Preparation of dark Fe/Mn/Zr-doped CaO-based heat carriers for solar-driven thermochemical energy storage

Thermochemical energy storage technology exhibits great potential due to its high efficiency, low-cost and widespread availability. Efficient solar energy storage necessitates both a high energy storage performance and optimal direct solar absorption for enhanced performance. However, the density decay due to sintering and weak solar absorption because of light color of CaO-based heat carriers significantly restrict the application of CaLbased energy storage technology. This study aims to synthesize CaO-based heat carriers possessing both high energy storage density and strong solar absorption. Fe/Mn ions and Zr-based supports were individually and simultaneously doped to enhance the cyclic energy storage performance of CaCO3/CaO materials more effectively. The Fe/Mn/Zr triple-doped material (Ca100Fe12Mn6Zr10) exhibited a substantial enhancement in average spectral absorption (reaching 51.42 %), an increase of nearly 5.5 times compared to pure CaO (9.33 %). This enhancement was attributed to the formation of dark Ca2Fe2O5, Ca2MnO4, and Ca4Mn3O10 compounds. Additionally, the formation of CaZrO3, Ca2MnO4, and Ca4Mn3O10, characterized by notable sintering resistance, served as a thermal stabilizer to enhance the cyclic stability. Consequently, the triple-doped CaO-based heat carrier achieved a high density of 1549.04 kJ/kg in the 10th cycle, retaining 97.24 % of its initial value.