Calorimetric examination of suitability of calcium, cobalt and nickel nitrate hydrates for thermal energy storage

Abstract

Selected nitrate hydrates were characterized using X-ray diffraction, helium pycnometer, Calvet-type calorimeter and simultaneous thermogravimetry coupled with differential scanning calorimeter. Melting and crystallization of all studied nitrate hydrates were observed by differential scanning calorimeter, and the supercooling (difference between melting temperature and crystallization temperature) was found to be more than 80, 14 and 37 °C for calcium nitrate tetrahydrate, cobalt and nickel nitrate hexahydrates, respectively. Those salts were mixed with selected nucleating agents to suppress supercooling and then tested under repeating of heating–cooling cycles. Calcium nitrate tetrahydrate showed reversible liquid–solid process with addition of CaO, Ca(OH)2, BaO, Ba(OH)2·8H2O, graphite and graphene, but the supercooling remains too high, and the enthalpy change significantly decreased. In the case of cobalt nitrate hexahydrate, the CaO, Ba(OH)2·8H2O and graphite are the most effective nucleating agent in amount of 1 mass%. The supercooling of nickel nitrate hexahydrate was effectively reduced by addition of CaO, Sr(OH)2 and graphite. The cobalt and nickel nitrates were tested with combination of two nucleating agents (1:1). The best results were obtained for composites with addition of 1 mass% of nucleating agent mixture consisting of both graphite and Ba(OH)2·8H2O for cobalt nitrate hexahydrate, and CaO with Mg(OH)2 for nickel nitrate hexahydrate.