Photo‐thermal conversion performance of mono MWCNT and hybrid MWCNT‐TiN nanofluids in direct absorption solar collectors

Solar energy is being widely investigated due to its abundant, renewable, and clean features. Photo-thermal conversion in solar collectors is a direct approach for harvesting solar energy. Compared to the commonly used surface-based solar collectors (SSCs), nanofluid-based direct absorption solar collectors (DASCs) are more efficient. In this work, water-based mono MWCNT and hybrid MWCNT-TiN nanofluids were prepared through a two-step method, aiming at light absorption enhancement of nanofluids for applications in DASCs. The optical and photo-thermal conversion performance of the MWCNT nanofluids was firstly evaluated. The results showed that the MWCNT nanofluids presented high solar absorption capability, and the highest photo-thermal conversion efficiency was obtained at 10 ppm. The TiN nanoparticles were subsequently incorporated to form the hybrid MWCNT-TiN nanofluids. The absorption performance of the hybrid nanofluids was further enhanced. The highest efficiency of the hybrid MWCNT-TiN nanofluids was achieved at the TiN mass fraction of 10 ppm, which was about 6.9%, 6.0%, and 3.8% higher than that of the MWCNT nanofluid at 2000, 4000, and 6000 seconds due to the localized surface plasmon resonance (LSPR) effect of TiN nanoparticles. It was also found that the collector efficiency dropped rapidly with the prolonged irradiation time owing to the convection and radiation heat loss at the elevated temperature. Therefore, effective measures should also be taken to reduce heat loss to maintain high photo-thermal conversion efficiency.