Modified thermal model to predict the natural ventilation of greenhouses: Fid-Bau Portal

Modified thermal model to predict the natural ventilation of greenhouses

Al-Helal, I.M. / Waheeb, S.A. / Ibrahim, A.A. / Shady, M.R. / Abdel-Ghany, A.M.

Highlights • 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.

Abstract Most of thermal models used to estimate the natural ventilation of greenhouses ( ${\dot{m}}_{a}$ ) use an average value for the cover transmittance (τ c) to estimate the input solar energy to the greenhouse. These models failed to predict ${\dot{m}}_{a}$ at low solar radiation flux due to the spatial variation of τ c in the greenhouse and the thermal inertia of soil. This study is to develop and validate a thermal model predict ${\dot{m}}_{a}$ 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 ${\dot{m}}_{a}$ 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 ${\dot{m}}_{a}$ and the temperature difference of air between inside and outside the greenhouse (ΔT) is the main driving force of ventilation. A correlation between ${\dot{m}}_{a}$ and ΔT was provided, it can be used to estimate ${\dot{m}}_{a}$ that required to maintain the air temperature in the greenhouse at a desired level.