Photothermal and Magnetic Recovery Properties of Cu/Fe ₃ O ₄ Nanofluids: Implications for Direct Absorption Solar Collectors

With the escalating global energy crisis and environmental issues, the development of high-efficiency and sustainable clean energy technologies has become a research hotspot. Solar photothermal conversion, especially in Direct Absorption Solar Collectors (DASC), has attracted extensive attention due to its broad application prospects. However, traditional photothermal materials generally suffer from high cost, poor stability, and difficulty in recycling, which limit the efficient utilization of solar energy. To address the aforementioned challenges, this study explored the photothermal conversion and magnetically controlled recovery properties of Cu/Fe 3 O 4 magnetic nanofluids dispersed in deionized water, which are promising photothermal working fluids for DASC. First, two types of Cu/Fe 3 O 4 nanospheres with different particle sizes ( A 1:76 nm, A 2:63 nm) were prepared via a hydrothermal method by regulating the Cu/Fe molar ratio (1:1) and the dosage of urea (15 mmol, 30 mmol). Subsequently, the photothermal performance of the prepared samples was investigated. The results showed that Cu doping significantly enhanced the absorption capacity of the nanofluids in the visible light region (400–600 nm) while retaining the absorption peak in the near-infrared region (800–1000 nm). Specifically, the photothermal efficiency of A 2 reached 84.2% at a concentration of 350 ppm, which was 5.9% higher than that of the pure Fe 3 O 4 nanofluid ( A 3, 78.3%). Finally, magnetic recovery experiments were conducted, and the recovery rate reached 92.4% within 50 min under a magnetic field of 200 mT. This study demonstrates that Cu/Fe 3 O 4 nanofluids possess both high-efficiency photothermal conversion and controllable magnetic recovery characteristics, providing an economical and sustainable solution for solar photothermal utilization systems.