Measurement and prediction of greenhouse ventilation rates

Abstract

Natural ventilation and leakage rates were measured in a multispan glasshouse with a mature tomato crop. The ventilation-rate measurements were made with four settings of the leeward ventilators (10, 20, 30 and 40% of the maximum opening). The influences of wind speed, wind direction and inside-outside temperature difference were analysed. The leakage and ventilation rate measurements were made with the dynamic tracer-gas method using N 2O. Wind speed was found to have a strong influence on both leakage and ventilation rates. No influence was observed from either wind direction or temperature difference, but the experiments were carried out in a windy area, where it was difficult to determine the influence of the latter parameter. A formula to predict the air exchange as a function of wind speed was obtained for each ventilator position. Another formula was derived for the ventilation flux per unit ventilator area and average wind speed as a function of the opening angle ( G( α) function). Ventilation rates were predicted using an energy balance model of the greenhouse. Good agreement was obtained between values predicted by the energy balance model and the measured values for larger ventilator openings, but at low ventilation rates the agreement was poor. The results obtained were compared with those obtained using models developed by other authors. This enabled the influence of side-wall permeability on ventilation rate to be established.