To ensure that the rectangular thermal energy storage unit (RTESU) can heat more cold outdoor air for practical applications, three kinds of storage 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, respectively. 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 height, uniform, 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 can increase the area dominated by NC. However, there is an opposite trend in the effect of enhanced heat transfer between the two methods for different heights in general. It is worth noting that the Nusselt number ( [[EQUATION]] ) is introduced for examining the heat transfer performance during the melting process with different unit configurations. And the correlation between the dimensionless numbers is applied to predict the variation of a liquid fraction under 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 improvement in the charging process as well as the maintenance of a higher heat release efficiency of about 84.5% can be achieved in the RTESU.
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