Influence of optical constants in carbon-based dispersions for enhanced solar evaporation

Abstract

Carbon-based nano- and microfluid dispersions show promise as effective media for solar-assisted desalination. They act as strong solar absorbers in the base fluid and increase its temperature by volumetrically heating the bulk of the fluid, leading to faster vapor generation. This radiative heating depends on the fraction of the solar radiation absorbed by the fluid dispersion system. This in turn depends on the optical constants, i.e., refractive index and attenuation (absorption) coefficient of the particles. Different carbon materials exhibit different values of the optical constants when exposed to the solar spectrum and so, the evaporation efficiency is different for different particles. An attempt is made to explore the influence of these optical constants on the evaporation rate and evaporation efficiency of water. The values chosen are within the upper and lower bounds of the refractive index ( $$1\le n \le 3$$ ) and attenuation coefficient ( $$0.001\le k \le 1$$ ), which are typical of carbon-based materials. It was found that the evaporation rate is higher for large attenuation coefficient for any refractive index. Also, at a relative refractive index of 1, the solar absorption coefficient is comparatively smaller than for other values, due to lower scattering efficiency. At high refractive index, the evaporation efficiency remained almost constant for volume fractions less than $$10^{-4}$$ (or 0.01%). In summary, water evaporation rate and evaporation efficiency are strongly influenced by the optical constants.