kinetic, chemical, economic and environmental characteristics. These have been listed by [PERSON] et al. [60] and [PERSON] et al. [74] as follows:  
* In terms of thermal properties, the PCM must have a phase change temperature corresponding to the operating temperature of the LHS application. Moreover, it must have a latent heat and a specific heat. PCMs with high thermal conductivity are advocated to facilitate phase transition and interstitial heat transfer;
* Regarding physical properties, PCMs should exhibit large density, small volume changes and low vapor pressure during the phase transition process while respecting the operating temperature range to limit containment issues;
* As for kinetics properties, subcooling and supercooling should be avoided, and a sufficient crystallization rate should be achieved;
* For chemical properties, PCMs should be compatible with the materials in the application. They should retain their chemical stability for long-term cycles (_no chemical degradation and breakdown_). In addition, the latent heat storage material should be a non-toxic, non-flammable, non-corrosive, and even less explosive substance. Moreover, charging and discharging periods must be fully completed;
* Low-cost PCMs should be preferred in terms of economic properties. For environmental properties, PCMs should have a low environment impact by being non-polluting and recyclable. Unfortunately, each PCM has its own characteristics that make it virtually difficult to provide a clear answer to all the listed criteria. As tacitly stated above, PCMs can be split into PCM solid/solid, PCM solid-liquid, PCM solid/gas and PCM liquid-gas. It is solid-liquid PCMs that are involved in this section and can be divided into organic, inorganic and eutectic, as depicted in Figure 11[3; 20; 24; 75; 76; 77]. In addition, an attempt to compare the groups that have been up-cited is provided in Table 5[24; 77]. Figure 11: PCMs’ subdivision based on phase transition [3; 20; 24; 75; 76; 77], reprinted/adapted with permission from Refs. [20; 24; 75], 2022, Elsevier. It is worth noting the PCMs' use has spread to various industrial sectors such as textiles, satellites, telecommunications and medicine thanks to the change in their physical states. For moderate melting temperatures (_below_\(15\)\({}^{\circ}\)C), PCMs are best suited for air conditioning and extreme cooling applications where the operating temperature matches that of melting. On the other hand, for extreme melting temperatures (_above_\(90\)\({}^{\circ}\)C), PCMs are used to prevent a sudden temperature rise which can cause fires [78; 79; 80; 81; 82]. Figure 12 portrays the enthalpy of fusion vs. the melting temperature of some existing PCMs [77; 78; 82; 83; 84]. Close examination of such a figure indicates that suitable PCMs for construction applications are those with a phase temperature between 18 and 40\({}^{\circ}\)C. Simply put, potential PCMs for building applications are paraffin, fatty acids, salt hydrates and eutectic mixtures. These are used (a) as passive thermal storage integrated within building elements, and (b) as independent storage units coupled with heating, ventilation and air conditioning (HVAC) systems. Figure 12: Melting enthalpy vs. melting temperature for some PCMs [77; 78; 82; 83; 84], reprinted/adapted with permission from Ref. [82], 2022, Elsevier. \begin{table}