Radiative control through greenhouse covering materials using pigmented coatings

Abstract

Greenhouses are built in hot climates to provide a cool environment suitable for growing out of season crops. Their temperature is usually controlled by utilizing air conditioning systems and shading the transparent cover material in peak sun hours. A new method to keep a cool greenhouse, and reduce energy consumption is proposed in this study. Using a new approach for pigmented coatings, that would reflect incident solar radiation in the infrared wavelengths (NIR), and allows the shorter visible wavelengths (VIS) to be transmitted. It is possible to reduce the heat gain inside the green house by radiation while keeping the useful wavelengths necessary for photosynthesis. A series of numerical calculations is performed, modeling the radiation transfer in the pigmented coating using the Radiation Element Method by Ray Emission Model (REM2). Mono dispersed particles are assumed, and Mie theory is used to estimate the radiative scattering properties of the particles. Different materials are studied as pigments and compared over a range of particle sizes and volume fractions. The calculated spectral reflectance curves were compared with experimentally derived ones of TiO2 pigmented coating, they differed in values but showed the same tendency. An optimization parameter is proposed and used to compare the different materials and parameters. Diamond particles are found to be the most appropriate candidate for the desired application, giving a high transmittance in the VIS region and high reflectance in the NIR region.