Measuring and predicting radiometric properties of reflective shade nets and thermal screens

Screen materials are being used extensively in agriculture for conserving energy by reducing night radiant heat loss, and for reducing solar heat load by day. Measurements of their radiometric properties are needed to determine the influence of screens on crops. Radiometric properties of five screens used in agriculture were determined from an analysis of measurements made with pyranometers, pyrgeometers, net radiometers and thermocouples. Long- and short-wave radiation balance equations were analysed for a number of different cases, including asymmetrical (two-sided) and symmetrical screens composed of one to three different materials. By using three screens of similar construction, but with different solidities and one containing an additional component, properties of the component materials were determined. Radiation balance equations were solved to calculate radiant fluxes, and transmittance, reflectance, and emissivity of the screens and their components. Results compared favourably with values reported for the same materials by the manufacturers and in the literature. The theoretical analysis does not consider geometrical properties of the screens, introducing a 10% inaccuracy in the results. However, differences between some previously reported values and those supplied by the manufacturers exceed 10%. (C) 1999 Silsoe Research Institute.