Modeling and Experimental Demonstrations Reveal Ragone Framework for Salt Hydrate Thermochemical Energy Storage

Salt hydrates offer the dual potential of high energy densities and low material costs for building heating applications. Heat is released and consumed with uptake and release of water (vapor) respectively by the salt without dissolution. However, the adoption of salt hydrate reactors is hindered by the limited knowledge on how power and energy tradeoffs translate from the materials to the reactor scale. Our study addresses this knowledge gap with integrated modeling and measurements, focusing on a packed bed reactor with SrBr2 vermiculite composite particles. Results reveal a mass transport limited regime producing steady, low power with larger energy capacity utilization, and a kinetics limited regime offering high, but variable power and smaller energy capacity utilization. We use a modified Damkohler number to predict the specific power and performance limiting factors, with knowledge of the inlet state, materials-specific kinetics and thermodynamics, and reactor design/operating conditions. Comparison between the reactor-scale (100 g) with the material-scale (1 g) test using SrBr2 vermiculite composite highlights that the kinetics and specific power decreases nearly tenfold from 35 W/kg to 3.9 W/kg at full discharge, likely due to the larger temperature and morphological variations at the larger scale. These discrepancies emphasize the need for improved characterization of composite materials’ temperature-dependent kinetics, equilibrium vapor pressure, and structural/morphological changes associated with water vapor uptake and release. Our research enhances the understanding of salt hydrate performance from materials to reactors, applies the Ragone framework to contextualize and benchmark against other energy storage materials, and suggests strategies for performance improvement.