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

Journal of Thermal Analysis and Calorimetry, Dec 2017

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.

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Daniel Lager, Wolfgang Hohenauer, Christian Knoll, Peter Weinberger, Andreas Werner. Methodology to determine the apparent specific heat capacity of metal hydroxides for thermochemical energy storage, Journal of Thermal Analysis and Calorimetry, 2017, 207-215, DOI: 10.1007/s10973-017-6883-2