Decorating CaO with dark Ca2MnO4 for direct solar thermal conversion and stable thermochemical energy storage

Calcium looping (CaL) process, a thermochemical energy storage technique, probably offers a mid-term if not near-term solution to the next generation Concentrated Solar Power (CSP) plants featured with high temperature over 700 degrees C. However, CaCO3 used in the conventional CaL-CSP system is usually white with little absorption of sun light, hindering its application in efficient volumetric solar energy conversion scheme. This work decorates CaO with dark Ca2MnO4 derived from natural manganocalcite, achieving direct solar absorption and simulta-neously enhanced anti-sintering ability due to the presence of dark Ca2MnO4. As a thermal stabilizer, Ca2MnO4 minimized the sintering-induced decay of the materials' energy storage density and ensured high and stable energy storage performance over prolonged operation cycles. Detailed electron microscopy-based analysis confirmed the compositional homogeneity which is identified, together with the porous structures, as the crucial feature to guarantee a stable performance. Over 20 cycles, CaO/Ca2MnO4 nanoparticles still remained energy storage density of 1932 kJ/kg, exceeding that of the limestone-derived reference material by nearly 100%. Additionally, the average solar absorptance of Ca2MnO4-stabilized calcium composite is remarkably enhanced in comparison with that of conventional limestone. This work paves the way for designing materials with high energy storage density and excellent cyclic stability for simultaneous solar thermal conversion and high -temperature thermochemical energy storage.

Automated summary

Calcium looping (CaL) process offers a possible solution for next generation Concentrated Solar Power (CSP) plants. However, the conventional CaL-CSP system has limited solar absorption capability due to the white nature of CaCO3. This study introduces dark Ca2MnO4 derived from natural manganocalcite to enhance solar absorption and anti-sintering ability of CaO. The presence of Ca2MnO4 stabilizes the materials' energy storage density and ensures high and stable energy storage performance over prolonged operation cycles.