Photothermal Conversion Characteristics of ZnO/MWCNTs Binary Nanofluids and ZnO/MWCNTs/Au Ternary Hybrid Nanofluids in Direct Absorption Solar Collectors

Herein, the oil‐based ZnO/multiwalled carbon nanotubes (MWCNTs) binary nanofluids and ZnO/MWCNTs/Au ternary hybrid nanofluids are proposed. The effects of nanoparticles concentration, and the ratio of ZnO and MWCNTs on the optical properties and photothermal conversion performance are experimentally analyzed. Compared to the base fluid (heat transfer oil), the transmittance of the nanofluids decreases with increasing nanoparticle concentration. The ZnO/MWCNTs nanofluids show slightly higher optical absorption performance than MWCNTs due to the multiple scattering caused by the ZnO hierarchical structure. The ZnO/MWCNTs nanocomposites decorate with gold nanoparticles (Au NPs) to form ZnO/MWCNTs/Au ternary hybrid nanofluids. The ZnO/MWCNTs/Au nanofluids have slightly higher absorption performance in the visible region due to the plasmonic effect exhibited of the Au NPs. The photothermal results show that the maximum value of photothermal conversion efficiency (91.2%) of ZnO/MWCNTs nanofluids is achieved when maintaining the solid content of MWCNTs at 0.01 wt% and the ZnO and MWCNTs ratio of 2:3. The photothermal conversion efficiencies of MWCNTs and ZnO/MWCNTs/Au nanofluids are enhanced by about 35.6% and 42.8%, respectively. This work demonstrates that ZnO/MWCNTs and ZnO/MWCNTs/Au nanofluids have potential application as working fluids for direct absorption solar collectors.