Energetic and exergetic performance of a direct absorption solar collector using Fe3O4/ATO hybrid nanofluid

The performance of low-temperature direct absorption solar collectors depends not only on solar absorption intensity but also on the spectral and spatial distribution of absorbed energy, which governs heat loss and thermodynamic irreversibility. In this study, the thermal and exergetic performance of a direct absorption solar collector using Fe3O4/ATO hybrid nanofluids was experimentally investigated. The total particle concentration was fixed at 0.1 wt%, and the Fe3O4 mixing ratio was varied to tailor the solar absorption characteristics of the working fluid. The Fe3O4/ATO hybrid nanofluid with a MXFe3O4 of 0.25 showed superior performance compared with water and single-component nanofluids. The maximum thermal efficiency increased by 0.044–0.114, while the heat loss coefficient decreased by 0.098–0.15 relative to water. The operable normalized temperature difference was extended to approximately 0.061, indicating an expanded operating range for effective thermal energy production. Entropy generation analysis further showed that the optimized Fe3O4/ATO hybrid nanofluid reduced thermodynamic irreversibility and achieved a maximum exergy efficiency of 0.094. In contrast, the single Fe3O4 nanofluid, despite its strong solar absorption, showed lower thermal efficiency than water, which was attributed to dominant near-surface absorption and the resulting increase in heat loss. These results confirm that optimized hybrid composition, rather than nanofluid application alone, is essential for converting broadband solar absorption into effective heat gain while suppressing heat loss. Therefore, optimizing the Fe3O4/ATO mixing ratio offers a practical working-fluid design strategy for low-temperature direct absorption solar collectors by enhancing useful heat gain while suppressing thermodynamic irreversibility.