Modelling energy transport through greenhouse screen materials for a standardized property assessment

In Dutch greenhouses one or more movable screens are used for energy saving cultivation techniques. Screens separate the lower warm and humid compartment with crops from the upper cold and dry roof compartment. Therefore, screens have an important role in energy saving due to insulation. The objectives of this project are to translate standardized measurements of different screen properties under lab conditions toward a model of energy fluxes around screens and overall energy saving under pre-defined greenhouse conditions. Different measurements of screen properties were carried out for different materials under dry and wet conditions: radiative properties such as transmission and emission for thermal infrared radiation; aerodynamic properties such as air permeability; humidity transport characteristics such as hygroscopy, the mechanism of humidity condensation on the lower side of the screen, transport through the material and evaporation at the upper side of the screen. A physical model of energy fluxes around a screen material has been developed. For that the greenhouse climate model KASPRO has been modified to consider the measured characteristics of screens in dry and wet conditions. The model is able to calculate the different components of the energy fluxes around the screens under various conditions. It translates measured screen properties toward overall potential energy saving in greenhouses. This helps to obtain an objective comparison of the performance of different screen materials under predefined conditions, lets growers make informed choices of investment and practical usage and helps screen manufacturers to develop improved materials. The results of different screens show that low emissivity and transmissivity for thermal infrared radiation of a screen is correlated to low radiative energy losses. Low air and humidity permeability is correlated to a low loss of sensible and latent heat.