Optimizing cyclic performance of latent heat thermal energy storage systems: A comprehensive analysis of eccentricity and novel longitudinal fin geometries in shell-and-tube systems

The current research on latent heat storage systems (LHSSs) with phase change materials (PCMs) lacks a comprehensive analysis of the combined effects of varied longitudinal fin shapes and upper/lower tube eccentricity. While individual studies have explored these methods for heat transfer enhancement, there is a gap in understanding their concurrent impact on both charging and discharging cycles. This work aims to bridge this gap by conducting a more thorough investigation to evaluate the cyclic performance of LHSSs across a wider range of fin designs/number and eccentricity configurations. Three downward eccentric designs (with eccentricities of 0.2, 0.4, and 0.6), one upward eccentric design (with an eccentricity of 0.2), and one concentric design are investigated. The enthalpy-porosity method is used to develop a transient, three-dimensional computational model that characterizes the melting and solidification dynamics of ten LHSSs, using RT-55 as the PCM. The liquid fraction, flow pattern, and PCM temperature are used to visualize the melting and solidification dynamics. Additionally, the performance of the LHSSs is analyzed in terms of the charging and discharging capacities and rates, dynamic effectiveness, and enhancement ratio. The downward eccentric units with eight fins and eccentricities of 0.2, 0.4, and 0.6 reveal reductions in the melting time by 24.0 %, 27.3 %, and 35.8 %, respectively, while the upward eccentric unit shows a 27.8 % increase in the melting time, compared with the concentric design. Conversely, the concentric design shows the shortest solidification time. The best LHSS (with eight asymmetric fins and an eccentricity of 0.6) demonstrates reductions in the melting, solidification, and total cycle times by 7.0 %, 16.5 %, and 13.8 %, respectively, compared with the unit with six asymmetric fins and an eccentricity of 0.6. Concomitantly, the former shows improvements by 17.9 %, 16.2 %, 9.3 %, 18.5 %, 11.0 %, and 21.4 % in the daily charging capacity, daily discharging capacity, charging rate, discharging rate, charging effectiveness, and discharging effectiveness, respectively, over the latter.