This study aims to improve the melting and solidification characteristics of a triple-pipe heat storage unit by
introducing a wavy middle plate with a sinusoidal fixed wavelength. The inner and outer tubes facilitate the flow
of heat transfer fluids (HTFs), while the middle tube houses the phase change material (PCM). Various scenarios
are examined to gauge the impact of the wavy central wall on the PCM’s phase change performance, comparing
liquid fraction and mean temperatures with cases featuring a straight middle plate. The primary goal is to
pinpoint the optimal scenario with the shortest discharging/charging times and the highest rates of heat release/
storage. The investigation commences by assessing the effects of middle wall thickness (2, 4, 6, and 8 mm) on the
PCM’s phase change characteristics, comparing liquid fraction and mean temperature with a straight middle
plate. Subsequently, the influence of various sinusoidal wavelengths (250, 125, 50, and 25 mm) on both melting
and solidification performances of the PCM is numerically examined to identify the optimum case. The research
also delves into the effects of various sinusoidal amplitudes (2.5, 5.0, 7.5, and 10 mm) on the PCM’s phase
change process. In the final step, the capability of the wavy middle plate, with various phase shifts (0◦, 90◦, 180◦,
and 270◦), in improving the PCM’s phase change characteristics is analyzed. Numerical results highlight that,
among the tested scenarios, the configuration with a 5 mm amplitude and a 25 mm wavelength (A5λ250) exhibits outstanding performance during both melting and solidification phases. A5λ250 showcases a 19.92 %
reduction in melting time and a 2.44 % reduction in solidification time compared to other cases, coupled with a
22.55 % improvement in heat storage and a 3.48 % enhancement in heat release. However, introducing a phase
shift in the melting process adversely affects system performance. Conversely, adjusting the phase shift of A5λ25
to 270◦ leads to enhancements in both solidification time and heat release rate. Specifically, solidification time
improves by 0.86 %, and the rate of heat release increases by 0.66 W compared to A5λ25–0.