Analysis of the crystallization and melting processes of phase change materials via X-ray computed tomography

Latent thermal energy storages (LTESs) are based on phase change materials (PCMs), which can store heat at a nearly isothermal temperature with a higher energy density compared to sensible systems. Contrary to sensible thermal energy storage, the level of charge of the systems, i.e., the amount of energy stored, is not proportional to temperature. X-ray computed tomography (XCT) approach is here proposed to measure the volumetric liquid fraction during solid-liquid phase change in non-metallic PCMs. The liquid fraction has a paramount role because it is the only parameter that gives a direct information about the amount of stored energy by the LTES. Moreover, the direct measurement of the volumetric liquid fraction evolution can be used to validate the numerical models and the collected data can become a new reliable validated dataset for the scientific community. Furthermore, the possibility to “see inside” the materials can help in understanding the underlying heat and mass transfer mechanisms of the phase change and can give new insights about the main issues of PCMs, such as salt hydrate segregation, subcooling, etc. In this work, XCT analyses are used as an innovative method to address all these activities and it is applied to different materials. Starting from ice-water, the study then moved to eicosane, an organic paraffin and ended with a couple of salt hydrates. In particular, calcium chloride hexahydrate was extensively analysed and sodium acetate trihydrate was also considered to investigated the segregation issue. The structure of the manuscript is the following: -Chapter 1: the topic of latent thermal energy storage is introduced with some discussions on experimental and numerical methodologies used to study phase change materials; -Chapter 2: an introduction to X-ray computed tomography technologies is presented and described; -Chapter 3: the numerical model used in the simulations run in this work is presented and discussed; -Chapter 4: the experimental and numerical analysis run on the ice/water PCM is presented; -Chapter 5: the experimental and numerical analysis run on the eicosane PCM is presented; -Chapter 6: the experimental and numerical analysis run on the calcium chloride hexahydrate PCM is presented; -Chapter 7: the work on calcium chloride hexahydrate follows up the previous chapter with different water concentrations experiments; -Chapter 8: a new material is experimentally analysed via X-ray computed tomography: sodium acetate trihydrate; -Chapter 9: conclusions.