Experimental investigation of spectrum selective nanofluids for thermal management of greenhouses in hot arid climates

The plants inside the agricultural greenhouses utilize only the visible spectrum during photosynthesis, and remaining solar spectrum causes a rise in the greenhouse temperature. This higher temperature affects plant growth and increases the cooling load of greenhouse significantly in hot arid climates. The spectrum selective nanofluids provide an efficient and sustainable solution to this problem by transmitting the visible spectrum and absorbing the near-infrared radiations in the solar spectrum. In this experimental study (which is one of the first studies on this specific application), TiN/water (0.005 wt%) and CuS-MgO/water (0.0025 wt%) nanofluids were prepared using two-step method, and CuS-MgO nanofluid has a nanoparticles composition of 99% CuS and 1% MgO. The stability of nanofluids and nanoparticle size distribution were examined. The optical transmittance of prepared nanofluids in the visible and near-infrared region for an optical path length of 10 mm was explored. The water and nanofluids flow through a hollow container (duct) attached in front of a greenhouse unit, and this setup was placed in front of a solar simulator. The experimental results showed that the application of optically tuned CuS-MgO/water nanofluid reduced the inside temperature of greenhouse unit under all irradiance and ambient temperature ranges. The cooling system gained an average of 25.8% less heat from the greenhouse unit when CuS-MgO/water nanofluid flowed through the duct compared to the empty duct case. The photothermal conversion efficiency of CuS-MgO/water nanofluid was 70.5%. The TiN/water nanofluid showed a crop growth factor of 83.7%, and the photosynthetic photon flux density inside the greenhouse unit was reduced without affecting the growth of many plants. In addition, the payback period of the nanofluid system (containing CuS-MgO/water nanofluid) was calculated as 0.49 years, and the application of CuS-MgO/water nanofluid can help to reduce the emissions associated with electricity production by reducing the electricity requirement for the cooling system of greenhouses.