# Demonstration of Phase Change Thermal Energy Storage in Zinc Oxide Microencapsulated Sodium Nitrate
## 3 Results
### Temperature-Dependent Thermal Properties of NaNO\({}_{3}\)-ZnO Microcapsules  
Latent heats of NaNO\({}_{3}\) solid-liquid transitions were determined by DSC experiments in microcapsules and pure salt (Figure 3). For microcapsules, the melting heat was 102.564 kJ/kg and the solidification heat was 94.5 kJ/kg. In addition, the specific heat at 284.8 \({}^{\circ}\)C was 0.824 kJ/kg. Pure NaNO\({}_{3}\) required 124.5 kJ/kg for solidification. For microcapsules, the temperature intervals for melting and solidification are [t\({}_{\text{onset}}\), t\({}_{\text{offset}}\)] = [303, 319] \({}^{\circ}\)C and [302.5, 285] \({}^{\circ}\)C, respectively (Figure 3A). This does not significantly differ from pure salt, (Figure 3B--our data). Thus, in the DSC experiments at 10 \({}^{\circ}\)C/min cooling rate, supercooling tends to be less pronounced in microcapsules, and clearly reduced compared with similar experiments on pure NaNO\({}_{3}\), as reported by [PERSON] in his Ph.D. thesis, Table 7.2.2.1. [5]. The higher solidification heat (modulus) reported there (167 kJ/kg) for NaNO\({}_{3}\) is partly related to the fact that our measurements were done for repeated melting-solidification cycles, with latent heat in the first-cycle consistently higher than the rest; converging to the reported value). The onset and offset temperatures (solidification) were used in simulations and in calorimetric calculations. In FEM simulations, the normalized DSC curve profile and solidification latent heat were principal determinants of the T4 temperature evolution, whereas calorimetric calculations used onset and offset temperatures from DSC to precisely identify the interval for latent heat contribution to the experimental T4 curve.  
Figure 4 presents the specific heat and the thermal conductivity of microcapsules in comparison with pure NaNO\({}_{3}\) (own experiment and from [PERSON] et al., 2013 [21]). In the  
\begin{table}
\begin{tabular}{c c c c} \hline
**NaNO\({}_{3}\):ZnO (Mass Ratio)** & **3.4:1** \(\div\) **4:1** & **4:1** \(\div\) **8:1** & **above 60:1** \\ \hline Count (no. of spots) & 9 & 3 & 2 \\ \hline \end{tabular}
\end{table}
Table 1: Analysis of microparticle composition stability by NaNO\({}_{3}\)–ZnO mass ratio after multiple phase-change thermal cycles.  
Figure 3: (**A**) DSC heating - cooling curves for NaNO\({}_{3}\)–ZnO microcapsules. (**B**) Comparison of DSC solidification curves (10 \({}^{\circ}\)C /min cooling rate) for pure NaNO\({}_{3}\) (Mohammad [5] and own data) and microcapsules.  
temperature interval from 303 to 285 \({}^{\circ}\)C the Hot Disk values are not shown (data from DSC were used in calculations). In the temperature interval 30-270 \({}^{\circ}\)C, the thermal conductivity of the microcapsules is around 0.3 W/m\(\cdot\)K. At 280 \({}^{\circ}\)C, a minor decrease in thermal conductivity was noticed, probably associated with phase transition for NaNO\({}_{3}\) crystalline structure. Above 300 \({}^{\circ}\)C, a sudden increase in the thermal conductivity was measured, correlating with PCM melting. The room temperature density of the microcapsule-based storage material after several melting-solidification cycles was calculated, considering the density decrease when increasing temperature, as reported for NaNO\({}_{3}\)[21]. A value of 1.8 t/m\({}^{3}\) was inferred and used for FEM simulations (at temperatures just below the liquid-solid transition temperature).  
Figure 4: Thermal properties of NaNO\({}_{3}\)–ZnO microcapsules in comparison with pure NaNO\({}_{3}\). Hot Disk determinations (own data) of volumetric specific heat (**A**) and thermal conductivity (**C**), in comparison with other works. (**B**) Specific heat of pure sodium nitrate and of a composite mixture of NaNO\({}_{3}\): diatomite (same weight ratio as microcapsules). External data are from [PERSON] et al., 2013 [21] and [PERSON] et al., 2017 [4].