Modeling the integration of a heat pipe evacuated tube system with paraffin for solar energy storage

Abstract

This article offers a numerical research into the melting within heat pipe-integrated evacuated tube solar collector, integrating advanced thermal enhancement techniques to improve energy storage efficiency. To optimize the thermal performance, two fin configurations—upward and downward—were incorporated into the phase change material (PCM) zone, where paraffin (RT27) was mixed with ZnO nanoparticles. The outputs demonstrated that the downward fin arrangement exhibited superior performance compared to the upward configuration. To further augment heat transfer, the downward fin was coupled with porous foam. The study employed SolTrace software to determine the heat flux received by the outer layer of the solar collector, ensuring precise boundary conditions for numerical modeling. The three-dimensional model was developed using ANSYS FLUENT, incorporating user-defined functions (UDFs) to dynamically capture the thermophysical property variations of the PCM. A piecewise linear approach was utilized to account for phase-dependent properties. Also, the density variation with temperature in the liquid phase has been applied ensuring a more accurate representation of natural convection effects. Simulation results revealed that transitioning from an upward to a downward fin configuration led to an increase of approximately 4.1 % in the liquid fraction (LF) and a 3.46 % rise in the average PCM temperature (T_PCM). Moreover, integrating the downward fins with porous foam resulted in a remarkable 24.85 % enhancement in the liquid fraction due to the superior thermal conduction characteristics of this configuration. This enhancement is particularly beneficial during the solidification process when solar irradiation is absent, ensuring a more stable energy storage unit. In the optimal arrangement, as the operating time increased from 20 min to 100 min, the temperature of the paraffin zone and the water zone rose by 15.31 % and 8.64 %, respectively, underscoring the system's effectiveness in heat retention and transfer.