Radiometric properties of screens and energy saving in greenhouses

In heated greenhouses energy screens are used to reduce energy use due to their insulating function. Materials with low thermal infrared (TIR) emissivity can further reduce the energy use by decreasing the amount of energy losses through TIR radiation. When developing new materials for thermal screens, the following questions arise: what is the role of emissivity of materials on energy saving? What other optical and radiometric material properties have the potential to improve the performance of energy screens? And what are the tradeoffs between various radiometric properties that influence the screen’s potential to reduce energy use or increase energy use efficiency? This study used model simulations for tomato greenhouses in the Netherlands to evaluate how radiometric properties influence screen performance. First, an analysis of various properties was performed, showing that the most important radiometric property of a screen influencing energy use and energy efficiency was the emissivity of the upper side of the screen. A second analysis examined modifying a screen’s photosynthetically active radiation (PAR) transmissivity and consequently modifying the strategy for use of the screen in daytime. Lastly, the interaction between emissivity, PAR transmissivity and near infrared (NIR) transmissivity was examined. In all simulations, screen properties such as air and humidity permeability were kept equal. The results show that lowering a screen emissivity from 0.5 (typical value for commercial screens) to 0.2 (a theoretical, improved value) has the potential to reduce energy use by ~10%. Increasing a screen’s PAR transmissivity from 0.7 (typical value for good commercial screens) to 0.9 (theoretical improved value), can increase energy use efficiency by about 2%. These results demonstrate the interaction of the various screen properties and their influence of energy use, highlighting the importance of low upwards emissivity and the need to further examine the interaction of screen properties, screen use strategy and other control and design decisions.