Carbon quantum dot nanofluids for spectral splitting photovoltaic/thermal systems: preparation, characterization, and performance

The spectral-splitting photovoltaic/thermal (SSPV/T) system is a promising approach for high-efficiency full-spectrum utilization of solar energy. In this study, four series of CQD nanofluids were prepared via microwave carbonization using polyethylene glycol 200, glycerol, and diammonium citrate as precursors, and their optical, thermophysical and rheological properties, as well as stability, were systematically characterized. A three-dimensional transient opto-electro-thermal coupled numerical model was established to investigate the thermo-electrical performance of CQD-nanofluid-based SSPV/T systems, and their techno-economic and environmental benefits were comprehensively evaluated. The results show that the MIX and CADS-DD series CQD nanofluids exhibit excellent spectral selectivity, and the MIX-0.25 and CADS-DD-0.05 nanofluids have the highest spectral filtering efficiency, which effectively suppress the temperature rise of photovoltaic (PV) panels and greatly improve their temperature uniformity. The embedded vacuum gap achieves effective thermal decoupling. In summer and winter, the total system energy output under the 10-fold concentrating mode is approximately 9 times and 29 times that of the non-concentrating case, respectively, with overall exergy efficiency ranging from 14.84% to 17.34%. The concentrating SSPV/T system based on the CQD nanofluid delivers superior economic and environmental benefits over conventional PV systems, showing promising engineering application prospects. Further outdoor prototype tests are required to verify its practical application.