PERFORMANCE ANALYSIS AND THERMAL IMPROVEMENT OF PARABOLIC TROUGH COLLECTORS

Abstract

Parabolic trough collectors (PTCs) are among the most widely utilized concentrating solar power technologies for thermal energy generation. However, their performance is limited by heat losses from the absorber tube and by relatively low internal convective heat transfer rates. Improving receiver thermal performance is therefore essential for increasing collector efficiency and reducing the cost of solar thermal energy production.Several approaches for improving the thermal performance of a parabolic trough collector were investigated through analytical modeling and Computational Fluid Dynamics (CFD) simulations. First, a comprehensive thermal model was developed to describe the heat transfer mechanisms occurring within the receiver, including solar energy absorption, conduction through the absorber wall, convection to the heat transfer fluid, and thermal losses to the environment. A three-dimensional CFD model was then developed in STAR-CCM+ and validated against published experimental data by Bharti et al. [1] for both bare and acrylic-covered absorber configurations operating at mass flow rates of 0.01 kg/s and 0.02 kg/s. Good agreement between numerical predictions and experimental measurements confirmed the accuracy of the modeling methodology and established a reliable baseline for evaluating enhanced receiver designs. Three receiver enhancement techniques were subsequently investigated: spiral absorber tubes, annulus vacuum insulation, and twisted tape inserts. The spiral absorber tube study examined the influence of coil diameter ratio and pitch ratio on heat transfer performance. The spiral geometry generated secondary flow structures that enhanced fluid mixing and increased heat transfer between the absorber wall and the working fluid. The best-performing spiral configuration increased collector thermal efficiency from 28.29% to 45.61% for the acrylic-covered receiver operating at 0.01 kg/s and from 23.13% to 42.58% at 0.02 kg/s, demonstrating that spiral absorbers provide a highly effective passive heat transfer enhancement technique. The second phase of the investigation evaluated the effect of vacuum insulation within the annular space between the absorber tube and the transparent envelope. Different vacuum levels and annulus geometries were examined to quantify their impact on thermal losses. The results showed that increasing the vacuum level reduced convective heat transfer across the annulus and improved collector efficiency. Complete vacuum conditions increased thermal efficiency from 28.29% to 30.90% at 0.01 kg/s and from 23.13% to 25.27% at 0.02 kg/s without introducing additional hydraulic losses. The third phase investigated twisted tape inserts as passive enhancement devices. Parametric studies were conducted to evaluate the effects of insert length, twist ratio, width ratio, and number of inserts on thermal and hydraulic performance. While twisted tape inserts promoted additional fluid mixing, the corresponding improvements in outlet temperature and thermal efficiency were relatively small compared with the associated increases in pressure drop. Consequently, twisted tape inserts were found to be less effective than spiral absorber geometries and vacuum insulation for improving overall collector performance. The results demonstrated that all enhancement techniques influenced receiver thermal behavior; however, spiral absorber tubes provided the most significant performance improvements. Vacuum insulation offered moderate thermal benefits without hydraulic penalties, while twisted tape inserts primarily increased pressure losses with limited thermal gains. Among the investigated designs, the spiral absorber configuration produced the highest thermal efficiency and showed the greatest potential for improving parabolic trough collector performance. This work contributes validated analytical and CFD methodologies for PTC analysis and provides a comprehensive evaluation of practical receiver enhancement strategies. The findings offer guidance for the design of more efficient solar thermal collectors and support the continued development of advanced concentrating solar power technologies.