Investigation on the enhancement of optical and solar thermal harvesting performance of water with Fe3O4 nanoparticle and external rotating magnetic field

•Light absorption and photothermal conversion performance were investigated experimentally.•PTCE of 0.075 wt% Fe3O4 nanofluid was 90%, which was 45.6% improved compared to DI-water.•Dipole interaction between Fe3O4 magnetic nanoparticles under a magnetic field produces active heat transfer.•0.075 wt%-Fe3O4 nanofluid improved greatly PTCE by an external rotating magnetic field. In this study, the light absorption characteristics and solar heat harvesting performance of water by adding Fe3O4 nanoparticles (NPs) were experimentally investigated under different external magnetic field (MF) strengths, and rotating MF speeds. As a result, the photothermal conversion efficiency (PTCE) of Fe3O4 NF exhibits higher light absorption characteristics than water at all concentrations. The PTCE of the Fe3O4 NF at the optimal concentration of 0.075 wt% was 90%, which was 45.6% higher than that of the base liquid. The optimal performance of 0.075 wt% of Fe3O4 NF was presented at an external MF strength of 500 Gauss and rotating MF speed of 200 rpm, resulting in a high PTCE of 96.1%, which was significantly higher than that of base liquid. Furthermore, it increased by approximately 6.73% compared to the PTCE of 0.075 wt% Fe3O4 NF without an external MF. The Fe3O4 NPs are reacted by the external MF and arranged on the MF line. As the external MF rotates, the Fe3O4 NPs move to form convection in the working fluid, improving the temperature uniformity of the working fluid. Therefore, the PTCE of the working fluid can be improved by adding Fe3O4 NPs into water and using external rotating MFs, which can significantly improve the performance of direct absorption-type solar collectors.