Advancing sustainable solar thermal energy: A comprehensive review of mono, hybrid, and ternary nanofluids in evacuated tube solar collectors

This is a thorough systematic review of the research on the usage of mono, hybrid and ternary Nanofluids (NFs) in Evacuated Tube Solar Collectors (ETSCs) through the synthesis of recent published studies between 2020 to 2026 as per the Preferred Reporting Items of Systematic Reviews and Meta-Analyses (PRISMA) guidelines. It is shown that Mono Nanofluids (MNFs), including multi-walled carbon nanotubes (MWCNT)/Water and Al 2 O 3 /Water obtained 73.5% of energy efficiency and from 26.4% to 55.83% of exergy efficiency depending on the utilised base fluids; Hybrid Nanofluids (HNFs) such as FeO 4 /MWCNT and MgO/MWCNT. Also, Ternary Nanofluids (TNFs), such as MWCNT/Al 2 O 3 /MgO and TiO 2 /Al 2 O 3 /ZnO provide the best performance with a thermal efficiency of 86.9% and with higher heat absorption rates than mono and hybrid counterparts by 56.15%. The review also establishes that these gains can be synergistically combined through modification of the system, e.g. porous media, longitudinal fins, and mechanical vibrations. Nevertheless, the residual weaknesses lie in the areas of standardized stability procedures, long-term stability and reliability in the real world environments, as well as the full life cycle analysis. The results verify that the NF-based ETSCs can be optimised to deliver specific thermal efficiency beyond 85%, making them as advanced, sustainable heating technology of decarbonized heating that can be applied to scalability in the future as long as the research tackles the issue of economic viability and the design of integrated multi-physics modeling systems.