Nitrogen-doped carbon quantum dots enable efficient photothermal conversion for direct absorption solar collectors

Direct absorption solar collector (DASC) using nanofluids is an efficient way to utilize solar energy. The heat collection efficiency in DASC is constrained by the stability and optical absorption capacity of nanofluids. Carbon quantum dots (CQDs) nanofluids are used in DASC due to the excellent dispersion stability. However, the optical absorption range of current CQDs is primarily confined to the ultraviolet region, resulting in the visible and near-infrared regions of sunlight being underutilized. To solve this problem, this study synthesized novel N-doped carbon quantum dots (FPNCQDs) using ammonium bifluoride (NH₄F) and perylene derivatives. The addition of NH₄F and perylene derivatives during synthesis increased the sp² conjugated structure and the content of graphite N in FPNCQDs. The doping of N, especially graphite N, significantly reduces the band gap by injecting excess electrons into the unoccupied π∗ orbitals. The temperature of FPNCQDs/EG nanofluids increased from 24 °C to 65.7 °C within 60 min, demonstrating excellent photothermal conversion performance. What's more, the FPNCQDs/EG nanofluids achieved high stability with nearly consistent transmittance over a 14-day storage test. Finally, a theoretical model for the photothermal conversion process of the nanofluids was developed to investigate the effect to carbon quantum dots on the solar collection performance. The simulation results indicated that the FPNCQDs/EG nanofluids at 10 ppm demonstrated the highest solar collection efficiency considering both photothermal conversion and heat loss from the surface. The novel FPNCQDs/EG nanofluid will be a promising photothermal fluids in direct absorption solar collector.