Photo-thermal conversion properties of hybrid NH2-MIL-125/TiN/EG nanofluids for solar energy harvesting

• NH 2 -MIL-125/TiN/EG hybrid nanofluids were prepared. • NH 2 -MIL-125 nanoparticles improved the photothermal stability of TiN nanofluids. • The final composite nanofluid can realise both outstanding photothermal properties and stability. • A photothermal conversion efficiency of 83 % is achievable for 60 ppm MIL/TiN (M5:T5) nanofluid. Nanofluids, engineered by dispersing nanoparticles into base fluids, have demonstrated significant potential in enhancing the performance of solar collectors. Their superior thermal conductivity and improved heat transfer properties lead to more efficient energy absorption and utilization in solar thermal systems. This study comprehensively investigated and compared the performance of ethylene glycol (EG)-based NH 2 -MIL-125/TiN (MIL/TiN) composite nanofluids, fabricated by a two-step method, with single-component nanofluids in photothermal utilization. The results illustrated that incorporating NH 2 -MIL-125 (MIL) nanoparticles enhances the stability of TiN nanofluids in EG. Additionally, the thermal conductivity and optical absorption properties of the hybrid nanofluids exhibited a positive correlation with the proportion of TiN nanoparticles. The results showed that hybrid nanofluids demonstrate excellent solar radiation absorption across a broad spectrum from UV to NIR, with minimal heat loss and significant depth penetration. The MIL/TiN (M5) hybrid nanofluid achieved a remarkable photothermal conversion efficiency of 83 %, outperforming that of single TiN and MIL-125(Ti) nanofluids by 19 % and 15 %, respectively. Furthermore, the efficiency distribution within the collector reached 100 % at a height of 3.5 cm with this hybrid MIL/TiN nanofluid, even at a low concentration of 20 ppm. This research highlights a promising nanofluid that can enhance photothermal conversion efficiency in solar thermal systems. It leverages the capabilities of nanoporous materials with superior photothermal properties and optimizes the structural design of solar collectors to minimize heat losses, making it suitable for various applications.