Enhancing optical and thermohydraulic performance of linear Fresnel collectors with receiver tube enhancers: A numerical study

Abstract

Linear Fresnel collector system is a type of solar thermal utilization installation. It is mainly used to focus solar radiation on the receiver tube to generate high temperature thermal energy which can be applied in many aspects. This study investigates five different types of receiver tube models using water as the flowing medium, which leads to improving heat transfer performance by means of being equipped with enhancers. The optical results and thermohydraulic performance of the Linear Fresnel collector system are evaluated through Monte Carlo Ray Tracing and Computational Fluid Dynamics (CFD) simulation methods. The obtained optical results reveal that the distribution of energy flux density concentrating on the receiver tube surpasses that of the parabolic trough collector system. In order to maintain an energy reception rate of 90.0% or higher, the installation height range of the receiver tube should be between 1468–1532 mm. When the sun-tracking error increases from 0.0° to 1.0°, the overall optical efficiency of the Linear Fresnel collector system decreases from 98.9% to 79.7%. The thermohydraulic performance shows that the enhancers can generate a swirling flow as its main feature, which effectively promotes the homogenization of the temperature field and destroys the boundary layer near the receiver tube wall, thus greatly improving the convective heat transfer in the receiver tube. However, the introduction of the enhancers also means that the pressure drop loss of the receiver tube increases. Furthermore, the impact of the Kenics mixer's enhancer on heat transfer and flow characteristics is further amplified as the rotation rate decreases, owing to its alternating and complex structure. Through a comparative analysis of the influence of different receiver tube models on Nusselt number (Nu), Frictional resistance coefficient (f) and comprehensive evaluation criterion (PEC) under four rotation rate conditions (y = 6.3, 8.1, 11.34, 18.9), the conclusions present that the receiver tube with an enhancer at a low rotation rate has a larger Nu than that at a high rotation rate, while the Frictional resistance coefficient changes little at different rotation rates. Therefore, the PEC is higher when the receiver tube is equipped with a low-rotation-rate enhancer. Moreover, the receiver tube equipped with a Kenics mixer at a rotation rate of 6.3 was found to be the overall optimum case, with the PEC value varying from 1.47 to 1.73.