Transfer coefficients of several polyethylene greenhouse covers

Abstract

An attempt was made to calibrate in situ a greenhouse model of moderate complexity. Such a model can be used as a tool in simulation and optimization studies related to energy conservation in greenhouses. A six-element, linearized, steady-state model of a double-cover, dry greenhouse, without a crop was developed. It was used to evaluate four convective heat transfer coefficients, two radiative transmissitivities, one radiative shape factor, and a heat flux. A small (20 m 2 floor area) greenhouse frame was covered consecutively with four different polyethylene covers. These were the four combinations of regular and high emissivity polyethylene sheets on the one hand, and single and double layer (inflated) covers on the other. Various heat fluxes and temperatures were measured. The data obtained for the four covers were analysed simultaneously. It was assumed that the radiative and convective coefficients were independent of each other, and that local coefficients had the same values for all covers. In particular, the inside and outside convective transfer coefficients were assumed to be the same for all covers. The results suggest that the basic assumptions of the model (linearity, independence of coefficients) are valid for engineering purposes. Values of the various convective coefficients are compared with results from other studies.