The efficacy of a parabolic trough solar collector (PTSC) was improved by using TiO 2 /DI-H 2 O (De-Ionized water) nanofluid. Working samples consisting of nanofluids with concentrations of 0.05%, 0.1%, 0.2%, 0.3% and 0.5% were compared with deionized water (the base fluid) at different flow rates under turbulent flow regimes (2850 ˂Re ˂ 7440). The experiments were designed as per ASHRAE 93 (2010) standards. Heat transfer and the flow characteristics of nanofluids through the collector were studied, and empirical correlations were developed in terms of the Nusselt number, friction factor, and performance index. The convective heat transfer coefficient was improved up to 23.84% by using TiO 2 nanofluids instead of the base fluid. It was found that TiO 2 nanofluid with a volume fraction of 0.3% (at a mass flow rate of 0.0689 kg/s) will provide the maximum efficiency enhancement in the PTSC (9.66% higher than the water-based collector). Consequently, the absorbed energy parameter was found to be 10.3% greater than that of the base fluid.

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Heat pipe-type evacuated tube solar collector (HP-ETSC) is one of the most efficient solar thermal collectors used to harness solar energy as it prevents various thermal losses. A heat pipe uses a working fluid in it so that, it transfers heat to water in heat exchangers. The Use of nanofluids (nanoparticle suspensions) instead of the normal working fluid in HP-ETSC is the current focus of research. Nanofluids help in increasing thermal conductivity, hence the efficiency of the ETSC heat pipes increases by great magnitudes. The main objectives of this investigation are to evaluate the performance of a thermosyphon-ETSC and to study the effect of using hybrid nanofluids of different concentration on the performance of the thermosyphon-ETSC. Three different percentages of titanium dioxide (TiO 2 ) + copper oxide (CuO) hybrid nanofluid were used and the composition were 0.5, 1, 1.5% v/v. The results of the experimental investigation are presented in this paper.