This paper discusses the synthesis and characterization of a novel hybrid nanofluid and its performance analysis on a parabolic trough direct absorption solar collector. Broadening the absorption spectra of working fluid using nanoparticles is the new research revolution for increasing the volumetric solar absorption efficiency. It is reported that plasmonic silver nanoparticles have higher absorption in visible spectra while antimony doped tin oxide has an absorption peak in the near-infrared region. Hence, antimony tin oxide/silver hybrid nanoparticle with broad spectral absorptivity was synthesized. Optimization of the nanofluid composition performed using response surface methodology yielded an optimized mass fraction of antimony tin oxide and surfactant, sodium dodecyl sulfate, as 0.1% each. The solar weighted absorption fraction of optimized nanofluid was obtained as 90.12%. Performance evaluation of the solar collector was based on ASHRAE standards 93-2010. The optical efficiency of the parabolic collector was calculated to be 75%. The maximum thermal efficiency obtained by the optimized nanofluid applied parabolic trough direct absorption solar collector was 63.5% at a flow rate of 0.022 kgs-1 and the highest exergy efficiency obtained was 5.6%. Thermal and exergy efficiency was observed to increase with increase in flow rate.

Augmenting thermal and optical properties of working fluids used in solar thermal conversion systems using hybrid nanomaterials is gaining prominence. In the present study photo-thermal analysis and thermal conductivity investigations were performed on SiO2/Ag-CuO binary water based nanofluid. The influence of particle concentration and surfactant concentration on thermo-optical properties were investigated using the design of experiment concept. Analysis of variance (ANOVA) was employed to study the significance of the process parameters on thermal conductivity and solar weighted absorption fraction of nanofluid. The statistical optimisation of the process parameters was done using the desirability function. The optimum combination of nanoparticles and surfactant that yield good thermal conductivity and solar absorption was found to be SiO2/Ag: 206.3 mg/litre, CuO: 864.7 mg/litre, and SDS (surfactant): 1996.2 mg/litre. The optimum mass fraction of constituents yielded a relative thermal conductivity of 1.234 and solar weighted absorption fraction of 82.82 %.

The evolution of nanofluids over the years have opened new research opportunities in the field of renewable energy. Research on the optical properties of nanofluids for application in direct absorption solar collectors (DASCs) is progressing at a burgeoning speed. In a direct absorption solar collector system, nanofluid with high optical absorptivity will convert the incident solar energy to the thermal energy of the fluid. The dispersed nanoparticles in the fluid acts in the process by the phenomenon of absorption and scattering. Studies conducted on the optical property characterization of monocomponent nanofluids have become saturated. Moreover, the photothermal efficiency of the nanofluid could be enhanced by using multicomponent nanofluids. Nanofluid prepared by varying materials, shapes, and size of nanoparticles could tune the absorption spectra of the bulk fluid to improve the photothermal efficiency. A hybrid nanocomposite can similarly enhance the absorptivity due to the synergy of materials present in the nanocomposite particle. In this review, a comprehensive survey on the synthesis and optical characterization of different blended and hybrid nanocomposite nanofluids have been performed. Besides, works on different mono nanofluids that have been studied over the years have been reviewed.

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Experimental investigations on the application of SiO2/Ag-CuO plasmonic nanofluid on direct/volumetric absorption parabolic solar collectors is presented in this article. The process variables for the preparation of nanofluid were optimised by employing the desirability function and response surface methodology (RSM). The optimisation was performed to achieve nanofluid with maximum possible thermal conductivity and solar absorptivity. The final solar radiation absorbed fraction and relative thermal conductivity noted for the optimised nanofluid was 82.84% and 1.234, respectively. The performance of the collector was evaluated at various flow rates from 60 lph to 90 lph, using water and optimised nanofluid as the heat transfer fluid. It is noted from the results that the thermal efficiency of the collector increases with the flow rate whereas, the exergy efficiency decreases for both water and nanofluid. The highest temperature difference of 11.27K was noted at 60lph for nanofluid which corresponds to a thermal efficiency of 57.47%. A maximum thermal efficiency of 64.05% was noted at 90 lph which corresponds to an enhancement of 48.19 % in comparison with water. Exergy efficiency of the nanofluid was enhanced by 9.4% at 60 lph, in comparison with water.