Salt inclusion and deliquescence in salt/zeolite X composites for thermochemical heat storage

Abstract Composites of commercially available zeolite Na-X beads and the salts NaCl, CaCl2, MgCl2, and MgSO4 (loadings up to 24 mol mol−1 or 18 wt.-%) were prepared by ion-exchange followed by incipient wetness impregnation and characterized by elemental analysis (XRF), XRD, N2 sorption, mercury porosimetry, and SEM. During loading of the zeolite with salt, the Donnan effect leads to an inclusion of salt ions into the zeolitic micropores. Salt is included up to a threshold loading of ∼20 mol mol−1 depending on the nature of salt. Thermochemical properties are determined at 30 °C and a humidity of 3–21 g kg−1. Both water loading lift and heat storage density decrease with increasing salt loading below the deliquescence humidity of the salt. Above the deliquescence humidity, absorption into a salt solution within the secondary pores of the zeolite beads contributes to the water uptake. Heat storage densities up to 275 kWh m−3 (153% of the salt-free zeolite) are reached at high salt loadings and sorption humidity.