Effects of the characteristics of silica microcapsules with nanoholes on the reaction rate of calcium chloride for chemical heat pump

Abstract

An optimized-double emulsion method for preparing hollow silica microcapsules with nanoholes was developed for encapsulating calcium chloride to enable its long-term use in chemical heat pumps. The hydration/dehydration of calcium chloride was achieved by permeating water vapour through the surface holes, allowing the thermal upgrading and storage of low-temperature waste heat. The effects of the solution mass and surfactant concentration during the fabrication process on the texture of the microcapsules and thermal properties of the composites were investigated. The results demonstrated the successful encapsulation of calcium chloride in the microcapsules to occupy the entire void and complete hydration/dehydration reaction. Changing the fabrication conditions affected the diameter, shell thickness and opening fraction of microcapsules, thereby further controlling the void fraction and hole specific surface area, which were proven to be key parameters for improving heat storage density and hydration output power density. Composite microcapsules with a heat storage density of 809.1 J·g⁻¹ at a void fraction of 54.7 vol% and a power density of 0.95 W·g⁻¹ at a hole specific surface area of 0.089 μm^-1 and satisfactory thermal stability after 50 cycles were obtained.