Enhancing solar absorption and reversibility in calcium looping-based energy storage via salt-promoted CaO

Calcium looping (CaL)-based solar to thermochemical energy storage is a promising option for long-term thermal energy storage in concentrated solar power generation. CaL is a chemical looping process involving reversible carbonation-calcination reactions among CaO, CO2, and CaCO3, which has distinct advantages, such as high energy storage density at high operating temperatures. However, the low optical absorption of CaO limits its application as a direct solar absorbing material, and the continuous reduction of reactivity through CaL reactions degrades the energy storage ability. In this work, we investigated the multi-doping of transition metals Fe, Co, Ni, and CaCl2 to improve optical absorption and promote the carbonation reactivity of synthetic CaO-based materials. The synthetic CaO materials were synthesized by acetic acid-based wet mixing and sol-gel method. Proposed materials exhibited significantly enhanced optical absorption up to 88%, preserved within 78 - 85% throughout 45 cycles of CaL reactions. The production and regeneration of Ca2FeO3Cl are expected to enhance optical absorption. The CaL reaction analysis revealed better carbonation conversion performance holding with cyclic stability, achieving 1.94 MJ/kg for 50 cycles or 1.17 MJ/kg for 100 cycles. The reaction mechanism was investigated using in-situ high-temperature XRD, and a salt-promoted carbonation mechanism is proposed, which consists of Ca2FeO3Cl carbonation and carbonation in a molten salt phase, improving the mass transfer of Ca2+O2- to adsorbed CO2. Based on the findings, we suggest the synergistic use of the CaCl2 multi-doping method for synthetic CaO designs.