Abstract
•A model was developed to predict the natural ventilation of greenhouses.•The model is able to predict ventilation rate at low or zero solar radiation intensity.•Wind at a speed <2.5ms−1 has no significant effect on the ventilation rate and the temperature difference do the main effect.•A correlation between the ventilation rate and temperature difference was provided.
Most of thermal models used to estimate the natural ventilation of greenhouses (m˙a) use an average value for the cover transmittance (τc) to estimate the input solar energy to the greenhouse. These models failed to predict m˙a at low solar radiation flux due to the spatial variation of τc in the greenhouse and the thermal inertia of soil. This study is to develop and validate a thermal model predict m˙a precisely. All modes of energy were treated, under the unsteady state conditions, at the outer surface of the cover to avoid the sources of error. The required parameters were measured for a naturally ventilated glasshouse. The predicted values of m˙a were in the range between 0.36kgs−1 and 1.65kgs−1 and showed good accordance with measured and simulated values in the literature. The results also confirmed that outside wind at a speed <2.5ms−1 has no significant effect on the value of m˙a and the temperature difference of air between inside and outside the greenhouse (ΔT) is the main driving force of ventilation. A correlation between m˙a and ΔT was provided, it can be used to estimate m˙a that required to maintain the air temperature in the greenhouse at a desired level.