## 4 Improvement of Thermal Performance
### Encapsulation
#### 4.1.2 Methods of Microencapsulation  
In physical methods, the material that creates the shell does not take part in any chemical reactions. The shell is mechanically applied on the core material by physical processes such as binding due to adhesion, drying, etc. There are a lot of physical methods, such as spray drying, vibrational nozzle, centrifugal extrusion, solvent evaporation, etc., although the method that is most used for microencapsulating PCMs is spray drying. The equipment and knowledge in this method is widely available. Additionally, the method can be easily scaled up, with average microcapsule size around some micrometers and efficient thermal performance; although, the high operating temperature, the agglomeration of microcapsules in the drying chamber, and the remaining uncoated particles are some its disadvantages [22; 29].  
In physicochemical synthesis methods, the capsule emerges through a physicochemical process. A physicochemical process is a combination of physical and chemical methods. The physical methods include processes such as cooling, heating, phase separation, etc., while the chemical methods include chemical processes such as condensation, hydrolysis, cross-linking, etc. Those methods combined result in microencapsulation. The most widely-used physicochemical methods are complex coacervation and sol-gel technique.  
The complex coacervation method is suitable for fabrication of microcapsules with shell of organic materials such as chitosan, gum Arabic, gelatin, etc. This method is based on phase separation. The shell polymer is dissolved in an emulsion with the PCM (core material). The shell is formed around the droplet of the core material by evaporation of the solvent. In another way, the formation of the microcapsule can be managed by electrostatic coalescing of two different organic polymers around the droplet, and then decreasing the temperature and pH of the emulsion to initiate phase separation [30]. On the other hand, the sol-gel technique is used for the fabrication of inorganic shell microcapsules, such as SiO\({}_{2}\), TiO\({}_{2}\), etc. The sol-gel technique includes a sol solution, which is a solution of the hydrolyzed precursor compound, and an emulsion of the PCM. When the sol is added  
Figure 3: Physical, chemical, and physicochemical methods of encapsulation.  
to the emulsion, the synthesis of the shell is happening on the PCMs droplet's surface, producing a gel of the microcapsules [31].  
Finally, in chemical methods, the shell results from polymerization on the surface of a droplet or a particle of the core material. The most important methods are emulsion polymerization, suspension polymerization, interfacial polymerization, condensation polymerization, and in situ polymerization [22; 31]. Suspension and emulsion polymerizations are widely used for PMMA and polystyrene shells, while the interfacial polymerization is preferred when the final shell consists of two or more monomers. In in situ polymerization, no monomers are used. The precursor compound hydrolys in situ, and it initiates the polymerization. It is used for the synthesis of organic shell materials such as polyurea-formaldehyde and melamine-formaldehyde. Finally, condensation polymerization can also be used for inorganic shells, such as SiO\({}_{2}\), and it is usually part of other synthesis techniques, such as sol-gel [32].