Optimization of a large-aperture parabolic trough solar system with threestage variable-diameter absorber tubes for enhanced efficiency and high outlet temperature

• A collector tube was formed by increasing plate reflector. • The novel tubes with variable diameters were made into 3-stages large-aperture PTCs. • Single loop length is 1314.5 m, shorted by >71.5 m. • Optical efficiency is 79.3 %, increased by >3.1 %. • Thermal efficiency is 62.9–72.8 %, increased by >4 %. In large-aperture parabolic trough solar power plants, improving the efficiency of heat collection elements remains a critical research challenge. This study proposes an integrated system consisting of a novel vacuum absorber tube combined with a flat secondary reflector, configured within a three-stage heating structure to form a single-loop parabolic trough solar collector system. The system includes three sequentially arranged absorber sections with diameters decreasing from 90 mm to 80 mm and finally to 70 mm, coupled with an 8 m-wide aperture concentrator. Under direct normal irradiance levels ranging from 400 to 1000 W/m², the system achieves an optical efficiency of 79.3 % and a thermal efficiency between 62.9 % and 72.8 %. The total loop length is 1314.5 m, representing a reduction of 71.5 m (5.2 %) compared to a conventional system using an optimal 90 mm absorber tube. Furthermore, the optical and thermal efficiencies are improved by 3.1 % and 4.0 %, respectively. Additionally, the system enables the heat transfer fluid to reach an outlet temperature exceeding 560 °C, significantly enhancing its applicability in high-efficiency power cycles. These results demonstrate the potential of the proposed multi-stage variable-diameter design to simultaneously enhance optical performance, thermal output, and economic feasibility in next-generation concentrated solar power applications.