Numerical analysis of an evacuated tube thermosyphon solar collector with hybrid nanoparticle-enhanced PCM storage and porous metal foam for improved thermal performance

• Evacuated zone modeling enhanced PCM melting, boosting liquid fraction by 9.17%. • Hybrid Ag–SiO 2 nanoparticles reduced melting time by about 5.46% in PCM tank. • Elliptical PCM container at 135° showed best melting, reaching full melt in 6403 s. • Adding porous metal foam cut melting time by 53.06%, yielding superior heat storage. This study presents a comprehensive numerical investigation of a thermosyphon-based solar collector integrated with a thermal energy storage system. The system operates through buoyancy-driven circulation, and the evacuated region surrounding the absorber is explicitly modeled using the Surface-to-Surface (S2S) radiation method to capture radiative exchange and minimize convective losses. A phase change material (PCM) container is incorporated within the upper water tank, and multiple geometries—circular and elliptical, each tested at different inclination angles—are examined to identify the most effective configuration. RT31 is used as the base PCM, and its thermal conductivity is enhanced with Ag–SiO 2 hybrid nanoparticles at a 0.02 vol fraction. The best-performing design is further improved by embedding porous metal foam to accelerate phase change and reduce thermal stratification. Model validation against independent experimental datasets confirmed strong agreement, demonstrating the reliability of the numerical approach. Results show that incorporating the evacuated zone increases the liquid fraction by approximately 9.17 %, while incorporating hybrid nanoparticles decreases the melting time of the circular container by approximately 5.46 %. Among all tested configurations, the elliptical container inclined at 135° provides the fastest melting, achieving full melting in 6402.7 s. Integrating porous metal foam into this optimized geometry further decreases melting time by 53.06 %, highlighting the synergistic benefits of combining NEPCM, optimized geometry, and porous media. The study introduces a compact and efficient thermal storage concept that addresses the low conductivity and slow melting rates of conventional PCM systems, offering improved heat charging performance and thermal stability for solar energy applications.