Thermal energy storage (TES) systems are a crucial component of solar energy harvesting cycles. Our objective in this study is to enhance the efficiency of a double-tube TES that utilizes lauric acid, a phase change material (PCM), and heated water as a heat transfer fluid (HTF). For the first time, this study examined the effects of utilizing a twisted inner tube inside a double-tube TES containing HTF. The finite volume method based on the enthalpy porosity model was used to investigate the effects of three different cross-sectional aspect ratios (AR = 0.5, 0.75, and 1) of the inner tube, at three different normalized locations along the vertical axis (Y = 0, −0.2, and − 0.4). The results were evaluated based on liquid fraction, temperature, velocity, total melting time, and average Nusselt number. The obtained results revealed that the AR effect of the inner tube varies depending on its location along the Y-axis. It is established that placing the tube at lower Y values in cases where a helical tube is present will yield significantly better results than higher Y values. Finally, the best case with the lowest total melting time occurred when the inner tube was located at Y = -0.4 and AR = 0.5. This case resulted in a total melting time decrease of up to 63 % compared to the reference case (AR = 1 at Y = 0).