Performance improvements of solar evacuated tube collectors a literature review

Evacuated-tube solar collectors have been shown to be effective medium-temperature solar thermal collectors, with improved insulation and less heat loss than flat-plate collectors. To enhance their usage and performance, various enhancement techniques have been explored. This review comprehensively integrates experimental and numerical studies of the five most frequently mentioned enhancement techniques: nanofluids, phase-change materials, reflectors/concentrators, design modifications and hybrid methods. Comparative evaluation reveals average efficiency improvements of 38.19% for hybrid, 28.03% for design modifications, 24.29% for phase change materials, 22.84% for nanofluids, and 20.40% for reflector/concentrators. The efficiency ranges reported in the studies vary widely, due to the strong influence of collector geometry, material characteristics and testing procedures. Optical misalignment and shading losses of the concentrator, dispersion stability and nanofluid abrasion, and cycling durability and leakage in the case of phase change materials are key issues. Along with reviewing the performance data, this study highlights key inconsistencies in the definition of baselines, test protocols and reporting approaches that affect the inter-study comparability. The novelty of the review is the aggregated efficiency gains reported for each category, the reporting of bias and the research agenda, which emphasises addressing the limitations of standardised testing, durability over time, and techno-economic and life-cycle considerations. Closing such gaps is crucial to make evacuated tube collectors trendsetting at the laboratory scale, and therefore affordable, effective and scalable for renewable energy applications.