A coupled optical-thermal-fluid-mechanical analysis of parabolic trough solar receivers using supercritical CO₂ as heat transfer fluid

Supercritical CO₂ (S-CO₂) is an attractive heat transfer fluid (HTF) candidate for application in parabolic trough receivers (PTRs). The thermal and mechanical performances are of serious concern for the PTR using S-CO₂ as HTF which suffers from the rigorous conditions of high operating pressure and non-uniform solar flux distribution. The present study develops an integrated numerical model for the optical-thermal-fluid-mechanical analysis of PTRs by coupling Monte Carlo ray-tracing method, finite volume method, and finite element method. It is found that the non-uniform solar flux distribution can induce significant secondary flow of S-CO₂ which improves the synergy of the velocity vector and the temperature gradient in the fluid, benefiting the convective heat transfer. However, both of the high operating pressure and the non-uniform solar flux distribution can induce remarkable mechanical stress in PTR, and the superposition of the operating pressure and the solar flux distribution aggravates the mechanical stress. It should be noticed that the operating pressure has no contribution to the axial stress component, while the radial stress component induced by the non-uniform solar flux distribution can be neglected. Flattening the solar flux distribution by adding a secondary reflector is recommended as an effective approach to reducing the thermal stress, and the maximum thermal stress can be reduced from 50 MPa to 10 MPa under the conditions studied in this paper.