A review of state-of-the-art in solid-gas thermochemical energy storage technologies for solar energy applications

Thermochemical energy storage systems (TCES) demonstrate a strong capacity to play an essential role in the transition to sustainable energy systems. Among these, solid-gas-based solutions have shown promise, offering high energy density, long storage capacity, and integration with almost any energy supply or demanding systems. Integration of solar systems with such systems is a favorable approach to addressing the challenges associated with solar energy intermittency, standby losses, scalability, and long (even very long) heat storage. This article focuses on solid-gas TCES technologies that integrate with solar energy for industrial applications, particularly in sectors requiring medium-to high-temperature heat, such as power generation, chemical processing, and carbon capture. The review covers any investigated or implemented solutions in this domain, including direct or indirect integration of solar energy, a variety of reactor and reactive material types, and reaction mechanisms. Recent advances in this area are highlighted, and research gaps and major issues with these systems are thoroughly discussed. The key conclusions include: i) solar TCES systems using materials such as Ca(OH)₂, SrBr₂, and metal oxides have reached a fair maturity; ii) material cyclability, heat and mass transfer, and system costs are major challenges to be addressed; iii) extensive research is still needed on developing composite materials with enhanced stability; iv) techno-economic analyses supported by detailed dynamic simulations giving insights into state transitions and off-design operation for such systems with a broader range of materials and at large scales can be great add-on to the literature; and last but not the least, v) pilot-scale demonstration of such integrations beyond laboratory scales and conditions are critically needed to push these forward in innovation and applied impact.