Photothermal efficiency enhancement of a nanofluid-based direct absorption solar collector utilizing magnetic nano-rotor

The concentration of nanofluids and receiver depth are important factors that significantly affect the efficiency of direct absorption solar collectors. In the current work, the impact of the two factors is investigated using reduced graphene oxide/ethanol glycol nanofluids. The results show uneven temperature distribution in the receiver at high nanofluid concentration or when the receiver is deep. To overcome these issues, a forced convection nanofluid absorption system using alpha-Fe2O3 magnetic nanoparticles as the nano-rotor, and driven by an external rotating magnetic field, was designed. The mechanism of the new system is compared with the conventional one in detail. The photothermal conversion efficiency of the binary nanofluids (alpha-Fe2O3-graphene oxide/ethanol glycol nanofluids) in the new system reaches 56.8%, which is 14.5% higher than non-external rotating magnetic field nanofluids, for an RGO content of 0.007 wt%. This enhancement is ascribed to two effects of alpha-Fe2O3: improving the optical absorption capacity of binary nanofluids and acting as a nano-rotor. The optimum photothermal conversion efficiency is achieved at an RGO/alpha-Fe2O3 ratio of 3 in the current system.