## 3 Latent Heat Storage (LHS) Method  
Latent heat storage is based on the amount of energy absorbed or released during the reforming of the phase structure of a phase change material (PCM) from one physical state to another during the phenomena of melting, solidification, gasification and liquefaction [19; 27; 56; 57]. Due to its high thermal energy storage density and nearly constant working temperatures, latent heat storage (LHS) technology has become a good solution for correcting the mismatch between energy supply and demand. At the start of the charging process, the temperature of the latent heat storage material evolves as it absorbs thermal energy, and when it reaches a specific range, the material begins to change the phase while storing heat. However, during the discharge period, the PCM switches to its initial state by releasing thermal energy. Unlike SHS, the phase transition process unfolds at a nearly constant temperature characteristic of the material physical state change [19; 57]. The heat storage capacity (\(Q\)) of an LHS system typically consists of two parts sensible heat (_before and after the phase transition_) and one part latent heat (during the phase transition) [3; 19; 27; 58]:  
\[Q=\underbrace{\int_{T_{1}}^{T_{m}}mc_{p}dT}_{\textit{Sensible heat}}+ \underbrace{m\Delta h}_{\textit{Latent heat}}+\underbrace{\int_{T_{m}}^{T_{2}} mc_{p}dT}_{\textit{Sensible heat}} \tag{1}\]  
where the mass (\(m\)), specific heat capacity (\(c_{p}\)) and temperature (\(T\)) are the properties of the PCM. \(m\cdot\Delta h\) is its latent heat. The indices 1, 2 and \(m\) indicate the initial, final and melting temperature, respectively. Recall that during the phase change operation, the temperature remains roughly constant with small volume changes, making the phenomenon simple and safe. The typical diagram of LHS is illustrated in Figure 8[3; 28]. The phase change process takes place in a temperature range between a solidus temperature, \(T_{s}\) (the first solid melts (_melting process_)/the first solid forms (_start solidification process_)), and a liquidus temperature, \(T_{l}\) (the last solid melts (_end of melting_)/the last solid forms (_solidification end_)). In this context, [PERSON] et al. [59] defined two cooling/heating PCM temperature ranges to minimize the annual energy consumption. During cooling,the maximum melting temperature of PCM is about 26 \({}^{\circ}\)C (melting range 24-28 \({}^{\circ}\)C), while during heating, the minimum melting temperature of PCM is about 20 \({}^{\circ}\)C (melting range 18-22 \({}^{\circ}\)C). As depicted, the PCM stores sensible heat when the PCMs temperature is outside the phase change temperature range and latent heat when its temperature reaches the phase change temperature. Moreover, the different phase changes in LHS can be in solid-solid, solid-liquid, liquid-solid, solid-gas, gas-solid, liquid-gas and gas-liquid forms. In the case of a solid-solid transformation, the crystalline structure of the material changes and continues to store heat [34; 60]. Generally, such a phase is characterized by low amounts of latent heat and volume changes compared to solid-liquid processes. For this process, organic solids such as pentaglycine, pentaerythriol, Li\({}_{2}\)SO\({}_{4}\) and KHF\({}_{2}\)[60; 61] were able to open up interesting opportunities. Leaks and encapsulation and a more flexible design are their main advantages [62; 63]. On the other hand, during solid-gas and liquid-gas transformations, a large quantity of latent heat can be acquired requiring large capacity containers due to the significant change in volume. Thereby, this solution is less usable for the building's thermal comfort [64]. For the sake of brevity, solid-liquid phase change processes remain economically competitive with other LHS solutions [64].