Moisture Reactivation of Calcium Sorbents for Long Duration Thermochemical Energy Storage

Calcium sorbents—mainly composed of calcium oxide and carbonate—exhibit great promise for thermochemical energy storage due to their high working temperatures, high energy density, good thermal conductivity, affordability, and non-toxicity. However, progressive performance degradation over long cycles hinders their practical application. In this study, we investigated the mechanism of 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 indicate that calcium sorbents with a larger surface area and smaller crystallite size typically show higher initial carbonation conversion rates, leading to more efficient thermochemical energy storage. Specially, 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 is significantly higher than the 65% conversion rate 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 exhibited more phenomenal degradation due to the sintering and aggregation, with the conversion rate rapidly dropping from 92% to 18% after 15 cycles. To address this issue, we proposed controlled moisture hydration treatments to reactivate the degraded sorbents. Rigorous studies were conducted to study the effects of hydration degrees on restoring surface area and crystallite size, while thermal analysis was performed to understand the improved degradation performance and overall energy storage performance. By reactivating the degraded sorbents with a hydration degree of 300%, the conversion rate was recovered to 87%, indicating an effective restoration of thermochemical energy storage efficiency. This demonstration of reactivating degraded sorbents offers a promising solution for improving long-duration thermochemical energy storage, benefiting renewable energy harvesting systems such as concentrated solar power plants.