Photothermal performance analysis of a parabolic trough direct-absorption solar collector with nanofluids

This study investigates the photothermal performance of parabolic trough direct absorption solar collectors employing nanofluids under non-uniform solar irradiation. A coupled optical-thermal-flow model is developed by combining Monte Carlo ray tracing and the discrete ordinate methods. The coupled model resolves the photothermal conversion process in nanofluids with accurate source term representation and high computational efficiency. The model incorporates optical property calculations of metallic oxide nanoparticles based on scattering field solutions validated by Mie theory, while empirical correlations are used to determine nanofluid thermal properties. A finite element framework is employed to solve the coupled radiative and convective transport equations, enabling quantitative evaluation of photothermal conversion efficiency. Results indicate that over 96 % solar absorption is found for the 0.20 wt% nanofluid with a radiation propagation distance of 4.8 mm. The maximum thermal efficiency of 48.3 % is observed for 0.2 wt% nanofluid at 80 L/h with an inlet temperature of 291.15 K. The proposed model reduces the computational cost, exhibiting strong generalizability for various direct absorption solar collector configurations.