Hydroxyl functionalized MWCNT nanofluids with excellent photothermal conversion performance in direct absorption solar collectors

Developing novel nanofluids with good stability, excellent photothermal conversion performance, and appro-priate thermophysical properties is of great importance in promoting the practical utilization of nanofluids in solar collectors. In this work, hydroxyl multi-walled carbon nanotube (MWCNT-OH) nanofluids are proposed, which can achieve a high photothermal conversion of 88.2 % at a concentration of 20 ppm under 1-sun irra-diation. Both the sedimentation experiment and spectrophotometer analysis prove that the prepared MWCNT-OH nanofluids have excellent stability. The influence of hydroxyl functionalization, MWCNT-OH concentrations, and temperature on the thermophysical and optical properties is analyzed. Moreover, the comparison between MWCNT-based nanofluids with different functionalization reported in the literature and our work is carried out, the results show that MWCNT-OH nanofluids in our work outperform MWCNT-based nanofluids with other types of functionalization reported in the literature. This work may promote the use of MWCNT-OH nanofluids as working fluids in solar collectors.