Thermal energy storage and recovery in a double-pipe latent heat storage (LHS) system with variable wavelength wavy channels were examined. The presence of wavy channels with variable wave-length makes it possible for the desired aggregation of the solid waves inside the Phase Change Material (PCM) domain. The PCM and the moving water are placed in the outer and inner sides, respectively, in the vertical double tube heat storage unit. Different configurations of variable sinusoidal wave-length were studied compared with fixed wave-length as well as straight channel unit. The wave-amplitude, temperature and Reynolds number of water were also analysed. The significant effects of channel waviness with variable wave-length are presented results in higher heat transfer inside and to/from the PCM domain. Increasing the water temperature and Reynolds number, as well as wave-amplitude, result in a shorter melting/solidification time. Based on the results of the selected case, the time of melting/solidification reduces by almost 70.8/42.8%, respectively, compared with straight wall case. Furthermore, the heat storage/recovery rate is 8.3/3.0W for melting/solidification, respectively, for the selected configuration which is equal to 2.4/1.7W for the straight channel case.
