Study on the Photothermal Performance Enhancement of Direct Absorption Solar Collectors by Heat-Transfer-Oil-Based MWCNT-TiN Composite Nanofluids

Abstract To improve the volumetric solar absorption capability of the working fluid in direct absorption solar collectors (DASC) and to extend its application to medium- and high-temperature solar-thermal utilization, mono multiwalled carbon nanotube (MWCNT) nanofluids and MWCNT-TiN hybrid nanofluids were prepared using Therminol 66 heat-transfer oil as the base fluid by a two-step method. Unlike previous MWCNT-TiN studies mainly based on aqueous systems, this work introduces the hybrid absorber into a low-polarity heat-transfer oil medium and employs oleic-acid-assisted dispersion and ultrasonication to improve particle dispersion in the oil phase. The optical and photothermal conversion performances of MWCNT nanofluids were first evaluated. The results show that the addition of MWCNTs substantially enhances solar absorption; the average transmittance of the nanofluid decreased from 68.43% for the base fluid to 33.44%, and the maximum apparent photothermal conversion efficiency reached 82.51% at 10 ppm MWCNT. TiN nanoparticles were then introduced into the optimized MWCNT nanofluid to prepare MWCNT-TiN hybrid nanofluids. The hybrid nanofluids exhibited further enhanced absorption in the visible–near-infrared range. At a TiN concentration of 10 ppm, the MWCNT-TiN hybrid nanofluid achieved the highest apparent efficiency of approximately 88.48%, which was 5.97 percentage points higher than that of the 10 ppm MWCNT nanofluid. This enhancement is mainly attributed to the strengthened broadband absorption in the visible–near-infrared region after TiN incorporation; this behavior is consistent with the reported plasmonic absorption characteristics of TiN, although the efficiency improvement is not attributed solely to a single LSPR mechanism. Supplementary stability evaluation of the optimized 10 ppm MWCNT–10 ppm TiN nanofluid showed that the UV–vis absorbance retention after 10 and 20 thermal cycles was approximately 98.0% and 95.5%, respectively. The particle-size distribution showed a Z-average of 66.533 nm, a PDI of 0.125, and a main peak at 75.529 nm, indicating good short-term optical and dispersion stability. This study provides a reference for the selection and application of oil-based hybrid nanofluids in medium- and high-temperature DASC systems.