The attempts of the CaO/CaCO3 TCES for CSP from materials to gas-solid reactors: A review

Thermochemical energy storage (TCES) based on calcium looping (CaL) is promising for high-temperature concentrating solar power (CSP) because of its high energy-storage density, low material cost, abundant precursors, and compatibility with efficient power cycles. Among Ca-based reaction pairs, the reversible CaCO 3 /CaO system is particularly attractive, however, its practical deployment is restricted by sintering, pore closure, product-layer diffusion, particle degradation, insufficient cyclic stability, and inadequate matching between material properties and reactor requirements. Previous reviews have generally addressed reaction pairs, material modification, or reactor concepts separately, leaving the connection between material evolution, reactor development, and industrial implementation insufficiently resolved. This review therefore provides an integrated assessment of CaO/CaCO 3 TCES from materials to reactors. Material degradation and particle engineering are examined alongside four modification routes: inert stabilizers, multivalent-metal and defect regulation, alkali-salt promotion, and coupled multifunctional formulations. Their effects on reaction kinetics, cyclic durability, energy density, heat transfer, solar absorptance, and mechanical stability are evaluated against active-CaO dilution, secondary-phase formation, promoter migration, and shaping losses. Fluidized bed, fixed bed, moving bed, rotary kiln, and related particle-flow reactors are subsequently compared in terms of operating conditions, gas-solid contact, heat and mass transfer, solids circulation, efficiency, and scale-up limitations. The analysis shows that no material or reactor provides a universal solution; composition, particle architecture, and shaping must be selected according to reactor hydrodynamics, thermal load, residence time, and mechanical requirements. A potential material-reactor co-design framework is proposed to guide the transition from powder-scale evaluation to continuous, scalable, and industrially relevant CaL-CSP systems.