## 6 Conclusions  
A thermal energy storage system is a type of a sustainable energy storage system that is based on the utilization of materials that can store thermal energy when increasing their temperature and release it when the temperature is reduced. Latent heat storage systems using PCM are based on the absorption or release of heat that takes place during the material's phase transition (latent heat of fusion). There is a wide variety of compounds that are used as phase PCMs. For building applications, solid-liquid PCMs are preferred because of their ability to absorb and release large amounts of energy within a narrow range of temperature. Solid-liquid PCMs utilized in buildings can be classified into three major groups: organic compounds, inorganic compounds, and eutectics of organic and/or inorganic compounds. Organic PCMs gather the most advantageous properties. The fabrication of composite PCMs enhances the thermal conductivity, thermal stability, and heat transfer rate, as well as impedes the leakage of the liquid phase. The encapsulated PCMs are composed of the core material (PCM) and the shell material, which can be inorganic or organic. A core-shell composite can have single or several cores within the capsule and multiple shells. The PCMs can be characterized by their size. The most interesting encapsulated PCMs are microencapsulated and nanoencapsulated PCMs, as the higher surface area per volume unit facilitates the thermal transfer and leads to better thermal performance. Encapsulation can be achieved with physical, chemical, or physicochemical methods. The most widely-used methods for micro- and nanoencapsulation are chemical methods, such as emulsion, interfacial, and suspension polymerization, and by the physicochemical sol-gel technique. Form-stable, or shape-stabilized, PCMs consist of a 3D matrix, or a porous structure, where the pure PCMs are entrapped. In polymer-based form-stable PCM composites, the PCM is entrapped inside a cross-linked polymer matrix. The synthesis of the polymer matrix can be achieved with many different methods, providing structural diversity and, as a result, the ability of fabricating a product with special specifications. Form-stable PCMs can also be fabricated with inorganic mineral porous materials, as well as with expanded graphite and perlite. Thermal conductivity in a TES system is a very important factor for the efficiency of energy storing and releasing. As a result, the enhancement of thermal conductivity of PCMs is requisite. The improvement of thermal conductivity can be achieved with encapsulation, addition of highly-thermal-conductive materials such as carbon-based nanomaterials, metallic or inorganic nanoparticles, and the fabrication of form-stable PCM. The application of TES systems in the building sector is a promising technology with many advantages. TES systems applied in buildings can be either active or passive. Active TES systems are characterized by forced convection heat transfer and, in some cases, also mass transfer. On the other hand, passive TES systems aim to create or maintain comfort conditions in the building, exploiting naturally-available energy sources, such as the solar energy or the wind, along with architectural design of building components. The main characteristic of these systems is that there is no requirement for mechanical equipment or additional energy due to the fact that the heat is charged or discharged only because of temperature fluctuations when the environment's temperature rises or falls beyond the PCM's phase change temperature. Conceptualization: C.V.P. and L.A.K. ; methodology: C.V.P. and I.A.K. ; software: C.V.P. and I.A.K. ; validation: C.V.P. and I.A.K. ; formal analysis: C.V.P. and I.A.K. ; investigation: C.V.P. and I.A.K. ; resources: C.V.P. and I.A.K. ; data curation: I.A.K. ; writing--original draft preparation: C.V.P. and I.A.K. ; writing--review and editing: I.A.K. and C.A.C. ; visualization: I.A.K. and C.A.C. ; supervision: C.A.C. ; project administration: C.A.C. ; funding acquisition: C.A.C.