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

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, N-2 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 micmpores. Salt is included up to a threshold loading of similar to 20 mol mol(-1) depending on the nature of salt. Thermochemical properties are determined at 30 degrees 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.