Melting enhancement of latent heat thermal energy storage by implementing new geometric design

Abstract

Latent heat thermal energy storage systems can manage peak energy demand with the unique thermo–physical properties of phase change material. However, the primary concern of using these systems is their low heat transfer rate. The present study attempts to increase the thermal performance of these systems using perforated stair fins and changing the enclosure’s geometry. For this purpose, parametric studies have been conducted to assess the effect of perforations on fins with different diameters (from dimensionless diameters of 1 to 2) and different numbers of perforations (from 6 to 24). Next, the effect of changing the enclosure's geometry to a parallelogram, trapezoid, and rectangle is investigated. Finally, by simultaneously applying the above–mentioned ideas, the performance parameters of the system are calculated and compared with the base case (enclosure with simple stair fins). The investigations are conducted using the finite volume method, and the material melting process is carried out based on the enthalpy–porosity method. All the simulations are three–dimensional and transient, with variable properties of material in solid and liquid phases. According to the results, using perforated fins intensifies the buoyancy force by increasing fluid displacement inside the enclosure. As a result, using 24 perforations on each fin with a D/δ = 2 improves the liquid fraction up to 9.7%. Moreover, the change of geometry reduces the critical area at the bottom of the system and improves the liquid fraction up to 5.4% inside the trapezoidal enclosure. By applying two ideas of using perforated fins and changing the geometry simultaneously, the mean Nusselt numbers for rectangular, trapezoidal, and parallelogram enclosures are significantly enhanced by 95, 114, and 133%, respectively. In these cases, the improvement in the heat transfer rate has led to a decrease in the melting time of the materials by 44.5, 47.5, and 51.5 min, respectively. These operational suggestions also enhance the critical performance parameters of the system, namely the stored energy and the mean charging power from 946 kJ/m and 124 W/m (in the base case) to 963 kJ/m and 211 W/m (70%) (in the rectangular enclosure with perforated fins).