Effect of agricultural covering material thickness on the overall heat transfer coefficient

This study evaluates the effect of the agricultural covering material thickness on the overall heat transfer coefficient in developing covering materials with high heat insulation. The thickness of a thin film, such as the covering material, minimally impacts conductive and convective heat transfer within and outside the greenhouse. Nevertheless, the effect of covering material thickness on radiative heat transfer remains unexplored. The higher the longwave radiation absorptance of the covering material, the lower the overall heat transfer coefficient due to the suppression of radiative heat transfer through the material. This study examined the relationship between the covering material thickness and the overall heat transfer coefficient for 24 types of polyolefin (PO) films in the market. An equation was used to estimate the overall heat transfer coefficients based on the longwave radiation absorptance of each covering material, which was measured using an emissivity meter. The tested PO films’ maximum, minimum, and average thicknesses were 0.16, 0.06, and 0.12 mm, respectively, with a standard deviation (SD) of 0.03 mm. The longwave radiation absorptance’s maximum, minimum, and average values were 0.74, 0.37, and 0.63, respectively, with an SD of 0.08. The maximum, minimum, and average values of the overall heat transfer coefficients were 7.22, 6.19, and 6.56 W m-2 K-1, respectively with an SD of 0.24 W m-2 K-1. A significant positive correlation was observed between film thickness and longwave radiation absorptance (R = 0.6468). There was a significant negative correlation between film thickness and the overall heat transfer coefficient (R = -0.6642). As film thickness increased, longwave radiation absorptance increased, suppressing radiative heat transfer through the covering material and decreasing the overall heat transfer coefficient. This study clarifies the mechanism by which the thickness of the covering material affects the overall heat transfer coefficient.