Evaluating the behaviour of a composite of CaCl2 and vermiculite for thermochemical adsorption energy storage: Experimental tests during the charging and discharging phases

• The kinetic hydration and dehydration onsets of a CaCl 2 -vermiculite composite are extrapolated. • Temperatures >100 °C allow to reach the anhydrous state of the composite at an absolute humidity of 7.3 g/kg of dry air. • Water adsorption rate decreases by 20 % after ten cycles. • The temperature difference between outlet air and the composite decreases up to 79 % in the last four cycles. Thermal energy storage plays a pivotal role in reducing the gap between energy supply and demand, as well as facilitating the integration of renewable energy sources. Thermochemical adsorption energy storage has the advantage of higher energy density compared to sensible and latent heat systems. For the application in buildings, salt hydrates combined in composite materials are among the most suitable thermochemical mediums, since they charge (dehydrate) at low temperatures. In this work, the charging and discharging behaviour of a thermochemical composite material consisting of vermiculite and CaCl 2 was assessed through experimental tests, performed with samples of different sizes. With the aim of aiding the design of an innovative thermal energy storage prototype in the context of the European project ECHO, this research investigated correspondence between the different experimental tests and signs of degradation of the selected material. First, the material was synthesized, and a thermogravimetric analysis was conducted to estimate the temperatures and absolute humidities characterising the onset of the different hydration and dehydration steps of CaCl 2 . Then, a thermobalance was employed to perform consecutive dehydration tests. Finally, ten cycles including discharging and charging phases were investigated using a small lab-scale setup, consisting of an open circuit with recirculation. Results agreed that temperatures higher than 100 °C are required to reach the anhydrous state at an absolute humidity equal to 7.3 g/kg of dry air. Moreover, degradation of the material was assessed in the last four cycles, showing that the adsorption rate decreases by 20 %. This degradation can be attributed to the lowered porosity of the material in the last cycles, which might have slowed down the air passing through the material bed. The findings of this research provide relevant insights into the behavior of the vermiculite-CaCl 2 composite as a candidate for thermochemical adsorption energy storage, thus contributing to the development and spread of these systems.