| Abstract |
The integration of the Ca-looping (CaL) process in concentrated solar power systems (CSP) has been identified as an interesting approach for thermochemical energy storage (TCES) systems, since the TCES material regeneration can use clean and unlimited solar energy. The CaL cyclic stability of CaCO3 is an important parameter for the large-scale implementation of this technology, but in literature little attention has been paid to the absorptivity of natural CaO precursors such as limestone and other CaCO3-based materials. The CaCO3/CaO particles are usually white, and their absorptivity is relatively low at the wavelengths that correspond to solar radiation, limiting their use in directly irradiated reactors. In this work, the heat storage density (HSD) and absorptivity of different waste resources and natural Portuguese geological materials were assessed and improved after doping with 10 or 20 wt \% of dark inert additives. A sludge and SiC were used to increase the CaL-TCES materials absorptivity at the wavelengths of solar visible radiation and to hinder the material sintering. It is found that the doping of the CaL-TCES materials with sludge hinders the sintering of the CaO particles, and after 20 carbonation-calcination cycles the HSD of the doped material is two times higher than the corresponding value for undoped material (400-408 vs 203 kJ/kg CaCO3). However, the sludge additive has a minimal impact on the solar absorptivity of the TCES material. On the other hand, the SiC addition to a waste of marble powder (WMP-Galrao) has a positive impact with an increase of 13 \% on the absorptivity of the TCES material but does not improve the HSD. The absorptivity of CaL-TCES materials is a relevant property that can be improved through the use of dark inert additives. These new results show that TCES materials with improved HSD can be obtained through the tailoring of marble and sludge waste-derived materials, but sludges from other sources, with higher solar absorptivity need to be evaluated. The use of wastes is relevant for the circular economy goals, and it is environmentally friendly. |
