## 2 Thermal Energy Storage Systems in Buildings
### Materials for Passive Thermal Energy Storage  
Capacity, charge and discharge rate, storage period, charge and discharge time, and cost are the most important criteria for the selection of storage materials [12]. As mentioned in the introduction, heat can be stored in a material by three mechanisms: sensible, latent, and thermochemical heat.  
Sensible heat storage is the easiest way to store energy and involves applying a temperature gradient to a material for storing heat or cold. The amount of heat stored depends on three parameters: the mass of the material, the heat capacity of the material at constant pressure, and the range of temperature variation. The materials generally used for sensible storage are clay, hollow bricks, concrete or stone, which are widely used in the building industry.  
Latent heat storage stores energy by taking advantage of phase change. The materials used for latent storage are known as PCMs. PCMs are classified according to several criteria, such as their nature (organic or inorganic), their origin (renewable or non-renewable), and their phase change temperature (low, medium, and high) or according to the nature of the phase change involved in the process (solid-solid, solid-liquid, liquid-gas). The solid-liquid phase change is, to date, the most widely used because of its high enthalpy and the absence of problems related to pressure [13]. [PERSON] et al. [14] proposed a classification of solid-liquid PCMs into three groups according to their nature: organic, inorganic, and eutectic. Organic PCMs include paraffin, fatty acids, sugar alcohol, and polyethylene glycol (PEG). Inorganic PCMs are salt hydrates, nitrates, and metallics having a high heat of fusion. Eutectic PCMs are mixtures of two or more PCMs and are classified as organic-organic, inorganic-inorganic or organic-inorganic.  
Finally, the last storage mode uses thermochemical materials that store energy through an endothermic/exothermic reaction/process by generating easily separable reaction products [15].  
The advantages and disadvantages of the different storage modes are presented in Table 1.  
\begin{table}
\begin{tabular}{c c l c} \hline \hline
**Storage Modes** & \multicolumn{2}{c}{**Advantages**} & \multicolumn{2}{c}{**Disadvantages**} \\ \hline \multirow{5}{*}{Sensible} & - & Low price & \multirow{5}{*}{-} & \multirow{5}{*}{Incompatibility to corrosion} \\  & - & Availability & & \\ \cline{1-1}  & - & Non-toxic & & - \\ \cline{1-1}  & - & Simple and already & & cycles \\ \cline{1-1}  & & commercialized & & - \\ \cline{1-1}  & & technologies & & \\ \hline \multirow{5}{*}{Latent} & - & High energy density & \multirow{5}{*}{-} & \multirow{5}{*}{Low thermal conductivity} \\  & - & Quasi-constant storage & & \\ \cline{1-1}  & & temperature & & \\ \cline{1-1}  & - & No overheating & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Advantages and disadvantages of the different storage modes [8; 16; 17].  
Recently, PCMs have been studied extensively in order to lift the locks and allow a massive deployment of this technology in buildings. Thus, the systems that will be presented in this study will be those incorporating PCMs only. Table 2 provides a comparative summary of the advantages and disadvantages of the different types of PCMs.  
Organic PCMs are the most widely used for thermal energy storage due to their suitable melting/solidification temperature, high storage density, chemical stability, and storage capacity [17].