# Inorganic Salt Hydrate for Thermal Energy Storage
## 3 Supercooling of Salt Hydrates  
Supercooling is a common phenomenon among salt hydrates due to their poor nucleating ability [20]. Some salt hydrates without a nucleating agent do not crystalize even when supercooling temperature is decreased under tens of degrees. This always brings undesirable effects to practical application [21]. For instance, when we impregnated salt hydrates into building materials using as wall boards, the phenomenon of supercooling impedes the solidification process of PCMs and then the storing heat in PCMs cannot release even when the surrounding temperature has reached their freezing point. Consequently, the PCM with high supercooling degree will not have functions to regulate the indoor temperature fluctuation and this lead to the lost of the meaning of latent thermal energy storage. Supercooling degree depends on several kinds of parameters, such as nucleating agents, cooling rate, stirring rate, stirring time, solution composition. To overcome the supercooling problems, a number of methods have been suggested. One of the most effective and economical ways is adding nucleating agents. Theoretically, the choice of nucleating agents should be close to the lattice parameter of the target materials. For instance, Borax is always regarded as the most suitable nucleating agents for Na\({}_{2}\)SO\({}_{4}\cdot\)10H\({}_{2}\)O, which can reduce supercooling of Na\({}_{2}\)SO\({}_{4}\cdot\)10H\({}_{2}\)O from 15 \({}^{\circ}\)C to about 2\(\sim\)3 \({}^{\circ}\)C [22]. [PERSON] et al. [23] investigated the surface effects of interaction between salt hydrates and nano-particles. Three nano-particles of aerosol SiO\({}_{2}\), RNS-A SiO\({}_{2}\) and liquid phase SiO\({}_{2}\) were compared by adding them to Na\({}_{2}\)SO\({}_{4}\cdot\)10H\({}_{2}\)O. Results showed that aerosol SiO\({}_{2}\) can easily suppress the supercooling. CaCl\({}_{2}\cdot\)6H\({}_{2}\)O is also mentioned frequently as a kind of fine PCMs. However, its supercooling degree is up to 20 \({}^{\circ}\)C. [PERSON] [24] used SrCl\({}_{2}\cdot\)6H\({}_{2}\)O (1.0 wt %), Ba\({}_{2}\)CO\({}_{3}\) (0.5 wt %), and K\({}_{2}\)CO\({}_{3}\) (0.5 wt %) as additives to CaCl\({}_{2}\cdot\)6H\({}_{2}\)O. It was found that SrCl\({}_{2}\cdot\)6H\({}_{2}\)O exerted the best effect on CaCl\({}_{2}\cdot\)6H\({}_{2}\)O, this result was consistence with paper [25]. [PERSON] [26] adopted sonocrystallization method which used ultrasound radiation for promoting CaCl\({}_{2}\cdot\)6H\({}_{2}\)O to crystallize and results indicated this technique \begin{table}
\begin{tabular}{l c c c} \hline \hline \multicolumn{1}{c}{**Eutectic Mixtures**} & \multicolumn{1}{c}{**Melting Temperature**} & \multicolumn{1}{c}{**Heat of Fusion**} & \multirow{2}{*}{**Reference**} \\  & & (\({}^{\circ}\)C) & (J/g) & \\ \hline
55\% CaCl\({}_{2}\cdot\)6H\({}_{2}\)O + 45\% CaBr\({}_{2}\cdot\)6H\({}_{2}\)O & 14.7 & 140 & [1] \\
75\% CaCl\({}_{2}\cdot\)6H\({}_{2}\)O + 25\% MgCl\({}_{2}\cdot\)6H\({}_{2}\)O & 21.4 & 102.3 & [16] \\
66\% CaCl\({}_{2}\cdot\)6H\({}_{2}\)O + 33.3\% MgCl\({}_{2}\cdot\)6H\({}_{2}\)O & 25 & 127 & [1] \\
40\% Na\({}_{2}\)CO\({}_{3}\cdot\)10H\({}_{2}\)O + 60\% Na\({}_{2}\)HPO\({}_{4}\cdot\)12H\({}_{2}\)O & 27.3 & 220.2 & [17] \\
47\% Ca({}_{2}\)(NO\({}_{3}\cdot\)10H\({}_{2}\)O + 33\% Mg(NO_{3})\({}_{2}\cdot\)10H\({}_{2}\)O & 30 & 136 & [1] \\
25\% Na\({}_{2}\)SO\({}_{4}\cdot\)10H\({}_{2}\)O + 75\% Na\({}_{2}\)HPO\({}_{4}\cdot\)12H\({}_{2}\)O & 31.2 & 262.3 & [18] \\
58.7\% Mg(NO_{3})\({}_{2}\cdot\)6H\({}_{2}\)O + 41.3\% MgCl\({}_{2}\cdot\)6H\({}_{2}\)O & 59 & 132.2 & [1,13] \\
50\% Mg(NO_{3})\({}_{2}\cdot\)6H\({}_{2}\)O + 50\% MgCl\({}_{2}\cdot\)6H\({}_{2}\)O & 58–59 & 132 & [1] \\
80\% Mg(NO_{3})\({}_{2}\cdot\)6H\({}_{2}\)O + 20\% MgCl\({}_{2}\cdot\)9H\({}_{2}\)O & 60 & 150 & [1] \\
53\% Mg(NO_{3})\({}_{2}\cdot\)6H\({}_{2}\)O + 47\% Al(NO_{3})\({}_{2}\cdot\)9H\({}_{2}\)O & 66 & 168 & [13] \\
14\% LiNO\({}_{3}\cdot\)3H\({}_{2}\)O + 86\% Mg(NO_{3})\({}_{2}\cdot\)6H\({}_{2}\)O & 72 & 180 & [1] \\ \hline \hline \end{tabular}
\end{table}