The use of renewable energy as an alternative energy source for fossil fuels is receiving a lot of attention. Among various renewable energy, solar energy is widely used in many fields due to its abundant reserves and low equipment cost compared to other energy sources. However, it is known that when a nanofluid is applied as a working fluid of a direct absorption solar collector applied to use solar energy, heat collection performance can be improved due to enhanced optical and thermal characteristics. However, the enhanced optical and thermal properties due to the nanofluid increase the heat loss to the outside of the working fluid, thus there is a limit to improving the heat collection performance.
To solve this problem, in this study, the light absorption characteristics and photothermal conversion performance of DI-water according to the addition of Fe3O4 nanoparticles were experimentally investigated under various external magnetic field strengths and rotation speeds. Fe3O4 nanofluids were prepared at concentrations of 0.025, 0.05, 0.075, and 0.1wt%, and as a result of the experiment, Fe3O4 nanofluids showed higher light absorption characteristics than DI-water, which is a base fluid, at all concentrations. In addition, as Fe3O4 nanoparticles were added to DI-water, it was confirmed that the thermal properties were improved by increasing the thermal conductivity and viscosity. The Fe3O4 nanofluid increased solar energy absorption due to its high light absorption and thermal properties, which also improved the equilibrium temperature of the working fluid and the efficiency of the photothermal conversion performance. At the concentration of 0.075wt% of the Fe3O4 nanofluid, the photothermal conversion efficiency was the highest of 90%. This is a 45.6% improvement compared to that of DI-water. As a result of applying an external magnetic field to the Fe3O4 nanofluid, the thermal heat transfer characteristics are improved. The optimal external magnetic field strength and rotational speed were investigated by applying external magnetic fields with various intensities and rotating magnetic with various speeds at the optimal concentration of 0.075wt%. As a result, when the intensity of the external magnetic field is 500 Gauss and the rotating magnetic field speed was 200 rpm, the photothermal conversion efficiency is remarkably improved to 96.1%. This is about 6.78% higher than that of 0.075wt% Fe3O4 nanofluid without applying an external magnetic field. In addition, as a result of increasing the rotation radius by moving the rotation axis of the external rotating magnetic field, the photothermal conversion efficiency of 0.075wt% Fe3O4 nanofluid was 97.5%, and it improved as the rotation radius increased. Through this study, the photothermal conversion performance can be improved when an external magnetic field and a rotating magnetic field are applied to the Fe3O4 nanofluid, which indicates that it is suitable for improving the heat collection performance of a direct absorption solar collector.