Abstract
It is impossible to directly measure the thermal radiative properties of shading nets because of the net's perforated structure and the difficulty to distinguish the transmitted, reflected, and emitted radiation on the net surface. In addition, Kirchhoff's identity was derived for an object in thermal equilibrium and exchanges only thermal radiation with its surrounding, and the assumption that is valid for an object exchanging solar and thermal radiation and convection with the surrounding is unclear. This study was to (i) develop a theoretical model to predict the absorptivity, alpha(n); transmissivity, tau(n); reflectivity, rho(n); and emissivity, epsilon(n), of plastic nets and (ii) examine the equality (epsilon(n) = alpha(n)) under natural conditions. Nets with different porosities, texture structures, and colors were tacked onto a wooden frame, fixed horizontally over a black substrate. Thermal radiation balance was applied to the net-substrate system. The model input parameters (i.e., the thermal radiation fluxes below and above the net; the net and substrate temperatures) were measured on sunny days. The estimated values of alpha(n) and tau(n) were in the ranges 0.41-0.82 and 0.16-0.55, respectively, whereas rho(n) <= 0.08 for the nets tested. Under the steady-state natural conditions, epsilon(n) = alpha(n) for a net in thermal equilibrium. However, the solar and thermal radiation absorbed by the net did not equal the emitted radiation.