Thermal performance and physicochemical stability of silver nanoprism-based nanofluids for direct solar absorption

To utilize nanofluids for direct absorption solar collectors (DASCs), they need to maintain their performance and physicochemical stability with exposure to solar radiation. In the present studies, three water-based nanofluids were characterized under light exposure, including silver nanoprisms (AgNPrs), silica coated silver nanoprisms (SiO2@AgNPr) and silica (SiO2) nanoparticles. Their temperature profiles and stability were monitored using simulated sunlight (SSL) and natural sunlight exposure (NSL), quantified by UV-vis spectroscopy and, in the case of SSL, characterized by transmission electron microscopy (TEM). With SSL both silver nanofluids showed an increase in maximum temperature of approximately 40-45 degrees C, with a photo-conversion efficiency of about three times greater than the SiO2 nanofluid and water base-fluid. Stability tests showed the SiO2@AgNPr nanomaterial to be morphologically unstable, with the AgNPrs etching over a period of several hours. The AgNPrs showed a higher tendency to aggregation than SiO2@AgNPr nanofluids when exposed to NSL sunlight over a two-week period. Contrarily, the latter exhibited notable changing in shape, consequently effecting the absorption band position. The results highlight strongly the need for stability trials under realistic conditions for the development of nanofluids for direct solar absorption.