HORTITRANS, A MODEL FOR PREDICTING AND OPTIMIZING HUMIDITY AND TRANSPIRATION IN GREENHOUSES

A new model is presented to predict humidity and transpiration directly as a function of the outside climate, with the particular objective of developing optimal control strategies for humidity in greenhouses. This model is of straightforward structure, while including the processes of transpiration, condensation, ventilation and humidification or dehumidification. The model allows the inside vapour pressure to be directly calculated as a function of the outside conditions and the greenhouse characteristics. It includes a linearized relationship for transpiration, which is a good approximation of a more detailed model. Condensation on the cladding is first calculated for the inside greenhouse air at saturation (100% relative humidity) and then corrected by a factor to account explicitly for the feedback effect of inside humidity on cladding temperature. Because of its simplicity, this model also explicitly determines the water and energy to be added to or extracted from the greenhouse air, in order to achieve given humidity or transpiration set-points. The HORTITRANS model predicted to within 8% vapour pressure, relative humidity, transpiration and condensation inside the greenhouse, when tested against measurements taken on a young and on a mature crop in a full-size greenhouse. The optimization of inside humidity shows that there is little need for dehumidification in a single glazed greenhouse. For double glazing, dehumidification could be cost effective at very low light levels, in particular just before sunset. © 1994 Silsoe Research Institute.