Experimental Determination of Characteristic Curves of Two Commercial Zeolites and the Composite of γ-Al2O3 and CaCl2 for Sorption Heat Storage

Thermal energy storage based on adsorption and desorption of water on zeolites promises high energy storage densities. The characteristic curve of an adsorbent links the adsorbed water volume to the adsorption potential. With a characteristic curve available, the heat of adsorption of the adsorbent/adsorbate pair can be calculated for different conditions encountered in specific applications. Thus, the maximum storable amount of thermal energy can be assessed at a material level. In this work, the feasibility to use an existing hydrothermal stability test setup to determine the required equilibrium data for the construction of characteristic curves is evaluated by using well-known zeolites 13XBFK and NaYBFK, both from Chemiewerk Bad Köstritz GmbH (CWK). For the two zeolites, characteristic curves were determined gravimetrically for an adsorption potential (ΔF) between 81 kJ kg-1 ≤ ΔF ≤ 2192 kJ kg-1. The obtained characteristic curves are in good agreement with existing ones from other experimental data and, thus, the hydrothermal stability test setup can be used to determine characteristic curves. In the next step the hydrothermal stability test setup was used to determine also the characteristic curve of granulated gamma-alumina impregnated with 7 wt% of calcium chloride (γ-Al2O3 + CaCl2) in the range 47 kJ kg−1 ≤ ΔF ≤ 913 kJ kg-1. With the help of the characteristic curves, a comparison of the composite of γ-Al2O3 and CaCl2 with the silica gel Grace 125 was made for three arbitrary scenarios with varying adsorption and desorption conditions. In the studied scenarios, the composite γ-Al2O3 + CaCl2 can reach water loadings that are between 12 % and 34 % higher than those of the silica gel.