Quantifying optical and thermal properties of innovative screen materials and their effect on energy saving and crop growth

In Dutch greenhouses one or more movable screens are used for energy saving cultivation. Screens separate the lower warm, humid compartment with crops from the upper cold, dry compartment of the greenhouse roof. Screens, especially multiple screens, affect the insulation value of the greenhouse. The energy saving potential is depending on material properties and on screen usage strategies. Radiometric properties including transmissivity and emissivity for thermal infrared radiation (TIR), aerodynamic properties consisting of air permeability, humidity transport characteristics involving hygroscopy, the mechanism of humidity condensation on the lower side of the screen, transport through the material, and evaporation at the upper side have been shown to save energy. Materials with low TIR transmissivity, emissivity and low permeability show highest energy saving. However, energy saving is influenced by the amount of hours screens are closed. If screens are closed beyond dark nighttime hours, optical properties (hemispherical light transmissivity, light diffusion, light spectrum) are important. If screens are closed during light daytime hours, these properties determine not only the amount of incoming sunlight energy but also the availability of photosynthetic active light (PAR) for plant growth and development. In this study properties of different innovative energy saving screen materials were measured using standardized equipment and protocols. With the help of model calculations using the adapted dynamic climate and energy model KASPRO these properties were translated to energy saving potentials in a tomato crop. Materials with different light transmissivity were studied assuming different outside radiation levels, inside artificial lighting and screening strategies. Consequences for energy saving, crop growth and development are analysed to let growers make informed investment and operational choices and help screen manufacturers to develop improved materials.