Spectrum splitting through CuS–ZnO/water hybrid nanofluid for agricultural greenhouse cooling applications: An experimental study

In the present work, CuS–ZnO/water hybrid nanofluids (in concentrations of 0.0025 mass% and 0.005 mass%) are synthesized using a two-step method with nanoparticles composition of 95% CuS and 5% ZnO. The optically tuned nanofluid filter on the agricultural greenhouse roof can reduce the cooling load by transmitting the visible spectrum and absorbing the near-infrared radiation in the solar spectrum. The size distribution of nanoparticles, stability and optical transmission of both concentrations in the visible and near-infrared regions are examined. Two hollow containers (i.e., ducts) with thicknesses of 4 mm and 8 mm are prepared. Each of these ducts is attached to a greenhouse unit and placed in front of a solar simulator. The experimental results reveal that applying CuS–ZnO nanofluid reduces the inside temperature of the greenhouse unit under all irradiance and ambient temperature ranges. The cooling system gains an average of 27.4% less heat from the greenhouse unit when the CuS–ZnO nanofluid flows through an 8 mm duct compared to no-fluid case (empty duct). The photothermal conversion efficiency of nanofluid is found to be higher than the one for water. The crop growth factor of 82.2% is obtained for 8 mm duct case, and the photosynthetic photon flux density inside the greenhouse unit is reduced without affecting the growth of many plants. Furthermore, the payback period of the nanofluid system (with 8 mm duct) is calculated as 0.42 years, and the application of optically tuned nanofluid can help reduce the cooling system's size and energy requirement for cooling.