Non-linear finite element modelling of light-to-heat energy conversion applied to solar nanofluids

Abstract

Nanoparticles (NPs) exhibit remarkable photothermal conversion efficiency under optical illumination. This light-induced heating on NPs is interesting in many different applications, such as solar radiation absorption in nanofluids, which the present work focuses on. Consequently, mastering the temperature increase undergone by NPs and the surrounding media is extremely relevant today. As nanothermometry measurements of a single NP are hard to obtain, numerical simulations can contribute to better understand the physical phenomena involved in light-induced heating. In this vein, the current work presents theoretical and numerical formulations to predict the heating of optically excited NPs. Theoretically, a thermodynamic approach is conducted to obtain balance and constitutive equations. These equations are numerically discretised in the finite element method and implemented into a research code. The main novelty of the present work lies in developing, from a multiphysics perspective, a time domain formulation capable of modelling instantaneous dissipation that can be easily extended to account for more physical phenomena. Finally, the numerical model is validated by comparing analytical and numerical results, and maximum values of 0.0014 (%) of relative error between them are reached. Then some different analysis are performed for gold, silver and graphite NPs of 20 (nm) in diameter to characterise the temperature increase they produce in the surrounding medium (water) when optically excited at a wavelength of 400 (nm) and a laser intensity of 5 × 104(W/cm2) –silver NPs exhibiting the most significant temperature increase. The influence of NP concentration on the increase of temperature in nanofluids is numerically assessed as well by testing values of NP concentration up to a maximum of 0.052 (%), which considerably enhances temperature increase. In conclusion, the present numerical tool could be used to predict light-induced heating in NPs, which could complement and reduce the number of experiments for optimising the photothermal efficiency of solar nanofluids.