MEASUREMENT OF OVERALL HEAT TRANSFER COEFFICIENT OF COVERING MATERIAL WITH THERMAL SCREENS FOR PLASTIC GREENHOUSE

Recent increases in fuel prices have forced growers to apply energy-saving measures in the air heated greenhouses. As a result, growers have been forced to consider innovative cover materials and they have been advised to install thermal screens. Materials used as greenhouse covers are not spatially homogeneous that’s why the selection of a covering is crucial for attainment of an optimal controlled environment. Several attempts have been already made to develop measurement procedures applicable under conditions resembling those of greenhouse production on purpose to investigate the U-values of glazing materials by using a hot box. The objective of the present study was to investigate the overall heat transfer coefficient of greenhouse covering materials with or without thermal screens commonly used in the country under external and real conditions by using the hot box method. The test conditions of eight treatments combined covering materials and thermal screens with and without shelter from the sky radiation were used to determine the overall heat transfer coefficient. The insulation effect of two layers covering material is about 36% comparing to one layer covering material and the coefficient is 3.0 (W•m^-2•K^-1) less than one layer. The decrease of the coefficient of one layer and two layers coverings by shelter was about 30% and 24% respectively. When we measure the overall heat transfer coefficients in a laboratory, it is necessary to establish the measuring system to be able to simulate the sky radiation.