Photothermal conversion efficiency of saline nanofluids: coupled effects of Fe₃O₄ nanoparticle mass fraction and...

Nanofluids have shown great potential for enhancing solar photothermal conversion efficiency, yet most studies have focused on pure-water-based systems, and the coupled effects of nanoparticle mass fraction and salinity remain poorly understood. In this work, Fe₃O₄-NaCl nanofluids (0-0.20 wt% Fe₃O₄, 0-8 g L^-1 salinity) were evaluated through outdoor solar-heating and indoor cooling experiments comprising 31 experimental groups. Two-way ANOVA, Pearson correlation analysis, a Ridge-regularized quadratic polynomial model, and random forest (RF) regression with leave-one-out cross-validation (LOOCV) were employed for statistical analysis and prediction. Salinity was identified as the dominant negative factor affecting η _nf (p < 0.001, partial η ² = 0.769, r = -0.825), whereas nanoparticle mass fraction showed no significant effect (p = 0.586). η _nf exhibited a strong positive correlation with the cooling constant B (r = 0.866) but only a weak correlation with the maximum temperature rise ΔT _max, indicating that heat dissipation behavior plays a more important role than peak temperature in determining photothermal performance. The optimized RF model outperformed the polynomial model, achieving a LOOCV R ² of 0.757 (RMSE = 0.0377, MAE = 0.0301), and feature importance analysis further showed that salinity-related descriptors contributed 71.6% of the total importance. Although the highest predicted η _nf was obtained under zero-salinity conditions, a practical operating window (2-4 g L^-1 salinity and 0.05-0.10 wt% Fe₃O₄) was identified for applications requiring saline adaptability. These findings provide quantitative insights into the photothermal behavior of saline nanofluids and guidance for their application in solar thermal utilization and low-salinity desalination.