Numerical modeling and thermal performance enhancement of a parabolic solar thermal energy storage system using SWCNT-enhanced phase change materials and thermoelectric generator

This research presents a new approach to enhance the efficiency of solar thermal systems using a double-pipe heat exchanger with a parabolic solar reflector with multiple thermal enhancement techniques. The phase change material (PCM) is paraffin wax (RT35-HC) that fills the inner tube of the heat exchanger. To address the inherently low thermal conductivity of paraffin, single-walled carbon nanotubes (SWCNTs) are dispersed within the PCM, while cross-shaped fins are strategically embedded to further enhance the melting and solidification processes. The outer tube is filled with water and improved by adding copper porous foam to develop the heat transfer significantly through combined convection and conduction. A thermoelectric generator (TEG) module is also mounted on the outer surface of the water tube to convert the waste thermal energy to electricity. SolTrace is utilized to accurately simulate the solar flux distribution from the parabolic reflector, and ANSYS FLUENT is employed to perform a comprehensive unsteady three-dimensional simulation to evaluate the thermal behavior and energy conversion efficiency of the system for various designs. Results indicate that the addition of SWCNT nanoparticles and cross-shaped fins in the PCM zone boosts the thermal efficiency, the stored energy, and the liquid fraction significantly by about 53.32%, 63.12%, and 74.21%, respectively. Using porous foam in the water zone only, without fins, causes considerable enhancements of 2.64% in the overall efficiency, 24.74% in the stored energy, and 28.66% in the liquid fraction. When all techniques are combined, the system shows significant enhancements with the overall efficiency, energy storage capacity and phase change progress being improved by factors of 1.67, 2.09 and 2.74 respectively, compared to the baseline case. The system behavior in the solidification step, without solar input, was also evaluated. It was observed that the use of porous foam reduces the freezing time by 16.17% with respect to fin-only configurations while the fastest solidification (3420 s) is obtained when all enhancement features are applied. The results highlight the effectiveness and novelty of combining multiple enhancement techniques in a solar PCM-TEG hybrid system that has the potential to enable more efficient, compact and multifunctional solar energy storage technologies.