Development and characterization of Ca(OH)₂ doped with expanded graphite for thermochemical energy storage: Integrating thermal conductivity, kinetics, and cyclic performance

Abstract

In light of the widespread adoption of novel energy sources, there is a pressing need for the rapid development of long-term energy storage technologies. While most existing energy storage technologies are prohibitively expensive, the thermochemical energy storage (TCES) system based on CaO/Ca(OH)₂ offers a high storage density of 1400 kJ/kg, low costs, and excellent cycle stability. The choice of expanded graphite (EG) as the matrix is based on its outstanding thermal conductivity and rich microstructure, which enhance heat and mass transfer. This study focuses on the thermochemical properties of Ca(OH)₂ doped by expanded graphite with stable morphology. Five samples with EG contents were synthesized, and characterized with regard to their microstructures, and their thermal and physical properties examined. Additionally, a reaction kinetic analysis of the thermal decomposition process was conducted. The results indicate that the composite with 9 % EG content stands out as the optimal thermal storage material, exhibiting a thermal conductivity of 2.5 W/(m·K), almost three times that of pure material. The activation energy of the composite was calculated to be 166.9 kJ/mol and the pre-exponential factor is 5.5 × 10¹¹ 1/s which is two orders of magnitude higher than the pure. Furthermore, the composite demonstrated exceptional stability during cyclic performance tests, maintaining a conversion rate of 70 % after 10 cycles, confirming its reliability. In conclusion, this research presents a novel approach for the large-scale, low-cost application of TCES in CaO/Ca(OH)₂ systems.