Investigation of thermo-optical properties and photothermal conversion performance of MWCNT, Fe3O4, and ATO nanofluid for volumetric absorption solar collector

For the use of volumetric absorption solar thermal collector (VASTC), the thermo-optical properties, photothermal conversion characteristics and economics of MWCNT (Multi-walled carbon nanotube) (0.0005–0.004 wt%), Fe3O4 (0.025–0.1 wt%), and antimony-doped tin oxide (ATO) (0.05–0.25 wt%) nanofluids (NFs) were comprehensively investigated. As a result, The improvement of thermal conductivity, specific heat, and viscosity of MWCNT, Fe3O4, and ATO NFs was insignificant in the concentration range where optical absorbance can improve. The photothermal conversion efficiency (ηPTC) and receiving efficiency (ηrec) were significantly improved according to the increase in the concentration. Among NFs, the 0.002 wt% MWCNT, 0.075 wt% Fe3O4, and 0.1 wt% ATO NFs had maximum ηPTC (93.3%, 90%, and 89.2%) and ηrec (83.7%, 81.4%, and 77.7%). Regarding dispersion stability, the optical transmittance of MWCNT, Fe3O4, and ATO NFs slightly increased, and long-term dispersion stability is required for the use of VASTC. Economically, the cost required per unit photothermal conversion energy (CUPTE) of 0.002 wt% MWCNT NF is 0.102–0.538 $/g, which is cheaper than 0.075 wt% Fe3O4 and 0.1 wt% ATO NFs. Overall, MWCNTs can be used as working fluid for VASTC due to their excellent optical absorption and low price once long-term dispersion stability is achieved.