Development of a method for evaluating heat insulation capacity of breathable thermal screens for greenhouses using a hot-box with heat balance analysis

In this study, we developed a method for evaluating the heat insulation capacity of breathable thermal screens. Heat transfer through breathable thermal screens involves radiation, convection, conduction, and ventilation. Of these, ventilation heat transfer is unique to breathable thermal screens, and warrants further understanding. Therefore, we evaluated the heat insulation capacity of aluminized (Screen_Al) and polyester (Screen_Poly) thermal screens. Ventilation heat transfer coefficient (kven), heat transfer coefficient of the nonventilated part of the thermal screen (k), and heat transfer coefficient including all heat transfer forms (kall) were measured via a hot-box with heat balance analysis. The kven of Screen_Al (1.5 W m-2 °C -1) was higher than that of Screen_Poly (1.3 W m-2 °C-1) because the ventilation rate of Screen_Al (4.2 m3 m-2 h-1) was higher than that of Screen_Poly (3.8 m3 m-2 h-1). The k of Screen_Al (4.1 W m-2 °C -1) was lower than that of Screen_Poly (6.0 W m-2 °C -1) because of the higher longwave radiation reflectance of Screen_Al. Finally, the kall of Screen_Al (5.7 W m-2 °C -1) was lower than that of Screen_Poly (7.2 W m-2 °C -1). In conclusion, we successfully demonstrated a method by which the heat insulation capacity of breathable thermal screens can be evaluated.