Multi-form heat storage performance of expanded graphite based CaCl2 composites for low-grade heat source

Abstract

Hydrated salt CaCl2 is a promising material for low-grade thermochemical heat storage. To improve cyclic stability of the material, porous matrixes are usually employed to synthesize​ CaCl2/matrix composites. However, the multi-step reaction of the salt and multi-form sorption of the composites increase the complexity during the practical application. This study proposed a novel method to study the multi-form heat storage performance. By combining TG/DSC measurement with Peak-fitting analysis, the multi-form mechanism of hydration/dehydration is revealed quantitatively. A series of EG/CaCl2 composites are synthesized with different salt content in the range of 23.8 wt% ∼57.8 wt%. All of the composites show well endothermic dehydration performance below 130 °C. A highest heat storage density of 1637.6 kJ/kg was obtained with a salt content of 48.1 wt% and a water uptake of 0.79 g/g. Water uptake of EG/CaCl2 composites is composed of chemisorption water loaded through reversible hydration reactions and solution absorption water introduced from salt deliquescence. The solution absorption plays nearly the equitant important role as chemical sorption, both in water adsorption ability and thermal storage capacity. The EG/CaCl2 composite show great support capacity for both salt and solution, making it an interesting material for low-grade heat storage.

Keywords

• Expanded graphite
• Calcium chloride
• Composite
• Multi-form sorption
• Thermal energy storage