Thermal performance analysis of a parabolic trough solar collector using supercritical CO₂ as heat transfer fluid under non-uniform solar flux

In this paper, an integrated numerical model was established to solve the complex energy transfer in a parabolic trough collector (PTC) by combining Monte Carlo ray tracing and finite volume method. Based on the model, the thermal performance of the PTC using supercritical CO₂(s-CO₂) as the heat transfer fluid (HTF) was studied and analyzed. The results indicate that the solar fluxes on the receiver walls are non-uniform, which results in the non-uniform temperatures on the walls and in the s-CO₂. Moreover, the optical efficiency of 84.19% is achieved at normal incidence. The circumferential temperature difference (ΔT_c) of the absorber is found to be within 18–60 K under typical conditions, and it decreases with increasing inlet velocity (v_in) and decreasing inlet temperature (T_in). Studies on the flow and heat transfer characteristic indicate that the secondary flow occurs on the cross-section due to the buoyancy. The velocity of the secondary flow in the pseudo-critical region can be one magnitude order larger than that in the high temperature region, which leads to strong heat transfer enhancement in the former. Studies on the energy conversion performance indicate that the PTC can achieve the collector efficiencies of 18.78–84.17% and 81.93–84.17% at the typical conditions for Brayton and Rankine cycles, respectively, where the efficiency increases with decreasing T_inand increasing v_in. Additionally, corresponding efficiency equations have also been obtained. The information from this study will provide a reference for the design and operation of the PTCs using s-CO₂as HTF.