Study and Characteriation of S309 and S309 multi-coated alumina as potential housing materials for Al-12wt%Si phase change material in thermal storage applications.

Kurzfassung

Thermal Energy Storage has become a reliable option that can act as a storage medium by using sensible, thermochemical or latent heat. In this Master Thesis, the Latent Heat Energy Storage (LHES) concept uses metallic Phase Change Materials (mPCMs) as storage materials. mPCMs can provide high energy storage density and its characteristic to store energy at a constant temperature corresponding to the phase transition temperature of the mPCMs. However, material compatibility between mPCM and its housing material is a key aspect affecting the performance and life of the storage concept. Al-Si eutectic alloy is widely researched and accepted as a mPCM storage material. Here, two housing materials as crucibles were used to verify their compatibility with Al-12.5wt%Si as mPCM. One of them was S309 and the other was S309 coated with alumina with an intermediate layer of Ni-Ti. Reaction Furnace experiments were performed wherein the mPCM and crucibles were tested under cycling and static conditions. These samples were then investigated for any potential reaction layers between their interfaces. Scanning Electron Microscopy in combination with EBSD analysis was applied to the processed samples post-experiment to identify reaction layers between the mPCM and the housing material. In this thesis, characterization of S309 by calculating its thermal diffusivity and specific heat by using Light Flash Analysis (LFA) and Differential Scanning Calorimetry (DSC) respectively was carried out. The same properties for Al-12.5wt%Si in addition to its enthalpy were also experimentally calculated and validated against previous findings. Furthermore, thermal contact resistance was found out for the concerned materials by using LFA. Lastly, simulation analysis was performed as an initial approach in Solidworks 2022 to analyse the heat transfer between the mPCM and housing material by using the required values obtained from LFA and DSC. The reaction experiments showed significant reaction of Al-12.5wt%Si with S309. Elements from the housing material diffused into the mPCM by forming intermediate layers between the two along with pores. This was observed for both, static and cycling experiments. Cracks were also seen to propagate through the mPCM for cycling experiments. Thus, this combination was not suitable for thermal storage. On the contrary, S309 multi-coated alumina did not react with Al-12.5wt%Si. Gaps were seen to form between the mPCM and crucible. At some points the mPCM was in contact with the crucible but the multi-coated alumina layer prevented the diffusion of elements thereby preventing the formation of reaction layers. Thus, S309 multi-coated with alumina can be used as a housing material with mPCM Al-12.5wt%Si eutectic alloy during static and cycling conditions. The thermophysical properties mentioned above were strongly related to thermal conductivity. Here, thermal conductivity was treated mathematically with the help of experimental values obtained from LFA and DSC. The thermal conductivity andthermal diffusivity value of alumina were the least in comparison with S309 and Al-12.5wt%Si. The results of properties obtained via experimentation were used for simulation purposes. The S309 multi-coated alumina showed irregular heat transmission due to the multi-layered alumina coating. On the contrary, heat was seen to steadily pass through for the uncoated S309 and Al-12.5wt%Si sample because of comparatively higher values of thermal diffusivity, thermal conductivity and specific heat of these materials. Thus, the compatibility of a eutectic Al-Si alloy as mPCM with S309 and S309 multicoated alumina as housing materials was investigated. Thermophysical properties of the materials involved were also experimentally and mathematically measured thereby using them as input for simulation analysis of the reaction furnace experiment. The heat transfer between the concerned materials was analysed.