Solar energy absorption in direct absorption solar collectors: A study on microscale energy transport using carbon-based nanofluids

This study investigated microscale energy transport in direct absorption solar collectors (DASCs) using graphite nanofluids, selected for their superior optical and thermophysical properties. The performance of graphite nanofluids was examined against metallic nanofluids (gold and aluminum) over a nanoparticle concentration ranges from ϕ_p=0.00001%-0.6%. The results showed that at ϕ_p=0.5%, the collector achieved absorption efficiency of ∼ 89 %, nearly twice that of aluminum (44 %) and gold (56 %), highlighting graphite's advantage in DASCs. A transient Eulerian-Lagrangian (E-L) approach was employed to analyze nanoparticle-radiation interactions and energy distribution at the microscale level. In terms of optical enhancement, graphite nanofluids showed a four-order magnitude increase in the extinction coefficient compared to the base fluid, significantly enhancing UV–visible absorption spectrum range. The proposed model was validated through analytical, numerical, and experimental case studies, demonstrating its accuracy in predicting microscale energy transport. The findings of this study emphasized the critical role of microscale energy transport in DASCs, particularly with the use of graphite nanofluids, emphasizing their viability for scalable solar thermal applications.