Core-shell structured nanofluids employment in solar water heating systems with natural circulation and their effect on heat transfer performance

In this study, the effects of nanofluids on the performance of the closed-circuit solar energy system were investigated by using three different nanofluids, each with a core-shell structure. The nanofluids used are based on SiO2 shell – TiO2 core, SiO2 shell – Fe3O4 core and SiO2 shell – ZnO core. Pure water was used as the base fluid in each nanofluid. In addition, 2 wt% mass concentration was used in the preparation of each nanofluid. An experimental set-up with flat-plate, natural circulation solar water heating system was prepared. Two identical experimental setups were prepared to examine the effects of nanofluids on thermal performance. In one of the experimental setups, pure water was used as the working fluid. In the other setup, nanofluids were used as the working fluid. Thus, the performance of the system with pure water was taken as reference for the comparison with nanofluids. In this respect, the effects of nanofluid usage on the system performance were investigated. By recording the time-dependent changes in the collector inlet temperature, collector outlet temperature, absorber surface temperature, storage tank temperature for the both setup, ambient temperature, solar radiation intensity and wind speed, the thermal performances of the both systems were examined and compared with each other. The results show that higher performances were obtained for Fe3O4-SiO2 based and ZnO-SiO2 based nanofluids. On the other hand, any performance enhancement could not be achieved with TiO2-SiO2 based nanofluid. It was determined that convective heat transfer coefficient decreased by 4.13% with TiO2-SiO2 based nanofluid. In contrast, in the system working with Fe3O4-SiO2 based nanofluid, the convective heat transfer coefficient increased by 8.60%. Similarly, in the system working with ZnO-SiO2 based nanofluid, 3.07% increase in the convective heat transfer coefficient was determined. Useful energy values obtained for TiO2-SiO2, Fe3O4-SiO2 and ZnO-SiO2 based systems; decrease by 1.97%, increase by 16.65% and increase by 5.4%, respectively. Collector efficiencies was also similar to these results. For TiO2-SiO2, Fe3O4-SiO2 and ZnO-SiO2, average collector efficiencies were decreased by 4.43%, increased by 16.25% and increased by 7.39%, respectively.