Hydrophobic interface-assisted casting of core–shell CaO-based pellets for thermochemical energy storage: Assembly mechanism for dark Mn-based shell

Calcium Looping (CaL) is a noteworthy high-temperature thermochemical energy storage technology applied in the Concentrated Solar Power (CSP), given its considerable theoretical heat storage density and the low-cost raw materials. The inherent poor solar absorptance to CaO-based materials, however, limit their use in volumetric collectors that operate in direct solar-driven heat storage mode. This research implements a collaborative strategy of external Mn-doping and internal Al-loading to fabricate CaO-based composite pellets using a hydrophobic-assisted casting method. The fabricated pellets, featuring a “black exterior, white interior” design, achieve both good light absorption and stable heat storage/release capabilities. The findings reveal that incorporating a hydrophobic layer with 60 wt% MnO 2 is advantageous for preserving the structural integrity of the Mn-based outer shell and for the elevation of its optical absorption capability. Therefore, the solar absorbance of the proposed Al-stabilized CaO-based composite pellets with a dark Mn-based shell is 86.39 %, and their compressive strength is 9.38 MPa, which is notably higher than the shell-free pure CaO-based pellets (19.36 %, 0.6 MPa). Moreover, after 40 cycles, the energy storage density of the preferred CaO-based composite pellets has increased by 44.3 % compared to the initial cycle, achieving 1.07 MJ/kg. The desirable “self-reactivation” phenomenon is primarily attributed to the synergistic co-evolution of the outward soft skeleton and the inward hard skeleton within the CaO-based composite pellets, which helps maintain a stable pore structure and accelerates the carbonation rate during high-temperature cycles.