Optimizing nanofluids for efficient, sustainable, and cost-effective direct solar thermal conversion

• Gum Arabic stabilized CuO/water nanofluids studied for DASC use. • Optical absorption and SWAF increased with nanoparticle loading. • 0.04% mass concentration showed 71°C peak temperature and 82.3% e effectiveness. • Reusability Factor (RF) and Expense Factor (EF) introduced. • Composite Sustainability Index (CSI) developed for holistic ranking. • 0.04% mass concentration identified as optimal under lab scale conditions. This paper focuses on the photothermal conversion, sustainability and economic performance of nanofluids applicable to Direct Absorption Solar Collectors (DASC).The photothermal characteristics of a well-stabilized CuO/Water nanofluids were studied at different mass concentrations within the range of 0.01-0.05% mass concentration under a custom-made solar simulator. The absorbance and extinction coefficient along with the solar weighted ab- sorption fraction (SWAF) of nanofluids increased by increasing the mass concentration. The maximum temperature rise up to 71°C was recorded in 0.04% which was 33% higher relative to the same with basefluid. The same concentration showed the highest photothermal conversion efficiency of 82.30% which is 54.50% higher than the same with basefluid. The reusability, degradability and cost-effectiveness of nanofluids were analyzed introducing the novel concepts of reusability factor(RF) and expense factor(EF).Nanofluids at higher concentrations exhibited lower degradability, leading to enhanced reusability with a peak RF of 0.8 at 0.04% and 0.05%.However,nanofluids at 0.04% came out to be highly cost efficient.Moreover,the impact of nanofluids on the sustainability of photothermal convection process was analyzed introducing another novel concept of composite sustainability index (CSI) through holistic approach which describes that nanofluids at 0.04% is the most sustainable among all concentration.Overall,0.04% mass concentration is recommended as the optimal mass concentration for DASC application. The study implies potential aspect of nanofluids in e cient and sustainable solar energy utilization.