Configurational assessment of solidification performance in a triplex-tube heat exchanger filled with composite phase change material

The melting/solidification performance of a latent heat based thermal energy storage (TES) system is impeded due to the poor thermal conductivity of phase change materials (PCMs). To circumvent this limitation, several approaches have been adopted such as incorporation of nanoparticles including carbon nanotubes, fins/extended surfaces, heat pipes, metal foam, etc. This work presents an innovative design optimization technique for the solidification (discharging) enhancement by embedding the porous copper metal foam in PCM (referred as composite PCM) inside a triplex-tube heat exchanger unit. Equal volume ratio (0.5 v/v) of PCM and composite PCM was considered inside the annular space. Fifteen different configurations (M-1 to M-15) of composite PCM under the four broad classes based on the relative positioning and arrangement of metal foam were investigated and compared. Transient drop in melt fraction during solidification, temperature contours, instantaneous solidification contours, and total energy change in the solid PCM have been discussed. Numerical experiments demonstrated that the segmentation of porous metal foam zone in TES units significantly improves the discharging performance. The results suggest that upon maintaining a direct contact of the composite PCM zone with the flowing heat transfer fluid further improves heat dissipation and leads to an enhancement of -3.2 times (M-15) as compared to the pure PCM case (M-1). The overall performance is improved when the metal foam is placed above the pure PCM in TES unit. Typical results predict -97.5% and - 91.1% reduction in solidification time for model M-2 (fully filled with metal foam of porosity 0.95) and M-11 (uniform segmentation of metal foam of porosity 0.95 into 4 zones), respectively, when compared with pure PCM (M-1). An evaluation of the rate of the total energy released per unit cost of material used during discharging reports that model M-11 outperforms (the maximum value of -8.9) all other models having identical mass of PCM and/or composite PCM.