Air exchange and ventilation efficiencies of a monospan greenhouse with one inflow and one outflow through longitudinal side openings

Until about a decade ago, greenhouse microclimates were analysed using a simplified approach assuming a perfectly stirred enclosure. Demands for efficient and sustainable greenhouse operations, stimulated intensive research that focused into the distributed microclimate within greenhouses. Since the distributed microclimate and the air-exchange rate are interconnected the latter is important. Three methods for determining the air exchange in a small naturally ventilated monospan greenhouse were compared: experiments, computational fluid dynamics (CFD) and a model which relates flow rate through openings to their pressure drops. A rose-growing greenhouse was ventilated via two longitudinal side openings, one on the windward and the other on the leeward walls. In the experiments ventilation rates were estimated by means of a tracer gas; in the CFD simulations the decay rate of a virtual tracer gas and calculated airflow rates through the openings were used and in the model, ventilation rates were calculated with an equation relating flow rate through the openings to their pressure drop, using local wind-pressure coefficients. All the methods agreed well up to a wind speed of about 4 m s−1; at higher wind speeds the ventilation rate values deduced from the decay of tracer gas, both in the experiments and CFD simulations, were much lower than those obtained with the other techniques. The concept of age of air was used to show that the lower ventilation rates at the high wind speeds were associated with imperfect mixing of the supply air with the greenhouse air, with consequently diminished air-exchange efficiency.