Performance Study of Cu Nanofluids in Spectral Beam Splitting Photovoltaic/Thermal Systems

Spectral beam splitting is a promising approach for thermally decoupling photovoltaic and photothermal processes in PV/T systems. However, existing liquid spectral splitters still suffer from insufficient short-wavelength absorption and/or the high cost of noble-metal nanoparticles. In this study, a water-based Cu@SiO2 nanofluid was developed as a low-cost absorption/transmission spectral splitting filter for monocrystalline silicon PV/T systems. The full solar spectrum considered in this system covers both photovoltaic and thermal utilization, while 750–1000 nm was selected only as the target transmission window for the c-Si cell. This window was chosen because c-Si cells can effectively utilize this near-infrared band, whereas the short-wavelength radiation can be absorbed by Cu@SiO2 nanoparticles for heat generation and the longer-wavelength infrared radiation can be mainly absorbed by the water-based fluid for thermal recovery. FDTD simulations, combined with Mie scattering theory and the Beer–Lambert law, were used to evaluate the optical response of Cu nanoparticles and optimize the particle size. Cu@SiO2 nanoparticles were synthesized using an ammonia-free coating method to suppress Cu oxidation and core corrosion, and the prepared nanofluids were tested outdoors under real solar irradiation. The results show that Cu nanoparticles with a diameter of 50 nm provide a favorable balance between absorption and scattering. At mass concentrations of 0.02, 0.035, and 0.05 wt%, the overall system efficiencies were 56.84%, 60.12%, and 63.38%, respectively. The highest electrical efficiency was 9.30% at 0.02 wt%, whereas the highest thermal efficiency was 48.25% at 0.05 wt%. These results indicate that Cu@SiO2 nanofluids can improve the energy utilization of the tested SBS-PV/T system, although the nanofluid concentration should be optimized to balance thermal gain and photovoltaic transmittance loss.