Abstract
The overall heat transfer coefficients of various agricultural covering materials were measured under winter nighttime downward longwave radiation from 212 to 297 W m−2 reproduced in an experimental device. Under the downward longwave radiation conditions of this study, the ranges of overall heat transfer coefficients of glass, polyolefin film, fluorocarbon resin film, polyvinyl chloride film, polyvinyl acetate film, and polyethylene film were 5.2–5.7, 5.5–6.4, 5.6–6.4, 6.0–6.9, 6.7–7.6, and 6.7–7.6 W m−2 °C−1, respectively. The overall heat transfer coefficient tended to increase as the downward longwave radiation decreased. The relationship between downward longwave radiation and the overall heat transfer coefficient could be approximated for each covering material by a straight line. Therefore, it should be possible to calculate the amount of overall heat transmission of a greenhouse more accurately by considering the downward longwave radiation and applying each material’s respective approximate equation. In addition, it was possible to approximate the absorptance (transmittance) of longwave radiation and the overall heat transfer coefficient in each covering material with a quadratic function. Therefore, it should be possible to estimate the overall heat transfer coefficient of covering materials with unknown heat insulation capacity under different downward longwave radiation by measuring the absorptance or transmittance of longwave radiation.
