Thermal Performance Enhancement of Solar Collectors Using Al₂O₃-TiO₂ Hybrid Nanofluids

Solar thermal energy collectors represent one of the most economically viable and deployable renewable energy technologies for India's climate context, with solar irradiance levels exceeding 5.5 kWh/m²/day across most of the Indian subcontinent. A critical constraint on solar thermal system efficiency is the limited thermal conductivity of conventional heat transfer fluids — water and thermic oil — which determines the rate of heat acquisition from the absorber plate to the working fluid and consequently the achievable system efficiency. Nanofluids — engineered suspensions of nanoparticles (1–100 nm) in base fluids — offer significantly enhanced thermal conductivity, specific heat capacity, and convective heat transfer coefficients compared to base fluids, potentially improving solar collector thermal efficiency by 5–25% depending on nanoparticle type, concentration, and collector configuration.This study presents a comprehensive experimental and computational investigation of Al₂O₃/water, TiO₂/water, and Al₂O₃-TiO₂ hybrid nanofluids across three solar collector configurations — flat plate collector (FPC), evacuated tube collector (ETC), and parabolic trough collector (PTC) — at five mass flow rates (0.5–2.5 L/min) and three nanoparticle volume concentrations (0.5%, 1.0%, 2.0%). Taguchi L16 orthogonal array design systematically identifies optimal parameter combinations for maximum thermal efficiency. CFD simulation using ANSYS Fluent 2023R1 with realizable k-ε turbulence model and discrete phase modelling validates experimental results. The Al₂O₃-TiO₂ hybrid nanofluid (1.5% concentration) in the PTC configuration achieves the highest thermal efficiency of 83.7% — a 22.5% improvement over base water in the same collector — while the thermo-economic analysis confirms superior performance factor (PF = ηth/ΔP) for ETC-hybrid combination at 2.0 L/min flow rate.