Improving the Performance of Direct Solar Collectors and Stills by Controlling the Morphology and Size of Plasmonic Core–Shell Nanoheaters

Abstract

Plasmonic nanofluids, based on metal nanoparticles (NPs), has received tremendous attention for its potential to increase the efficiency of solar energy harvesting and harnessing systems. The ability to manage their optical absorption by tuning localized surface plasmon (LSP) bands is the reason why metal NPs are considered excellent nanoheaters with unique thermo-optic properties. In this work, we demostrate the influence of the tuning of plasmonic nanofluid absorption bands in different spectral regions, by modifying the morphology and size of the core–shell NPs, on the efficiency of plasmonic nanoheaters to heat fluids and generate steam. Five plasmonic nanofluids containing spherical Au@SiO2, rodlike Au@SiO2, with three different aspect ratios, and spherical SiO2@Au nanoshells were fabricated and characterized to study the local heating induced by plasmon-enhanced light absorption. Gains of up to 28.3 times in the nanofluid temperature increase in direct absorption solar collectors (DASCs) and 7.5 times in the amount of steam generated in the solar ethanol distillation were measured from control over LSP resonances of spherical and rodlike core–shell NPs. Energy distribution analysis shows that plasmonic nanofluids present an efficient energy transfer management, dedicating ∼72% of the absorbed energy to heating liquids at low levels of solar irradiance. However, at high solar irradiances, the good spectral matching between the plasmonic nanofluid LSP bands and the solar irradiance spectrum promotes strong local heating around the core–shell NPs, allowing local temperatures above the boiling point to be reached. Under these conditions, plasmonic nanofluids spend a small amount of energy to heat liquids and they transfer ∼83% of the absorbed energy to generate steam. Thus, a 7.7-fold increase in solar ethanol vaporization rate was achieved. The experimental results, understood from the optical properties of core–shell plasmonic NPs by using the Maxwell–Garnett theoretical model, corroborate the importance of fabricating nanoheaters with projected geometries to maximize the efficiency of solar collectors and stills.