Measurement and simulation of the ventilation rates in a naturally ventilated Azrom type greenhouse in Zimbabwe

A simple greenhouse ventilation model, based on the stack and wind effects (the main driving forces for natural ventilation) was adapted, calibrated and validated using measured air renewal rates in a three span naturally ventilated Azrom type greenhouse in Zimbabwe. Crop transpiration rates were monitored using stem heat balance sap flow gauges installed on the main stems of rose plants to continuously monitor whole-plant transpiration (WPT). This allowed continuous and automatic determination of full scale air renewal and leakage rates using the water vapour balance method. The model was fitted to experimental data of ventilation rates, and discharge and wind effect coefficients were determined. The results show a good fit between measured and predicted values (R2 = 0.80 and 0.82 for winter and summer, respectively), although there is a general over-estimation of the greenhouse air renewal rates, particularly during the night. The model, nevertheless, adequately describes the natural ventilation process in the greenhouse all year round. The model can be used as a design tool to evaluate and optimise the effects of different ventilation configurations and strategies on greenhouse air renewal rates, and as a component in a greenhouse climate model in order to further evaluate the effects of ventilation strategies on the inside greenhouse and crop microclimate, and thus lead to better greenhouse climate control.