Compressed reactive unit of CaCO3 and Co3O4 with improved solar absorption and energy density for thermochemical energy storage

Thermochemical energy storage (TCES) under direct irradiation mode is a promising technology that promotes the widespread utilization of high-temperature concentrated solar energy. However, poor cyclic stability and solar absorption for calcium carbonates, high cost for cobalt oxides, and safety issues for powder-form materials hinder the practical applications of the TCES systems. This work innovatively proposed a compressed reactive TCES unit composed of 10%-volume of modified Co 3 O 4 in the center and 90%-volume of modified CaCO 3 around the center. The modified calcium-based compacted material (CaCO 3 M-p) exhibits 20.5°C lower onset temperature, 13.9 kJ mol −1 lower activation energy, and faster kinetics than the commercial CaCO 3 powder (CaCO 3 -u) in the energy storage process. Both the cyclic stability in 15 cycles and solar absorption of the CaCO 3 M-p are boosted, which are 1.25 times and 18.3 times those of the CaCO 3 -u, respectively. While compaction reduces the reactive performances by narrowing the porous, the Mn/Mg co-modification has a more significant effect in increasing the above properties by creating lattice defects and introducing impurities. The modified cobalt-based compacted material (Co 3 O 4 M2-p) presents similar reactive and optical properties as raw Co 3 O 4 powder, yet it elevates the average conversion rate in 30 cycles to 1.75 times that of compressed raw Co 3 O 4 . Modification by microcrystalline cellulose is the key to this improvement, as it reduces activation energy and promotes decomposition kinetics for compacted Co-based materials. The compressed reactive TCES unit demonstrates outstanding energy storage density and power. The 10%-volume Co 3 O 4 improves external radiation absorption and reinforces internal heat transfer, enhancing the unit’s energy storage rate. Moreover, Co 3 O 4 also contributes to energy density, raising the total density to 0.4217 MWh m −3 , exceeding the 0.2468 MWh m −3 limit of CaCO 3 powder. The compacted materials and composite unit advance the practical use of TCES in solar energy storage.