## 3 Phase Change Materials
### PCM Properties  
For a material to be used as PCM, there are some thermal, physical, chemical, and kinetic properties that have to be satisfied. In addition, the final decision for a PCM to be manufactured for thermal storage also takes into account the environmental impact and the economics. Regarding the thermal properties, firstly, the phase transition temperature has to be in the desired operating temperature range in order to aver that the absorbing and releasing of heat happens in the desired range for the application temperature. Another very important characteristic for PCMs is high thermal conductivity of both liquid and solid phases, which is required in order to support the absorbing and releasing process of thermal energy. The latent heat of fusion per unit volume also has to be high, so that the storage density is higher and sensible storage can be achieved. In addition, high specific heat offers extra sensible heat storage. Finally, and also very important, is for the melting to happen congruently, in order for the phase change to be reproducible with stable cycling and long life [11,14,19]. On the physical properties needed, an important requirement is the minimum volume change during phase transition as well as small vapor pressure at the temperature of operation. Also, it is desired that the material has high density [14,19]. The kinetics of \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Inorganic** & \begin{tabular}{c} **Melting** \\ **Temperature** \\ **(\({}^{\circ}\)C)** \\ \end{tabular} & \begin{tabular}{c} **Melting** \\ **Hanplay** \\ **(J/g)** \\ \end{tabular} & **Ref** & \begin{tabular}{c} **Eutectic** \\ **Mixtures** \\ \end{tabular} & \begin{tabular}{c} **Melting** \\ **Temperature** \\ **(\({}^{\circ}\)C)** \\ \end{tabular} & \
\begin{tabular}{c} **Melting** \\ **Enthalpy (J/g)** \\ \end{tabular} & **Ref** \\ \hline CaCl\({}_{2}\)-12H\({}_{2}\)O & 29.8 & 174 & [8] &
\begin{tabular}{c} Capric \\ Acid \\ Capric \\ Capric \\ Capric \\ Capric \\ Capric \\ Capric \\ Acid \\ Cipric \\ Acid \\ Cipric \\ Acid \\ Cipric \\ Acid-Lauric Acid \\ Cipric \\ Acid \\ Cipric \\ Acid-Lauric Acid \\ Cipric \\ Acid-Lauric Acid \\ Cipric \\ Acid-Lauric Acid \\ Cipric \\ Acid-Stearic \\ Cipric \\ Acid-Plamitic \\ Acid \\ Cipric \\ Acid-Plamitic \\ Acid-Stearic \\ Cipric \\ Acid-Plamitic \\ Acid-Stearic \\ Cipric \\ Acid-Plamitic Acid \\ Cipric \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Plamitic Acid \\ Acid \\ Cipric \\ Acid \\ Acid \\ Acid \\ Acid \\ Acid \\ Acid-Myristic Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Acid-Stearic \\ Cipric Acid-1-1 \\ dodecanol & 27 & 126.9 & [4] \\ dodecanol & 30 & 136 & [17] \\ Mg(NO\({}_{3}\))\({}_{3}\cdot\)6H\({}_{2}\)O & 30 & 200.5 & [17] \\ CH3 COONa\(\cdot\)3H\({}_{2}\)O & 26–28 & 188 & [17] \\ + KCl+H\({}_{2}\)O & 26–28 & 188 & [17] \\ Na\({}_{2}\)SO\({}_{4}\cdot\)10H\({}_{2}\)O- & 32.52 & 226.9 & [5] \\ Na\({}_{2}\)HPO\({}_{4}\cdot\)12H\({}_{2}\)O & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Thermal properties of inorganic materials and eutectic mixtures used as PCMs. the charging/discharging cycle are also considerable because the insufficient nucleation rate leads to supercooling of the liquid phase, which is undesirable because the melting and solidification process take place in different temperatures. Therefore, high nucleation as well as enhanced crystal growth are required [8; 14]. Apart from that, several chemical properties have to be satisfied by a PCM. Firstly, the freezing and melting cycle has to be completely reversible and the material should not degrade after a big number of cycles. Additionally, the material has to be noncorrosive to the environment used [11; 14]. Finally, the materials used as PCMs have to be nontoxic, inflammable, and nonexplosive, both for safety and environmental reasons.