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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.