Simultaneous charging and discharging of metal foam composite phase change material in triplex-tube latent heat storage system under various configurations

Phase change material (PCM) has high latent heat on one hand albeit low thermal conductivity on the other hand which restricts its utilization in thermal energy storage applications. Therefore, to improve thermal performance of PCM, various techniques have been employed. This numerical work intends to estimate the effect of copper metal foam (MF) in the seven various configurations (M1-M7) of triple-tube heat exchanger (TTHX) under simultaneous charging and discharging (SCD) conditions using heat transfer fluids (HTF) both the sides. Five distinct configurations with equal volumes of PCM and composite PCM (CPCM) have been considered for optimization standpoint. RT55 (melting temperature = 327 K) is taken as PCM. Based on thermo-physical properties of PCM and thermal boundary conditions on the heated tube, the dimensionless controlling parameters such as the Rayleigh number (Ra), Prandtl number (Pr), and Stefan number (Ste) were taken as 1.79 x 10(5), 30, and 0.21, respectively. Typical results on melt fraction, latent heat storage, temperature contours, and steady-state melt fraction and corresponding melting time have been reported. Performance yielded by all the configurations was compared for a fixed duration of 2 h. The positioning of MF largely affects the heat transfer mechanism in the latent heat storage unit. Results show that the bottom-side positioning of MF can boost the heat storage due to enhanced buoyancy-induced convection. Among all the models, M3 predicts the highest steady-state melt fraction (lambda(ss) approximate to 0.62) in the shortest steady-state melting time (t (ss) approximate to 66 min), followed by model M6 (lambda(ss) approximate to 0.58, t (ss) approximate to 65 min). The optimized design (model M3) shows similar to 75 % latent heat storage enhancement than pure PCM (M1) case. Interestingly, one may also achieve similar to 17.2 % higher enhancement using model M3 than M2 but with only half of the mass of MF than that used in full porous configuration (M2).