A method for determining the long-wave radiative properties of a plastic shading net under natural conditions

Abstract

The long-wave radiative properties of a plastic shading net are essential to determine the thermal performance of a particular structure covered with this net. Nets are non-homogeneous, three-dimensional and semi-transparent materials, and therefore the radiative properties of a net cannot be measured in a laboratory using instruments designed for homogeneous materials. In addition, net radiometers cannot be used to measure these properties directly due to radiation emitted from the net surfaces. No suitable technique is presently available for determining the long-wave properties of shading nets. This paper describes a simple method for determining the total hemispherical long-wave radiative properties (i.e., transmittance, reflectance and absorptance or emittance) of nets. Nets with colors, porosities, texture type and nominal shading factors that are most commonly used in hot regions were selected for testing. The measurements were performed under equilibrium conditions (i.e., nighttime, steady-state and uniform sky thermal irradiance). The results show that the behaviors of plastic nets under thermal radiation conditions were different from those of translucent homogeneous materials. Net temperature and porosity were more important than texture type and color in determining transmittance and reflectance. The net temperature drop at night (610.5 °C) caused shrinkage in the net. This resulted in a decrease in porosity, which in turn caused a decrease in transmittance and an increase in reflectance. For the nets tested, transmittance decreased from 1.2% to 6.3% per 1 °C decrease in temperature. The reduction in transmittance was equal to the increase in reflectance. Emittance of the net depends on net temperature, color, porosity and texture type, although, at low temperature, none of these effects was significant. © 2011 Elsevier B.V. All rights reserved.