## 3 Phase Change Materials
### Classification of PCMs
#### 3.1.1 Organic Materials  
Organic PCMs can be paraffinic, such as paraffin wax, which is the most commercially used organic PCM for building applications, or they can be nonparaffinic, such as fatty acids, fatty acid esters, glycolic acids, alcohols, etc. Paraffinic PCMs consist of hydrocarbon chains, while nonparaffinic also contain electronegative atoms in their chains [8,9]. Paraffin waxes consist of hydrocarbon chains ranging from 8 to 15 carbon atoms, and pure paraffin of hydrocarbon chains range from 14 to 40 carbon atoms. Thermal properties of some paraffin waxes and pure paraffins are presented in Table 1.  
Nonparaffinic PCMs can be fabricated from biosourced raw materials, such as animal fats, and vegetable oils, such as beef tallow, margarine, coconut oil, castor oil, etc. These materials are sustainable, biodegradable, nontoxic, and less flammable. On the other hand, their major drawbacks are that they are more corrosive than paraffin and more expensive [14,15]. Some nonparaffinic PCMs are presented in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multirow{2}{*}{**Chemicalformula**} & \multirow{2}{*}{**Melting Temperature (\({}^{\circ}\)C)**} & \multirow{2}{*}{**Melting Enthalpy (J/g)**} & \multicolumn{2}{c}{**Thermal Conductivity (W/(m.K))**} & \multirow{2}{*}{**Density (kg/m\({}^{3}\))**} & \multirow{2}{*}{**Ref**} \\  & & & & & & \\ \hline n-Tetradecane & C\({}_{14}\)H\({}_{30}\) & 5.5 & 228 & - & - & [8] \\ n-Pentadecane & C\({}_{15}\)H\({}_{32}\) & 10 & 205 & - & - & [8] \\ n-Hexadecane & C\({}_{16}\)H\({}_{34}\) & 18.0 & 210.0–238.0 & 0.2 (solid) & 760.0 (liquid, 20.0 \({}^{\circ}\)C) & [10] \\ n-Heptadecane & C\({}_{17}\)H\({}_{36}\) & 19.0 & 240.0 & 0.2 & 776.0 (liquid, 20.0 \({}^{\circ}\)C) & [10] \\  & & & & & 774.0 (liquid, 20.0 \({}^{\circ}\)C) & [10] \\ n-Octadecane & C\({}_{18}\)H\({}_{38}\) & 28.0 & 200.0–245.0 & 0.15 (liquid, 40.0 \({}^{\circ}\)C) & 70.0 \({}^{\circ}\)C) & [10] \\  & & & & 0.36 (solid, 25.0 \({}^{\circ}\)C) & 814.0 (solid, 20.0 \({}^{\circ}\)C) & 20.0 \({}^{\circ}\)C) & [10] \\  & & & & & 750.0 (liquid) & \\  & & & & & 870.0 (solid) & \\ n-Nonadecane & C\({}_{19}\)H\({}_{40}\) & 32.0–33.0 & 222.0 & 0.18 (liquid, 60.0 \({}^{\circ}\)C) & 780.0 & [10] \\  & & & & 0.26 (solid, 19.0 \({}^{\circ}\)C) & & \\ n-Eicosane & C\({}_{20}\)H\({}_{42}\) & 36.0–37.0 & 247 & 0.42 (solid) & - & [10] \\ n-Heneicosane & C\({}_{21}\)H\({}_{44}\) & 39.0–41.0 & 201.0 & - & - & [10] \\ n-Docosane & C\({}_{22}\)H\({}_{46}\) & 44.0 & 249 & - & - & [8] \\ n-Tricosane & C\({}_{23}\)H\({}_{48}\) & 47.5 & 232 & - & - & [8] \\ n-Tetracosane & C\({}_{24}\)H\({}_{50}\) & 50.6 & 255 & - & - & [8] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Thermal properties of some paraffins.  
Organic PCMs exhibit a great number of advantages. First of all, the phase change temperature rises in proportion to the number of carbon atoms in the chain, providing availability in a broad range of temperatures for different applications. Another advantage of this type of PCMs is that they are noncorrosive, chemically inert, and thermally stable in the temperature range needed, therefore, no degradation problems occur during their use. Additionally, they demonstrate a congruent melting process which means that the phase change happens repeatedly without phase segregation. Furthermore, they have high latent heat, good nucleation properties, low liquid phase sub-cooling capability, and minimal volume variation. Finally, they are compatible with most of the construction materials, they can be recycled, and their cost is relatively low. On the other hand, organic PCMs also exhibit some disadvantages based on their low energy storage capacity, low density, low thermal conductivity, and high flammability [8; 9; 14; 15].