Study of the solidification behaviour of phase change materials by in-situ X-ray computed tomography

Calcium chloride hexahydrate (CaCl2.6H2O) is a stablished inorganic salt hydrate-based phase change material (PCM) with interesting properties for low temperature latent heat thermal energy storage (LHTES) applications. However, its phase transition during melting and solidification exhibits a complex behaviour with supercooling and phase segregation effects, which critically limit their practical applications. Experimental approaches enabling tracking of the solidification process in this PCM are thus crucial for a deeper understanding of these adverse effects and to design efficient LHTES. In this study, a novel method based on dynamic X-ray computed tomography (XCT) is used to monitor the solidification process in CaCl2.6H2O through sequences of XCT images. The method was tested with a sample of 60g of CaCl2.6H2O with density of 1.71 g/cm3 and purity higher than 98%. The results reveal that solidification of CaCl2.6H2O occurs through the fusion of crystalline structures initially appearing at the side and the bottom surfaces of the sample which grow over time. In the experiment performed, the solidification process is found to be incomplete, which is frustrated after 385 min of starting the solidification. This frustration was preceded by a change in the total solidification rate which is here estimated from the transient liquid volume fraction curve computed via image processing analysis. Quantitative estimation for the crystals size is also reported.