Phase Change Materials (PCMs) have attracted the attention as Thermal Energy Storage (TES) systems for their capability to store energy as latent heat of transformation. Among PCMs, also metals, as pure or alloys, are considered: they offer the possibility to store heat through their solid-liquid transformation at temperatures above organic PCM upper service limit (considered at 100°C). However, they show abrupt changes in their thermophysical properties during the phase change. Moreover, their spread is limited by the difficulty in handling them in the molten state due to element segregation tendency, and possible strong interaction with environment/containers. These problems can be solved by considering Miscibility Gap Alloys (MGAs); the system segregates in two phase, of which the low melting phase can act as PCM; while the higher temperature melting one, if well surrounding the active phase, can act as container, obtaining Composite Phase Change Materials (C-PCMs). Al-Sn binary system is an example, but the aforementioned described microstructure is not easily reached. By the addition of Si as alloying element to Al-Sn system, the miscibility gap in the solid state can be maintained, while the activation temperatures and the microstructural features after solidification can modified.
The authors explored the production of Al-Si-40%wtSn alloys, in view of the use as C-PCM. Water granulation process was considered. Granules of different size were produced, and their cooling rate was estimated by means of Finite Element simulations.
The microstructural suitability for C-PCM purposes was discussed on the bases of OM and SEM analyses on both as produced and thermally cycled granules. DSC and dilatometry cycles were performed for the thermal characterization. It resulted that Sn additions lowers and widens the solidification ranges of the produced alloys. As PCM, they activate in the temperature range of 200-230°C, and offer the possibility of storing about 90 J/cm3. Dilatometric tests confirmed their form-stability, i.e., the capability of a PCM to maintain its volume during a phase change.