Carnallite double salt for thermochemical heat storage

Abstract

The potential use of carnallite (KMgCl3·6H2O) for low temperature thermochemical heat storage has been evaluated. Carnallite is an incongruently soluble double salt and its possible decomposition and dehydration reactions have been carefully evaluated from available thermodynamic data and the phase diagram has been constructed. The dehydration and rehydration reactions have been experimentally studied and the reaction products have been characterized using thermogravimetric mass spectrometry, water vapor sorption, X-ray powder diffraction, scanning electron microscopy, Raman microscopy and calorimetry. The results of the experimental investigation of these reactions confirm the model predictions and clearly show that a thermochemical cycle including carnallite and its dihydrate (KMgCl3·2H2O) is very promising for low temperature domestic thermochemical storage. In comparison to both pure MgCl2·6H2O and also other salts mentioned in the literature, the double salt carnallite offers significant advantages. It has a higher deliquescence humidity and, thus, is much less sensitive to over-hydration and liquefaction. It can be easily dehydrated at only 100 °C and is much less prone to hydrolysis (HCl release). Carnallite has also a good reversibility between dehydration and hydration and the re-formation of carnallite by rehydration is easily achieved at low water vapor pressure and faster reaction rate than the hydration of MgCl2·2H2O to MgCl2·6H2O. The enthalpy of hydration is slightly higher for the hydration of KMgCl3·2H2O than for the hydration of MgCl2·2H2O. The only price to be paid is a slightly reduced storage density of the double salt due to its larger molar volume. However, the theoretical storage density of 1.52 kJcm−3 is still excellent. Also, carnallite is a low-cost material as it is available as industrial waste material or can be easily synthesized.