Avaliação experimental do uso de nanofluidos em coletores solares de placa plana modificados para absorção direta

Abstract

The present work was developed to contribute to the knowledge about the use of nanofluids in solar collectors. The production of nanofluids, the evaluation of the stability of the samples produced and tests on two experimental benches were carried out. The first one was designed to expose nanofluids to solar radiation and investigate parameters of energy absorption and conservation, resulting in the selection of an ideal material and nanoparticles volumetric concentration to use in a solar collector. The second bench was built aiming the use of the previously selected nanofluids as photothermal conversion medium in a reduced scale modified flat plate solar collector for water heating. Nanofluids of graphene, ZnO-Ag, gold and MWCNT in water were tested in the first bench at low volumetric concentrations (between 0.0001 and 0.0300%). Only the gold and MWCNT nanofluids samples remained stable after exposure to solar radiation, presenting a stabilization trend of the specific absorption rate and the stored energy ratio with the concentration increase, indicating the existence of a limit concentration for applications, determined in this work as being 0.0020%(Gold-water) and 0.010%(MWCNT-water). Among the samples tested, MWCNT in water showed greater stability, better results, and lower costs, motivating its selection for tests on the second experimental bench at concentrations of 0.0005%, 0.0008% and 0.0010%. Three collectors were tested simultaneously, the first with a traditional flat absorber plate, the second without the flat plate and with nanofluid added around the heating coil, and the third compound by the flat plate and the nanofluid added around the heating coil. The collectors operating with nanofluids had better performance in the concentration of 0.0010%, presenting average temperature variation (7.19°C and 7.21°C) higher than the traditional model (6.49°C) and similar average efficiencies (0.43, 0.45 e 0.47). On the other hand, the worst performances were observed in tests at intermediate concentration (ϕv=0.0008%), presenting average efficiencies up to 33% lower than the traditional collector. The modified collector that used an absorber surface combined with nanofluids as photothermal conversion medium showed the highest temperature variation averages, however, it was observed in the tests, with all three concentrations, the degradation of the nanofluid samples present in its interior. The traditional collector and the modified one operating only with nanofluids (without flat absorber plate) presented, respectively, the best and worst average efficiencies. Finally, it was concluded that filling the solar collectors with nanofluids implies in greater radiation absorption area, however, it also resulted in lower average efficiencies and in total energy gains that, at most, were equal to those of the traditional model.