Modified thermal model to predict the natural ventilation of greenhouses

• 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 <2.5 m s−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 0.36 kg s−1 and 1.65 kg s−1 and showed good accordance with measured and simulated values in the literature. The results also confirmed that outside wind at a speed <2.5 m s−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.