Tailored silver/graphene nanoplatelet hybrid nanofluids for solar applications

Hybrid nanofluids, suspensions of nanocomposites or nanoparticle-decorated complex structures in conventional fluids, are focusing the attention of the research in the recent years because of the potential synergistic effects among particles. Nevertheless, deep studies based on systematic experimental investigations are missing. Thus, in this work, a propylene glycol:water 10:90 wt% mixture was proposed as base fluid while various ratios (0:1, 1:4, 1:1, 4:1 and 1:0) between the constituents present in the hybrid nanoadditive (silver/graphene nanoplatelet with different functionalizations) were swept. Eleven nanofluids with three total nanoadditive concentrations of 0.05, 0.1 and 1 wt% were designed. The stability of the samples was assessed by dynamic light scattering measurements over the time. Experimental optical, rheological and thermophysical profiles jointly were analysed with the aim to produce a comprehensive elucidation of the new hybrid nanofluids. Optical properties were investigated through optical spectroscopy, rheological properties through rotational rheometry, thermal conductivity by means of transient hot-wire method, and density through oscillating U-tube technique. The optical transmittance results for all analysed nanofluids confirmed that more than 90% sunlight extinction is achieved for a path length lower or equal to 6 mm. Its spatial distribution in the nanofluid volume can be tailored acting on the concentration of each nanoadditive, allowing the design of customized miniaturized systems. Newtonian behaviour was evidenced for all samples, with viscosity increases lower that 9% for all 0.1 wt% nanofluids, those with optical applications. Thermal conductivity results showed practically no variations with respect the base fluid for all samples with this total concentration.