A novel multilayered nanofluid-based direct absorption solar collector for enhanced thermal trapping and efficiency

This work introduces a novel multilayered nanofluid-based direct absorption solar collector (DASC) designed to overcome the efficiency limitations of conventional single-layer designs. The multilayer configuration employs weakly absorbing media in the upper layer to suppress surface-localized heating, while stronger absorbers are positioned in deeper layers to volumetrically capture transmitted radiation. A comprehensive mathematical framework, incorporating wavelength-dependent optical properties and validated against established numerical and experimental benchmarks, was developed to analyze the coupled radiative and thermal behavior of the system. The results demonstrate that positioning weakly absorbing nanofluids (e.g., water or alumina) in the top layer maximizes the thermal-trapping effect, shifting the temperature peak into the interior and yielding efficiency gains of up to 60 % relative to single-layer collectors. An optimum top-layer thickness of about 5 % of the total channel height was identified, thin enough to maintain radiative transmission yet sufficient to shield the exposed boundary from peak heating. The proposed design also showed reduced sensitivity to wind-induced losses, with thermally-trapped peak temperatures confirmed by intensity and temperature contour maps. The analysis further shows that these systems benefit not only from optical trapping but also from hydrodynamic tuning, which enhances efficiency by up to 10 % by limiting wall heating and associated losses. The findings establish multilayer DASCs as a robust and efficient design strategy, offering a clear performance advantage over conventional single-layer configurations for practical solar-thermal applications.