A review on different profiles of absorber tube in parabolic trough solar collectors

Parabolic trough solar collectors (PTSCs) represent one of the most widely used concentrated solar energy technologies; however, their overall efficiency remains constrained by thermal losses and limited convective heat transfer within the absorber tube. Although extensive research has focused on advanced working fluids and system-level optimization, absorber tube geometric modification as a passive, scalable enhancement strategy has not been systematically and comparatively synthesized. This study provides a geometry-centered, design-oriented review of recent experimental and numerical studies on modified absorber tube profiles. Enhancement strategies are classified into four main categories: internal inserts, finned tubes, structured wall modifications, and non-conventional geometries, establishing a structured comparative framework absent in existing PTSC reviews. For each category, impacts on thermal efficiency, Nusselt number, pressure drop penalty, and overall thermo-hydraulic performance are critically evaluated. A comparative synthesis is conducted to identify representative performance ranges and typical trade-offs, rather than isolated peak values. In addition, a publication trend analysis of studies is conducted to reveal a growing shift toward hybrid enhancement approaches integrating geometric modification with nanofluids and phase change materials (PCMs). This review identifies critical gaps related to techno-economic feasibility, long term durability under cyclic thermal loading, and scalability, underscoring the need for multidisciplinary optimization frameworks to bridge laboratory-scale innovation and industrial PTSC deployment.