Numerical study of a nanofluid-based receiver for linear Fresnel collectors

Abstract

In concentrated solar technologies, the use of volumetric receivers instead of surface receivers may help to achieve higher solar collection efficiencies. This paper reports a simplified model that computes the concentrated solar radiation absorption and energy transport by advection in the volumetric receiver of a linear Fresnel collector. The proposed linear Fresnel collector is intended for use in industrial solar heat processes. The receiver consists of a rectangular channel whereby flows a nanofluid composed of thermal oil and graphite nanoparticles. The bottom wall of the channel is a glass, while the upper wall is a highly reflective surface that is well insulated. The light absorption into the nanofluid was solved using the Rayleigh dispersion, while the convection was solved with a simplified velocity profile for turbulent flow (power law) and incorporating a radiative source term in the energy equation. All equations were discretized by using finite volumes and were coded in Python language. Receiver efficiencies in the range of 92–96% were found for temperatures of 403–343K. The results show that the volumetric fraction (optical depth) drives the way the radiation is absorpted, and in consequence, the temperature profile and the receiver efficiency. For low optical depths, the solar absorption is low, resulting in high reflected radiation losses. Contrarily, high optical depths mean high solar absorption, consequently, the temperature close to the glass increases, and the convection losses are higher. The results suggest that values of optical depth close to 2.3 are optimum to obtain the highest receiver efficiency.