ISHS Acta Horticulturae 174: Symposium Greenhouse Climate and its Control
AN ANALYSIS OF THERMAL SCREEN EFFECTS ON GREENHOUSE ENVIRONMENT BY MEANS OF A MULTI-LAYER SCREEN MODEL |
| Author: | M. Okada |
| Abstract:
A heat balance model has been developed to evaluate the effectiveness of thermal screens. The model was based on the following major assumptions, 1) steady heat balance, 2) no latent heat flow, 3) no air infiltration and 4) no crops inside. Radiation balance equations are expressed in a matrix form, so that choice of the number of screen layers is arbitrary. The whole energy balance including convection and conduction is calculated numerically as temperatures being unknown variables. The model experiment indicated that the heat conductance from the inside air to the external cover decreased monotonically as RPI (Radiation Property Index) of the screen increased. The RPI is defined as a * (reflectivity of the outside screen surface) + (1-a) * (reflectivity of the inside screen surface) - (transmissivity of the screen), and 0.7 was observed for an empirical constant a. In case of double or triple screens with different films the effectiveness became higher when the film with the largest RPI was used at the outermost layer. When a high reflective aluminized film was used at the top layer, the supplemental layer from double to triple screens did not make a distinct improvement of the effectiveness. | |
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