## 2 Thermal Energy Storage Systems in Buildings
### Passive Storage Systems Incorporating PCMs
#### 2.2.9 Shape-Stabilized  
Shape-Stabilized PCMs are obtained by impregnating PCMs into porous building materials. This technique stabilizes the PCMs and prevents leakage problems during the phase change process due to capillary force, surface tension, hydrogen bonding interaction, and other interactions between the porous matrix and the PCMs confined in the nanopores of the building materials [49].  
In general, two main techniques are used to infiltrate liquid or solid PCMs into a porous support to fabricate shape-stabilized PCMs composites: the two-step impregnation method and the one-step in situ synthesis method.  
The two-step technique involves impregnating liquid PCMs into the nanopores of the porous support. Various impregnation methods have been used to synthesize shape-stabilized PCMs. The preparation processes generally involve the following steps: (1) design and synthesis of the porous supports; (2) addition of the porous support to the PCM solution, which is absorbed into the porous supports, and (3) obtaining the final composite PCMs after complete evaporation of the solvent [49]. Depending on the impregnation process, this method can be divided into direct impregnation and vacuum impregnation. [PERSON] et al. [50] analyzed the impregnation rate of a PCM in vacuum and non-vacuum processing. It was found that the impregnation rate was close to 50% in the vacuum treatment compared to only 30% in the non-vacuum conditions.  
In the one-step method, the PCMs are encapsulated in situ during the formation of an interconnected network of porous supports [49].  
There are a wide variety of materials used for shape stabilization of PCMs, including polymers, porous materials, and nanomaterials. [PERSON] et al. [46] classified these materials into five major groups: micro-encapsulation, polymer matrices, nanomaterials, porous materials, and solid-solid PCMs.  
Table 3 summarizes the advantages and disadvantages of the different techniques for incorporating PCMs into buildings. Due to the simple and inexpensive manufacturing techniques, the very high impregnation rate compared to other techniques as well as the wide variety of support materials available, shape stabilization was chosen to impregnate PCMs in our future studies.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Techniques** & \multicolumn{2}{c}{**Advantages**} & \multicolumn{2}{c}{**Disadvantages**} \\ \hline \multirow{3}{*}{Direct incorporation} & - & Easy technique & - & Reduction of mechanical properties \\  & - & Economical technique & - & Impact on the hydration process \\  & & & - & Leakage of PCM \\ \hline \multirow{3}{*}{Impregnation} & - & Easy technique & - & Long-term leakage of PCM \\  & - & Economical technique & - & Incompatibility with building materials \\  & & & - & Corrosion of reinforced steel when \\  & & & - & incorporated into concrete elements \\ \hline \multirow{3}{*}{Micro-encapsulation} & - & Improvement heat transfer & - & Limitation of the volume of PCM that can be used \\  & - & Reduction of volume variation of PCM & - & Complex and costly manufacturing process \\  & - & Possibility of incorporation into & - & Requirement for a non-permeable \\  & - & various materials & & capsule to prevent leakage \\  & - & Reduction of phase separation and & - & Some production methods may have \\  & - & Improvement cycle stability & & harmful by-products \\  & - & Improvement of thermal reliability & & \\ \hline \multirow{3}{*}{Macro-encapsulation} & - & Improved environmental compatibility of & - & Moisture transfer problem since the PCM \\  & & materials & & is macro-encapsulated \\ \cline{1-1}  & - & Improved handling of PCMs during & - & More work to integrate into the \\ \cline{1-1}  & - & production & & building envelope. \\ \cline{1-1}  & - & Increase in the amount of PCMs used & - & Low thermal conductivity \\ \hline \hline \end{tabular}
\end{table}
Table 3: Advantages and disadvantages of the different techniques for incorporating PCMs into buildings [9; 18; 47; 49].