A method to quantify the energy-saving performance of greenhouse screen materials

Abstract

Screens used in practice are made from various material compositions (woven fabrics, knitted fabrics, foils, open or closed structures, transparent, diffuse, aluminized, various colours) and for various purposes (energy saving, reduction in light sum, or diffuse light obscuration). An important goal of the use of screens in Dutch greenhouses is to save energy. Unfortunately, to date, there is no objective method to determine the energy-saving performance of a material under standardized conditions. Energy-saving rates are estimated by manufacturers using different methods. Growers have no way of comparing material performances independently in order to make a proper investment decision. In the current research, the goal was to develop a method to quantify the energy-saving performance of greenhouse screen materials under standardized conditions. The method is based on the scientific literature and expertise of different screen producers and growers. The research focused on the three main aspects that affect energy saving of a screen: 1) thermal radiation losses, determined by the emissivity and reflectivity for thermal infrared radiation; 2) air permeability, which determines heat convection losses, characterized at a wide range of air velocities to account for velocities by buoyancy through materials as well as for velocities by forced convection caused by internal fans; and 3) water vapour permeability, which determines latent heat losses, determined under temperature, humidity and air velocity conditions normally encountered in commercial greenhouses. For all aspects, different measurement methods were compared to choose the best method based on reproducibility, accuracy and practicability. Screen material properties were then fed into both steady-state and validated dynamic greenhouse climate models to calculate overall screen energy saving under well-defined conditions. In the current research, different screen materials from different producers were investigated. The paper describes the methodology developed and shows data on different screen materials.