Calcium-based composites for direct solar-thermal conversion and thermochemical energy storage

Calcium-Looping (CaL) is considered as a promising process for thermochemical energy storage in the 3rd generation Concentrated Solar Power plants using a supercritical carbon dioxide power cycle. Here we propose, for the first time, a novel strategy to directly absorb solar energy using calcium-based composite thermochemical energy storage (TCES) materials. The main novelty lies in the binary metallic element doping of the calcium-based raw materials to enhance their direct interactions with solar radiation photons for light capturing. In particular, the use of the metallic element doping is to form high light-absorbing material for addressing the unfavorable intrinsic optical absorption properties of the calcium carbonate for photon management. A simple and facile sol-gel method was used to synthesize the CaO composites doped with Mn and Fe. Such binary metallic element doping into the calcium-based materials not only boosted the solar absorption but also improved the cycling stability of the TCES material in the integrated CaL-CSP system. The synthetic composites were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The effects of thermochemical cycling on the microstructures and properties of TCES material were investigated. Furthermore, the underlying mechanisms were discussed. The results indicated that the enhancement of the optical absorption for the composites originated from metal oxides of Fe and Mn with high light-absorbing ability. The synergistic effects of small grain size and the reinforced skeleton structures could impede agglomeration and collapse of the composites, thereby improving the cycling stability.