Experimental study of the optical properties of nanofluids as potential candidates for direct-absorption solar energy applications

Nanofluids, which are colloidal suspensions of solid nanoparticles in conventional heat transfer fluids, distinguish themselves for their peculiar optical properties: in fact, even a very little volume of nanoparticles can significantly enhance the sunlight absorption capability of the base fluids, making nanofluids interesting candidates for different solar energy technologies. In particular, the present work investigated the optical properties of different nanofluids, trying to assess their potential for two applications: Direct Absorption Solar Collectors (DASCs), and Direct Solar Steam Generation (DSSG). The experimental research took place at the National Institute of Optics, National Research Council of Italy (CNR-INO) in Florence, Italy. Concerning the first application, the study was focused on the measurement of the spectral extinction coefficient of different types of nanofluids, by means of a double beam spectrophotometer featuring a spectral range of about 190÷3300 nm. The nanofluid samples contained carbon based, ceramic, and iron oxides nanoparticles, with mass concentrations ranging from 0.005% to 0.1%. As for the second application, the experiments involved the use of a solid state Nd:YAG laser to irradiate the samples with high intensity pulses at 355, 532, and 1064 nm. These high levels of input energy allowed to evaluate whether the nanofluid samples exhibited a nonlinear behavior, i.e., whether their transmittance began to drop after a threshold value of the incident energy itself: in literature, this kind of behavior is sometimes ascribed to nonlinear scattering of light due to vapor bubbles surrounding the nanoparticles. Finally, the present work applied the optical properties of some of the previously investigated nanofluids to a numerical analysis, with the purpose of evaluating the temperature distribution inside a cylindrical enclosure containing a nanofluid, exposed to electromagnetic radiation coming from the top. Using the finite elements software COMSOL Multiphysics®, it was possible to obtain some preliminary results which may help to identify the most relevant nanofluids’ parameters affecting the temperature distribution.