Preparation of microencapsulated KNO3 by solvothermal technology for thermal energy storage

In this study, ZnO has been used as inorganic shell material (in situ synthetized) for the encapsulation of KNO3, an inorganic molten salt commonly used in concentrated solar plants applications. The thermal stability of microparticles encapsulated by using a solvothermal process has been optimized by adjusting the parameters affecting the properties of the microparticles, such as the core:shell ratio and the temperature during the microencapsulation process. The energy stored and released after each thermal cycle was evaluated by differential scanning calorimetry. Chemical composition of microparticles was evaluated by infrared spectroscopy and inductively coupled plasma spectroscopy, as well as morphology was characterized by scanning electron microscopy. Results have shown the solvothermal synthesis as a feasible process for the microencapsulation of molten salts by obtaining KNO3 particles covered by ZnO microcrystals. These particles have thermal energy storage and release capacities and temperatures similar to those of raw KNO3, being the temperature used during the solvothermal process the parameter determining the thermal stability of the microparticles, as demonstrated by carrying out durability tests through consecutive heating-cooling thermal cycles (250-400 degrees C).