Synergistic enhancement for spectral absorption and thermochemical heat storage of hydrophobic layer assisting-casted CaO-based pellets

Thermochemical energy storage (TCES) is essential for high-temperature energy storage in 3rd-generation concentrating solar power plants. CaO-based materials are promising candidates for such systems due to their high theoretical energy storage density, good high-temperature resistance and low raw material prices. However, CaO-based materials are prone to high-temperature sintering and have poor solar absorptance, restricting their capability to harvest solar heat directly. Herein, hydrophobic layer-assisted casting approach was employed for preparing CaO-based pellets wrapped by a light-absorbing shell. The synergistic optimization of outer shell light-absorbing components and internal Al-based stabilizers contributes to improved optical absorption, cyclic heat storage capacity and mechanical durability of the pellets. The MnFe 2 O 4 /graphite hydrophobic layer outperforms Fe 3 O 4 /graphite and MnO 2 /graphite counterparts in improving the optical absorptance and shell structural strength of CaO-based TCES pellets. A MnFe 2 O 4 /graphite volume ratio of 1:2 yields an average shell optical absorptance of 85.3% and a 2.6% shedding rate after 20 min of testing. Furthermore, the Al-based inert stabilizers effectively enhance the mechanical strength (4.28 MPa) of the modified pellets. After 29 cycles, the pellets achieve an energy storage density of ∼1.13 MJ/kg, representing a 4.4% improvement over the first cycle. This improvement derives from the uniformly distributed inert framework, which inhibits CaO grain sintering/agglomeration, retains porosity, and ensures sufficient gas-solid interfaces and mass transfer pathways.