Thermal–Hydraulic–Exergy Trade-offs in Nanofluid–Passive Turbulator Receivers of Parabolic Trough Solar Collectors: An Evidence-Harmonized Critical Review

Abstract

Nanofluids and passive turbulators are increasingly used in parabolic trough solar collectors (PTSCs) for improving the heat transfer at the receiver side. Nevertheless, some of these gains have been reported as much greater as a result of variations in operating conditions, receiver geometry, reference configurations, and performance measures. In this critical review, the thermal–hydraulic–exergy trade-offs of combinations of nanofluids and turbulators are analysed with the help of evidence–harmonized critical review (EHCR) framework. The framework is based on four assessment tools: Evidence Grading (A–D), a Baseline Equivalence Test (BET), a Comparability Index (CI), and a Conflict Ledger. The tools evaluate the quality of evidence, consistency of baselines, comparability of studies, and conflicting evidence for selected experimental and numerical investigations. The synthesis shows that many configurations to enhance heat transfer lead to a corresponding increase in hydraulic resistance, and configurations that maximize the Nusselt number do not necessarily maximize the performance evaluation criterion (PEC) or exergy efficiency. Recent studies also support the evidence that the conditions under which the thermal, thermo-hydraulic, and exergy optima occur may differ. Thus, claims to universal superiority are not substantiated when there are no equivalent bases, specific goals, and conditions of operation. In addition, if there is any combined enhancement, it cannot be confirmed as a true synergistic effect unless four reference configurations are performed: base-fluid/smooth, nanofluid/smooth, base-fluid/turbulator, and nanofluid/turbulator. The matched-control experiments, uncertainty quantification, nanofluid stability, pumping-power evaluation, system-level economic and environmental evaluation, and experimentally verified multi-objective optimization should be emphasized for future investigations. This framework can give a well-structured framework to interpret the reported improvements and to identify transferable enhancement strategies.