Charging and discharging in thermal energy storage unit with fin-stone hybrid structure for enhancing heat transfer of phase change materials.

• A fin-stone hybrid structure enhances heat transfer of phase change materials. • Charging and discharging times are saved by 67% and 73%, respectively. • Satisfactory temperature stability and more yearly supplied energy are achieved. This work proposes a fin-stone hybrid structure integrating fins (popular thermal enhancers) and natural stones (widely used sensible heat storage media) to enhance the heat transfer of phase change materials for on-site thermal energy storage applications, with advantages of low cost, environmental friendliness, and easy accessibility. 3D numerical models of charging and discharging in shell-and-tube heat storage units with various configurations, including fins, the fin-stone hybrid structure, stones, and no heat transfer enhancement, were constructed, and the performance evaluation and comparison were carried out. Compared to fins, fin-stone hybrid structures with 20 mm-, 30 mm-, and 40 mm-sized stones shorten the charging time by 67%, 54%, and 56%, and the discharging time by 73%, 60%, and 46%, respectively. Small stones have better heat transfer enhancement, which is attributed to the small volume, large surface area, and contact with the tube and fins. The advantage of the fin-stone hybrid structure, i.e. the shortening of phase change time, is more significant in charging than in discharging, in comparison with stones, as both heat conduction and natural convection are enhanced. Moreover, the hybrid structure exhibits satisfactory temperature stability with a 48.9 °C temperature change in charging and 37.2 °C in discharging, each lower than the fins, which is beneficial to stabilise the heat transfer fluid outlet temperature. The yearly supplied energy of the hybrid structure with 20 mm-sized stones is 121% and 72% more than that of fins and stones, respectively. [ABSTRACT FROM AUTHOR]