Simultaneous impregnation and microencapsulation of CaCl₂ using silica gel and methyl cellulose for thermal energy...

Thermal energy storage utilizing the adsorption of moisture from air is a promising energy storage technology due to its high energy density and minimum heat losses. Salt hydrates and salt hydrate composites, such as calcium chloride (CaCl₂) and CaCl₂-based composites, have shown favourable energy storage properties in this area of research. However, these materials have shown issues with stability due to swelling and deliquescence. In this work, CaCl₂ was stabilized using three methods: impregnation into silica gel, encapsulation in methyl cellulose, and both impregnation and encapsulation stabilization techniques used simultaneously. Therefore, three CaCl₂-based composites were synthesized. For the first composite, silica gel was impregnated with CaCl₂. For the second composite, CaCl₂ was encapsulated by methyl cellulose. For the third composite, silica gel was impregnated with CaCl₂ and the CaCl₂ was encapsulated with methyl cellulose. These samples were structurally characterized using scanning electron microscopy as well as Brunauer-Emmett-Teller (BET) to  determine surface area, pore size distribution and nitrogen adsorption isotherms at 77 K. Water vapour adsorption isotherms were also determined at 25 °C for different relative humidities by dynamic vapor sorption (DVS). Similarly, LiCl-based composites were also synthesized and examined in this work, but issues of deliquescence, swelling, and agglomeration made the materials impractical to work with. To determine the prepared materials' thermal energy storage performance, 2-6 g of each sample was tested in a lab-scale apparatus. This process uses the exothermic adsorption of moisture from ambient air in an open thermal energy storage system. The CaCl₂ impregnated silica gel that was encapsulated in methyl cellulose showed reasonably high stability and energy storage performance after 3 hydration and dehydration cycles with minimum agglomeration. An energy storage density of 241 kWh/m³ (0.87 GJ/m³) and specific energy of 630 Wh/kg (2268 kJ/kg) was achieved with this material for 90% inlet relative humidity after a regeneration at 90 °C.