Photothermal characteristics and recovery rate of NiFe2O4 nanofluids under magnetic field regulation

Abstract

Due to energy shortages and environmental pollution, solar energy has attracted extensive attention as a clean and sustainable energy source. Direct absorption solar collectors (DASCs) show promising application prospects due to low heat transfer losses, and their performance improvement relies critically on high-efficiency heat-collecting working fluids. In this study, water-based NiFe₂O₄ nanofluids with various concentrations were prepared using a two-step method. Their stability, physical characteristics, and photothermal and recovery properties under magnetic field regulation were experimentally investigated. The results demonstrate that the NiFe₂O₄ nanofluids exhibit excellent dispersion stability and outstanding light absorption performance in the wavelength range of 300–1100 nm, with absorbance significantly enhanced as concentration increases. Without a magnetic field, the photothermal conversion efficiency reaches 74.19% at 250 ppm. Under a 40 mT vertical magnetic field, the efficiency increases to 84.01%, while a horizontal magnetic field shows no significant effect. The results of the magnetic recovery cycle experiments show that the magnetic field intensity is positively correlated with the recovery rate. Under a magnetic field of 200 mT, the recovery rate reaches 85.22% at 30 minutes and 95.87% at 60 minutes. Moreover, the material maintains excellent photothermal performance after 5 cycles. This study confirms that NiFe₂O₄ nanofluids possess both high-efficiency photothermal conversion and favorable recoverability under magnetic field control, providing important theoretical and experimental support for the performance optimization and sustainable application of DASCs.