Characteristics of the direct absorption solar collectors based on reduced graphene oxide nanofluids in solar steam evaporation

Abstract

For high-temperatures surfaces, surface absorbers possess a higher evaporation rate while volumetric receivers have a higher solar vapor generation efficiency by localizing high temperatures to the interior of the receiver and thus reduce surface losses. In this work, the solar vapor generation rates and efficiencies of several direct absorption solar collectors were compared by using reduced graphene oxide/graphene oxide and silver nanofluids as evaporators, two of which deserve to be noticed. One is the hybrid nanofluids containing reduced graphene oxide decorated with silver nanoparticles in volumetric solar absorbers and the other one is hybrid nanofluids containing reduced graphene oxide with silver floating on the surface, combining surface absorbers and volumetric ones. The results show that relative efficiency monotonously increases with the light density (e.g. correspondingly changes within 61.5% at 1 sun, 62% at 2 suns, 64% at 3 suns and 69% at 4 suns for 0.45 mg/ml reduced graphene oxide) and nanofluid concentration. The further experiments reveal that nanofluid-assisted vapor generation efficiencies of hybrid nanofluids containing reduced graphene oxide with silver floating on the surface were higher, reaching 91.6% at 3 suns, than the others due to its higher absorbance and plasmonic effect of the nanoparticles and high thermal conductivity of graphene nanosheets. This demonstration of hybrid nanofluids containing reduced graphene oxide with silver floating on the surface holds the promise of significantly expanding the potential applications in desalination, water treatment and power generation.