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Summary Hybrid nanofluids with Therminol‐55 as base fluid and TiO2 and Al2O3 as nano additives are investigated for parabolic trough collectors. Stable mono nanofluids based on Therminol‐55 with Al2O3 and TiO2 nanoparticles, and hybrid Al2O3–TiO2 nanopowder as additives are prepared by the two‐step method. Oleic acid has been made as a surfactant. The impact of nanoparticle content and fluid temperature on thermal conductivity was studied using the transient hot‐wire technique. Thermal conductivity exhibited an increase with an increase in both temperature and nanoparticle concentration. Al2O3–TiO2 Therminol‐55 hybrid nanofluids exhibit the highest increase of 33.5% in thermal conductivity followed by 27.06% in Al2O3–Therminol‐55, and lowest rise of 21% is found in TiO2–Therminol‐55 nanofluids. Synergetic effect of Al2O3 and TiO2 nanoparticles, increased Brownian motion, and filling of vacant spaces by smaller‐sized TiO2 nanoparticles in Al2O3–TiO2 hybrid nanofluids lead to effective thermal network for enhanced thermal conductivity. Maximum thermal conductivity of 0.15 W/m‐K is observed in the case of hybrid Al2O3–TiO2 Therminol‐55 nanofluid with 0.5 wt% concentration. Augmented thermal conductivity of HNF's leads to the efficient photothermal conversion in concentrating solar collectors. Further, a correlation for thermal conductivity with variation in nanoparticle content and the fluid temperature is proposed with R2 = 0.92.

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