Methodology to determine the apparent specific heat capacity of metal hydroxides for thermochemical energy storage

Abstract

Thermochemical energy storage uses reversible thermochemical reactions to store and release heat, representing a promising technology for energy conservation and utilizing fluctuating renewable energy sources and waste heat. Many recent studies have focused on determination of the enthalpy of reaction of possible thermochemical materials (TCM) based on thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). So far, comparatively few attempts have been made to characterize the apparent specific heat capacity at constant pressure c_{text{p}}^{text{app}} left( T right) of the investigated TCM. The purpose of this study is to outline a measurement and analysis procedure to evaluate c_{text{p}}^{text{app}} left( T right) of powdery TCM. The procedure is presented focusing on two metal hydroxides Ca(OH)2 and Mg(OH)2. Preliminary TGA experiments were conducted to identify reaction-free temperature intervals and mass change. Starting from the metal hydroxide, subsequent DSC experiments with two consecutive heating and cooling cycles were carried out to determine c_{text{p}}^{text{app}} left( T right) of the initial hydroxide and the oxide product. Three separate DSC runs for each candidate enable an evaluation of measurement uncertainty, and c_{text{p}}^{text{app}} left( T right) results were compared to available literature data. Preliminary TGA experiments have shown that the applied heating rate β has a strong effect on the measured dehydration reaction. This result influences the consecutive c_{text{p}}^{text{app}} left( T right) interpretation of the metal hydroxides. Analysis of the measured c_{text{p}}^{text{app}} left( T right) data compared to literature show good agreement for both metal hydroxides and oxides. Overlapping endotherm effects, which are not part of c_{text{p}} left( T right), have to be considered for further thermal conductivity calculations.