# Demonstration of Phase Change Thermal Energy Storage in Zinc Oxide Microencapsulated Sodium Nitrate
## 4 Discussion  
PCM microencapsulation aims to enhance the properties of materials for thermal energy storage and holds potential to provide effective solutions for applications in heating systems (Huang 2019) (Huang 2019). Microencapsulation of NaNO\({}_{3}\) in ZnO shell for high temperature thermal energy storage may reduce the investment cost of using high-grade stainless steel ([PERSON] et al., 2014) ([PERSON] et al., 2014) to contain the highly corrosive salt ([PERSON] et al., 2018) ([PERSON] et al., 2018). In [PERSON] et al., 2018 ([PERSON] et al., 2018), a solvothermal process was used to produce nanometer-thick zinc oxide shells around potassium nitrate cores, obtaining microcapsules of few microns size. In the present paper, preparation of larger size (100 to 300 \(\upmu\)m) microparticles is reported, each encapsulated in a layer of ZnO nanoparticles. Due to changes in the microencapsulation protocol (solvothermal synthesis was eliminated), the ZnO envelopes reported here typically show highly variable thickness, ranging from few nanometers to few microns. Initially uneven, the shell ZnO layer of microcapsules seems to uniformly distribute upon melting-solidification, and thus resists multiple charging-discharging cycles. In this view of a potential for improvements in ZnO deposition, a 4:1 NaNO\({}_{3}\) : ZnO mass ratio may be lower than optimal, where optimization means that for complete covering of the microparticle, the ratio should maximize the PCM mass on the expense of shell thickness. Despite this suboptimal PCM:shell ratio, our novel simplified protocol for microencapsulation may minimize the initial investments in specialized equipment for manufacturing. The observed thermal stability of microcapsules reported here may be explained by the fact that melting-solidification of PCM is able to select, after few cycles, structurally stable microcapsules. With the leaking PCM, compaction of the storage material at macroscale may occur, increasing the thermal energy density in storage cycles. The temperature dependency for density, thermal conductivity, and specific heat of pure NaNO\({}_{3}\) is available from multiple sources ([PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2017) ([PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2017). Therefore, it was possible to compare our data (Figure 4B) in the same graph with results reported by [PERSON] et al., 2018 ([PERSON] et al., 2018) for NaNO\({}_{3}\) and for a mixture of diatomic: NaNO\({}_{3}\) at the same weight ratio as our NaNO\({}_{3}\):ZnO (4:1). From Figure 4A, which shows the volumetric specific heat, it is possible to calculate the specific heat (gravimetric) of the material by considering the temperature dependency of the material density. Since the density for the microcapsule-based material was not  
Figure 9: FEM simulations of T4 (red segmented line) in the self-discharge experiment, compared with experimental data (filled circles) for T1 (blue), T2 (gray), and T4 (yellow): (**A**) 19.23 kg microcapsules load and (**B**): 22.07 kg load. measured for high temperatures, a temperature variation similar to that described for NaNO\({}_{3}\)[21] was assumed. In the temperature interval from 50 to 284 \({}^{\circ}\)C, a value between 300 and 1020 J/kg\(\cdot\)K resulted from the calculations of the specific heat of microcapsules; particularly important at 284 \({}^{\circ}\)C, where the specific heat for microcapsules is 801 J/kg\(\cdot\)K. FEM simulations (Figure 9) validated 750 J/kg\(\cdot\)K as the only value of the specific heat that allows consistent overlap below the solidification T\({}_{\text{offset}}\) with the experimental T4 slope (for both tank loads). For Equation (3) in the adapted calorimetric method, the 824 J/kg\(\cdot\)K value (measured by DSC) was used; this is near 801 J/kg\(\cdot\)K value deduced from volumetric specific heat, and close to the 750 J/kg\(\cdot\)K value from FEM validation.