# Demonstration of Phase Change Thermal Energy Storage in Zinc Oxide Microencapsulated Sodium Nitrate
The thermal properties of the storage material may be combined with appropriate design of the storage tank using in silico experiments of computational fluid dynamics (CFD) ([PERSON] et al., 2013) [11]. The finite element method (FEM) with common software packages (Comsol Multiphysics) was used to compare various inorganic salt PCMs and different suitable PCM-encapsulation materials to render economically viable solutions that would meet the requirements of thermal energy storage (TES) for CSP ([PERSON] et al., 2017) [9]. Micro and nanoencapsulation has been frequently reported for low temperature regime PCMs, mainly using organic polymers as shell materials. For high temperature applications (e.g., CSP), manufacturing micrometer-scale shells that are stable over many PCM melting-solidification cycles is challenging for at least two reasons: (a) coatings made by the versatile organic polymer technology do not withstand high temperatures; and (b) water-based chemistry to prepare shells may dissolve the PCM core, which is typically an inorganic salt/salt mixture [12]. SiO\({}_{2}\) coating of micrometer-size alkaline nitrate cores was recently demonstrated by several groups ([PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] and [PERSON], 2020) [12; 13; 14]. A method for microencapsulation of nitrate salts in zinc oxide shells was developed within a project funded by the National Authority for Scientific Research in Romania (project acronym ENERHIGH), as described in our previous papers ([PERSON] et al., 2019; [PERSON] et al., 2018) [15; 16]. Briefly, in that work, KNO\({}_{3}\)-ZnO microcapsules were prepared from raw materials (zinc nitrate tetrahydrate and potassium nitrate) by the solvothermal method. Spherical morphology of the ZnO-covered KNO\({}_{3}\) microparticles was observed after spray drying, with particle sizes of only a few microns. The aim of this work is to evaluate the capacity of NaNO\({}_{3}\)-ZnO microcapsules to store thermal energy and the possibility to retrieve it at high temperatures. To improve the scalability of microcapsule fabrication, a novel protocol for microencapsulation was elaborated and tested, consisting in direct ZnO coating of NaNO\({}_{3}\) particles using commercially available NaNO\({}_{3}\) grains (relatively large, up to 0.5 mm size) and ZnO powder (0.1 to 2 \(\upmu\)m particles) as raw materials. Up to 22.1 kg of NaNO\({}_{3}\)-ZnO microcapsules were thus prepared. Results from experiments in a 16.7 L cylinder-shaped prototype tank are presented, which show that microcapsules store significant amounts of thermal energy in solid-liquid NaNO\({}_{3}\) transitions. To facilitate further development of novel PCM materials for energy storage, a platform for combined in silico prototype-scale investigation was built. For a simplified tank FEM model, thermal transfer is presented in simulations of tank discharge and compared with real temperature data from prototype tank experimentson microcapsules. The goal is to understand and predict material performance in thermal storage applications, starting from material characterization by laboratory methods.