Optical and photo-thermal conversion characteristics of recyclable nano-ferrofluid under the external magnetic field

The efficient conversion of solar energy through photo-thermal processes holds significant importance in realizing cleaner and sustainable energy provision. To optimize the solar photo-thermal conversion performance, Water-Ethylene glycol-based Fe3O4 nano-ferrofluid was applied under the action of an external Magnetic Field (MF) in this work. The saturation magnetic intensity of prepared Fe3O4 nanoparticles with the diameter of 12.61 nm is 51.38 emu/g. The optical properties of the prepared nano-ferrofluid were investigated in the concentration range of 0.025-0.1 wt%. The results indicated the optical transmittance of nano-ferrofluid decreased with accumulation of concentration, and the absorption peaks were around 220 nm, 500-600 nm, as well as 970 nm. It is shown that both vertical and horizontal MF would improve the photo-thermal conversion efficiency of Fe3O4 nano-ferrofluid within intensity range from 5 mT to 60 mT because the chain-like structure formed by the nanoparticles enhanced the thermal conductivity, and the vertical MF was more beneficial with the improvements of 44.91%-70.28%. Moreover, the photo-thermal conversion efficiency of nano-ferrofluid gradually rises with the increase of vertical MF intensities. However, there is the optimal MF intensity for the horizontal MF. As a result, the nano-ferrofluid with 0.1 wt% concentration has the maximum enhancement of 70.28% at the vertical MF of 60 mT. In addition, the recovery properties of nanoparticles were also studied under the action of MF, and the recovery efficiency of 82.4% was obtained after the action of MF with 200 mT for 100 min.