Sub cooling of PCM due to various effects during solidification in a vertical concentric tube thermal storage unit

Phase Change Materials (PCMs), in the solidification/melting temperature range of 19 °C–22 °C, have been considered suitable for the free cooling application of buildings. Further, these phase change materials operate over a narrow range of temperatures, and hence, the sub cooling effect of the PCM plays an important role in the efficient design of heat storage devices. In the present experimental work, the sub cooling effect of PCM-RT 21 was studied in a vertical concentric tube heat storage unit. It was observed that the PCM experienced different cooling rates at different axial heights, with the highest rate near the Heat Transfer Fluid (HTF) entry region. Also, the higher cooling rate increased the sub cooling effect and lowered the solidification temperature of the PCM. The experimentally observed solidification temperatures under different cooling rates correlated well with the values arrived at, using the DSC analysis. The free convection during the liquid sensible cooling commenced the nucleation process at all the radial locations of the PCM at a particular axial height, at the same time, without much difference in the onset of the solidification temperature. The time span of the temperature plateau during the latent heat release increased on increasing the radial location of the PCM from the heat transfer surface, as the heat transport rate decreased. It was further found that when the driving temperature potential between the PCM and the HTF was high, the increase in the inlet velocity of the HTF beyond a certain value had a negligible effect during the liquid sensible cooling and the subsequent solidification process.