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Exergy efficiency, Nusselt number, friction factor, pressure drop, thermal and frictional entropy generation of water-based nanodiamond + Fe3O4 nanofluid flow in a tube and with various coiled wire inserts have been studied experimentally under turbulent and constant heat flux boundary conditions. The experiments were conducted in the Reynolds number range from 2000 to 22,000, particle concentrations of 0.05%, 0.1% and 0.2% and coiled wire inserts of different $$p/d$$ values of 3.67, 2.34 and 1.00, respectively. Results indicate that at 0.2% vol. and Reynolds number of 20,095, without coiled wire inserts, the heat transfer coefficient, Nusselt number, friction factor, pressure drop and pumping power are enhanced to 44.36%, 29.55%, 11.1%, 29.58% and 39.49% over the base fluid data. Similarly, at 0.2% vol. and Reynolds number of 20,095, with coiled wire inserts of $$p/d$$ = 1, the heat transfer coefficient, Nusselt number, friction factor, pressure drop and pumping power are further enhanced to 107.19%, 66.36%, 38.84, 64.44% and 76.54% over the base fluid data without inserts. The thermal entropy generation is decreased to 30.80% and it is further decreased to 46.34% at 0.2% vol. and Reynolds number of 20,095 with coiled wire inserts of $$p/d$$ = 1. The exergy efficiency of water is 18.95%, and it is increased to 24.06% for 0.2% vol. and it is further increased to 51.85% for 0.2% vol. and Reynolds number of 20,095 with coiled wire inserts of $$p/d$$ = 1. The study indicates that the hybrid nanofluids with coiled wire inserts are guaranteed choice for augmenting the exergy efficiency of flow through tube.

Energy demand is high in all parts of the world, mostly in all industrial sectors. To meet the energy demand the fossil fuel is the only way. Due to rapid industrial growth and use of fossil fuel result in global warming and environmental pollution. Moreover, the limited availability of the fossil fuels, it is necessary to depend on the renewable energy sources. Promising renewable energy in the world is solar energy, which is available largely on the earth surface. The solar energy can be converted into thermal energy in the solar flat plate collector. The collector thermal efficiency is purely depends on the working fluid used in it. Most of the studies revealed that replacing the working fluid with high thermal conductivity fluids called as nanofluids and hybrid nanofluids can improve the collector thermal efficiency. Few decades back studies have been conducted with nanofluids in solar collectors. Currently the researchers are working on solar collectors for further improvement of its efficiency using hybrid nanofluids. In this review paper, we will discuss about the synthesis of hybrid nanoparticles, hybrid nanofluids, characterization, thermophysical properties, and application of hybrid nanofluids in solar flat plate collector under natural and forced circulation of fluid. The research gap in the solar collector is also discussed in this article. This paper also explains about the heat transfer capabilities of hybrid nanofluids especially used solar collectors.

An experimental investigation was conducted aiming to determine the heat transfer, friction and instantaneous collector thermal efficiency of a thermosyphon (natural circulation) solar water heating system using as working fluids of water and the Cu/H2O nanofluid. The Cu nanoparticles were synthesized using the chemical reduction method and characterized by the x-ray diffraction and transmission electron microscopy techniques. The stable Cu/H2O nanofluid was prepared for the volume concentrations of 0.1% and 0.3%. The empirical correlations developed for Nusselt number and friction factor for the Cu/H2O nanofluid fit the experimental data with a deviation of less than ±3.5% and ±2.5%, respectively. The results of present experimental investigation were presented at various Reynolds number and particle volume concentrations under thermosyphon conditions. The comparison indicates that the heat transfer enhancement obtained with the Cu/H2O nanofluid for the thermosyphon is higher than that for the plain tube collector and increases with the increase of particle volume concentration. The overall thermal performance of the thermosyphon increases when the operating fluid is Cu/H2O nanofluid as compared to water.

The thermal effectiveness of solar water heaters can be enhanced if passive heat transfer enhancement techniques are used. Among the most effective passive heat transfer enhancement techniques are the increase of the working fluid thermal conductivity and of its flow turbulence. In this paper, Al2O3 nanofluids and twisted tape inserts are the passive techniques used to enhance the heat transfer and, consequently the thermal effectiveness of the solar water heater. In the solar water heating system considered in this study, the collector is essentially mimicked by a tube with or without a twisted tape, with water or nanofluids flowing through it. Results of the heat transfer experiments indicate that for a Reynolds number of 13000 the heat transfer enhancement for 0.3% volume concentration of nanofluid is 21% for the plain tube and it is further enhanced to 49.75% when a twisted tape of H/D = 5 is inserted in the tube. The maximum friction penalty of 1.25-times was observed for 0.3% nanofluid with H/D = 5 when compared to water in a plain collector. The thermal effectiveness of the plain collector is enhanced to 58%, when the 0.3% nanofluid is used and it is further enhanced to 76% with a twisted tape of H/D = 5 at a mass flow rate of 0.083 kg/s. Solar water heaters, in which the collectors have twisted tape inserts and use nanofluids, have thermal performance increases that largely outweigh pressure drop losses. Under the same operating conditions, the nanofluids/twisted tape inserts collector outperforms that with water and no twisted tapes.

The collector efficiency, heat transfer, energy, and environmental emissions of water -based nanodiamond (ND) nanofluids circulate in flat plate solar collector (FPSC) has been analyzed experimentally. The analysis was performed at 0.2%, 0.4%, 0.6%, 0.8% and 1.0% particle concentrations, the stability and thermo-physical properties were also analyzed. The thermal conductivity and viscosity enhancements are 22.86% and 79.16% at particle loading of 1.0% against water data at 60 °C. The water and 1.0 vol% of ND/water nanofluids in collector shows efficiencies of 53% and 74%, respectively, which leads to 39.62% enhancement. The collector area was decreased to 28.66% with the use of 1.0 vol% of nanofluid against water data. The collector weight is decreased to 35.81 kg, at 1.0% particle loading, whereas the collector with water weighed 50 kg. The embodied energy of 1.0 vol% of nanofluid is decreased to 1039.51 MJ, whereas, the collector with water its embodied energy is 1451.4 MJ. The CO2 emissions were noticed as 249.98 kg by using 1.0 vol% of nanofluid in FPSC. Additionally, the Nusselt number and heat transfer is augmented to 32.31% and 52.33% at 1.0% vol% with a friction factor penalty of 1.26-times at a Reynolds number of 12,766 against water data. The obtained data is validated with literature and the general form of regression equations was developed in order to evaluate the Nusselt number and friction factor.

The energy and cost saving of flat plate collector using Al2O3/water nanofluids and with wire coil with core rod inserts were studied experimentally. The experiments were conducted for various particle loadings of nanofluid (0.1%, 0.2% and 0.3%) and nanofluid with various p/d values of wire coil with core-rod inserts (p/d = 1.79, 2.54 and 3.24). The potential increase of collector efficiency is converted into useful size reduction, weight reduction, cost, and energy saving of the collector. With the use of nanofluids in the collector its efficiency is enhanced. The maximum collector efficiency is 37.73% for particle loading of 0.3% in the collector at a flow rate of 300 lit/hr in validation with water collector. By providing wire coil with core-rod inserts, p/d value of 1.79 with 0.3% particle loading of nanofluid, the collector efficiency is further incremented to 64.15% at a flow rate of 300 lit/hr. Meanwhile, at the same experimental conditions, using 0.3% nanofluid, a maximum reduction in the collector area is 27.66%, and it is reduced to 39.33% for nanofluid of 0.3% with p/d of 1.79 insert. The original cost of the water collector is 223.88$, which is reduced to 161.94$ for 0.3% nanofluid and further reduced to 135.82$ for 0.3% nanofluid with p/d of 1.79 insert. Simultaneously, the embodied energy of materials used in the collector is reduced by using nanofluids and inserts in the collector. Besides, the heat transfer and friction factor are also evaluated, and the data is fitted.