Experimental Investigation on Ag NPs-rGO-water/ethylene-glycol Hybrid Nanofluids Used in Solar Applications

In the current study, an experimental study on synthesis, characterization, stability and thermo-physical properties of silver nanoparticles - reduced graphene oxide dispersed in water and ethylene glycol mixture (Ag NPsrGO/W + EG) is presented. Ag NPs-rGO sheets (1:1) synthesized and characterized by SEM method were prepared using a mixture of water and ethylene glycol (W + EG (1:1)) as base fluid and low-viscosity sodium carboxymethyl cellulose (CMCNa) as surfactant. Dynamic light scattering (DLS) measurements were performed to estimate particle size distribution of the Ag NPs-rGO sheets in the nanofluids. Polydispersity index (PI) was also used to estimate the average uniformity of particle size in the hybrid nanofluids. The measurements of the thermo-physical properties were carried out at temperatures varying from 293.15 K to 323.15 K and concentrations of Ag NPs-rGO sheets of 0.050, 0.075 and 0.10 wt% respectively. The results indicated that the thermophysical properties of Ag NPs-rGO/W + EG hybrid nanofluids were generally dependent on concentration and temperature. The thermal conductivity was enhanced by about 1.13 times at 323.15 K and 0.10 wt%. The maximum increase in viscosity and density were 2.33 and 1.066 times compared to the base fluid at 293.15 K and 0.10 wt%. Based on the density measurements, the specific heat values of the Ag NPs-rGO/W + EG hybrid nanofluids were computed, recording a reduction up to 6.541 % at 323.15 K and 0.10 wt% compared to the water+ethylene glycol solution. The thermo-physical properties were compared to theoretical models and experimental results from literature. Based on the experimental data, new correlations for thermo-physical properties were established. Furthermore, this study analyzes the effectiveness of Ag NPs-rGO/W + EG hybrid nanofluids in a flat-plate solar collector (FPSC) in both laminar and turbulent flows by calculating the thermal efficiency.