Development and characterization of hybrid water-based nanofluids for solar thermal applications

Hybrid nanofluids enhance performance compared to mono nanofluids. In this work, two different combinations of nanoparticles were mixed to produce water-based Al 2 O 3 -SiO 2 and Al 2 O 3 -CBox hybrid nanofluids at 2 wt.% and 4 wt.%. Particle size distribution, colloidal stability, rheological behavior, thermal conductivity, specific heat capacity, and optical properties were experimentally measured from 30 ºC to 70ºC. Morphological and size analysis indicate that adding SiO 2 and CBox to Al 2 O 3 does not significantly alter the size of the clusters, being all samples electrostatically charged and stable. The Al 2 O 3 -SiO 2 nanofluids demonstrated the highest thermal conductivity enhancement (15.51%) with a moderate viscosity increase (14.89%). Conversely, Al 2 O 3 -CBox exhibited a lower thermal conductivity enhancement (4.62%) but with a significant improvement of solar-weighted absorption, enhancing their potential as efficient solar absorbers in direct absorption solar collectors. Additionally, the specific heat capacity is fairly unchanged, having no impact on their thermal storage capacity. • Water-based hybrid nanofluids were developed for solar thermal applications. • All nanofluids are colloidally stable with enhanced thermal and optical properties. • Al 2 O 3 -SiO 2 nanofluids are the best candidates for heat transfer purposes. • Al 2 O 3 -CBox nanofluids are the best candidates for direct absorption collectors.