Enhancing the Thermal Efficiency of Longitudinally Finned Parabolic Trough Solar Receivers

The objective of this study was to investigate the conjugate heat transfer in a longitudinally finned parabolic trough solar thermal receiver tube, taking into account the non-uniform heat flux distribution along the tube circumference. Both smooth and longitudinally finned stainless steel receiver tubes, with Syltherm 800 oil as the heat transfer fluid, were numerically simulated using Ansys Fluent 2023R2 with periodic boundary conditions for Reynolds numbers ranging from 20,000 to 60,000. Two fin configurations were investigated in the study: two fins - of little practical interest but very useful to gain physical insight into the problem - and six fin receiver tubes. In addition, sub-cases within the two- and six-fin configurations were analyzed by varying the position and height of the fins. It was found that fins on the bottom side, exposed to a higher heat flux, mainly enhanced conduction, while fins on the top side mainly enhanced convection by directing higher velocity values toward the bottom side. To reduce hot spots and improve thermal efficiency, fins should be located in higher heat flux areas, and shorter fins on the bottom side and longer fins on the top side are recommended. Finally, the performance evaluation criteria were assessed using both the inner Nusselt number, based on the convective heat transfer coefficient as commonly reported in the literature, and an overall Nusselt number, based on the total heat transfer resistance of the receiver. For the most favorable two-fin and six-fin cases, the maximum temperature in the receiver decreased by a maximum of 16.7 K and 30 K, respectively.