# Demonstration of Phase Change Thermal Energy Storage in Zinc Oxide Microencapsulated Sodium Nitrate
## 2 Materials and Methods  
**Chemicals**. Analytical grade reagents were used in all experiments. Sodium nitrate was purchased from Roth, Germany. Zinc oxide was provided by Chimreactiv, Romania. **Microcaps**. NaNO\({}_{3}\) was encapsulated in ZnO shells at 4:1 mass ratio, using a simplified protocol derived from our previous work ([PERSON] et al., 2018) [16]. Basically, commercially available NaNO\({}_{3}\) powder grains, ZnO, and ethanol were mixed and stirred to achieve homogenous dispersion of components. The mixture was then subjected to a thermal treatment for 1 h at 200 \({}^{\circ}\)C in atmospheric air. Microcapsules were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy to determine their structure and demonstrate encapsulation. **Chemical analysis** of the thermal energy storage material was determined by flame atomic absorption spectrometry (ISO 11047:1999) for Na and Zn and wet chemistry (ISO 5664: 2001) for NO\({}_{3}\)- Samples were taken from the tank at 14 different positions in the storage material. **Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS)** were performed on microcapsules using an SEM-EDS Quanta 250 scanning electron microscope (FEI Company, Eindhoven, The Netherlands) equipped with the Element Silicon Drift Detector Fixed and Element EDS Analysis Software Suite APEX(tm) 1.0, EDAX, Mahwah, NJ, USA. Thus, by EDS, the atomic composition of a selected spot was determined. In the report from the instrument, the abbreviations C K, O K, ZnL, NK, NaK, and AIK are used to indicate that the data are based on detection of energies for specific orbital transitions, corresponding to elements: C, O, Zn, N, Na, and Al, respectively. The elemental composition of each spot is reported as relative weight (%), atom concentration (%), or the net intensity (a.u.) of the respective element-specific peak. For microcapsules, the carbon % reported in analysis is background from the sample immobilization device, and was subtracted from further calculations. **Thermal analysis**. Thermal conductivity and thermal diffusivity/specific volumetric heat determinations were made by the transient plane source method (TPS-2200 from Hot Disk, Sweden, model 5082 mica sensor). In laboratory determinations, microcapsule powder was poured to fill a 200 mL stainless steel beaker, the sensor was embedded in the middle, the assembly was placed in the empty (air-filled) storage tank, then exposed to multiple temperature cycles in the 25 to 350 \({}^{\circ}\)C temperature interval. The thermal conductivity and specific volumetric heat curves were recorded only after the second cycle of microcapsule melting-solidification, when the microcapsule material was a compact porous material block; thus, the thermal parameters are representative for the material formed after multiple energy storage cycles. Temperature monitoring was achieved by tank thermocouples and corrected to report in-place temperatures using calibrated readings for the sensor resistivity. Differential Scanning Calorimetry (DSC) was performed using two instruments: SET-SYS Evolution from Setaram and DSC F3 Maia from Netzsch, Germany. Microgram amounts of NaNO\({}_{3}\) and microcapsules were heated-cooled in cycles to determine melting and crystallization temperatures and enthalpies. **Prototype tank**. The thermal storage tank is a right circular cylinder (referred to as the tank cylinder), 0.55 m length, 0.2 m diameter, made of 2 mm-thick stainless steel (SS 316), accommodating temperature sensors in inbuilt sheaths protruding transversally up to the longitudinal axis. A thermal conductivity sensor is located in a holder at the center-bottom of the tank. The cylinder is wrapped in 0.2 m thick Ravaber mineral foam insulation.