To increase the storage capacity and air flow rate of the rectangular thermal energy storage unit (RTESU) for enhancing its application capability, three modular units with air flow rates of 60 m3/h, 120 m3/h, and 180 m3/h for heights of 150 mm, 300 mm, and 450 mm are proposed. Besides, the effect of natural convection (NC) on the charging and discharging process is examined during the modular expansion of the RTESU. The melting performance including melting duration and phase change process is numerically investigated by varying the configurations such as module configuration, non-uniform, and eccentric tube arrangements. The results show that NC has a significant impact on the charging process. It is further demonstrated that the charging process can be enhanced by about 14.58 % and 13.61 % when non-uniform and eccentric tube arrangements are implemented, respectively, as they both increase the area dominated by NC. However, the non-uniform tube arrangement for acceleration of the melting process is gradually enhanced with the increase in storage unit height compared to that of the eccentric tube arrangement. It is worth noting that the Nusselt number (Nu) is introduced for examining heat transfer performance during the melting process with different unit configurations. And the correlation between the dimensionless numbers is applied to predict the liquid fraction for different configurations. For the heat release process, the performance including air outlet temperature, heat extraction, and heat release efficiency of the modular RTESU is evaluated. It is found that the NC has a negligible effect on the discharging process, which is dominated by heat conduction. With the modular expansion, an increase in air flow rate and heat storage capacity as well as the maintenance of a higher heat release efficiency of about 84.97 % can be achieved in the RTESU.
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