Moisture Reactivated Calcium Sorbents for Long Duration Thermochemical Energy Storage

Calcium sorbents—mainly composed of calcium oxide and carbonate—exhibited great promise for thermochemical energy storage (TCES) due to their high storage temperatures, high energy density, good thermal conductivity, affordability, and non-toxicity. The thermochemical energy is stored based on a reversible carbonation/calcination chemical reaction of calcium oxide at high temperatures with CO₂ absorption/desorption. However, calcium sorbents experience reactivity degradation over long cycles due to the sintering effect and particle aggregation, resulting in progressive performance decay that hinders their practical application. Recent studies demonstrated various strategies to mitigate this sorbent degradation at both material and system levels. Particularly, reactivating sintered sorbents using hydration intrigues emerged interest as it effectively restores the surface area and porosity of CaO for TCES efficiency at a low cost. However, the practical implementation of this approach is still constrained by the tradeoff between reactivation efficiency, kinetics, and energy penalty from the extra drying process when converting hydrated CaO (Ca(OH)₂) to CaO.
In this study, we investigated the mechanism of TECS performance degradation by studying the microstructure changes of calcium sorbents over multiple cycles, including variations in particle size, crystal size, and surface areas. Our results indicated that calcium sorbents with larger surface area and smaller crystallite size typically show higher initial carbonation conversion rates, leading to more efficient TCES. Our results showed that a nano-sized calcium sorbent with a surface area of 12.4 m²/g and crystallite size of 28 nm displayed an initial conversion rate of 92%, which was significantly higher than the conversion rate of 65% in a micro-sized calcium sorbent with a surface area of 0.3 m²/g and crystallite size of 64 nm. However, nano-sized sorbents also exhibited more phenomenal degradation, with the surface area rapidly dropping from 12.4 m²/g to 0.6 m²/g after 10 cycles, corresponding to a conversion rate degradation from 92% to 29%. To address this issue, we proposed controlled moisture hydration treatments to reactivate the degraded sorbents with a minimized energy penalty. Materials characterizations, such as scanning electron microscopy, X-Ray diffraction analysis, and Brunauer-Emmett-Teller analysis, combined with thermal analysis were conducted to study the effects of hydration degrees on restoring surface area and crystallite size on mitigating the degradation performance and improving overall TCES performance. Upon increased hydration degrees, the sintered CaO surface cracked under the stress generated from hydrated Ca(OH)₂ volume expansion, forming a porous structure with a large surface area. Particularly, reactivating the degraded sorbents with a high hydration degree of 300% resulted in a most effective surface area restoration of 11.1 m²/g with a recovered conversion rate of 87%, indicating a successful TCES efficiency restoration. This demonstration of reactivating degraded sorbents offers a promising solution for improving long-duration TCES, benefiting renewable energy harvesting systems such as concentrated solar power plants.