Effects of Vent Configuration and Span Number on Greenhouse Microclimate under Summer Conditions in Eastern China

The effects of vent configuration and span number on the microclimate in multi-span greenhouses were investigated. A three-dimensional (3-D) computational fluid dynamics (CFD) model was constructed based on an 11-span plastic greenhouse cultivated with 0.2 m-high lettuces. The model was verified with the temperature profile measured in the greenhouse. Then, it was used to explore the effects of vent configuration and span number on greenhouse climate. Simulations show that different vent configurations result in very different microclimate fields. Roof plus side opening has the best cooling and homogeneity performance and is thus recommended. The side openings, the first two windward roof openings and the last two roof openings play a central role during the ventilation process. A minor modification of the ventilation system induces a temperature decrease of 0.1 K and a humidity elevation of 0.3% and increases greenhouse climate homogeneity. These simulations also show that the ventilation rate exponentially decays with an increase of span number once side ventilators are opened, whereas it is maintained at 0.028 m3m-2s-1 with roof ventilation. Greenhouse climatic parameters and standard deviations are less influenced by span number than vent configuration. Finally, an analysis of inside climatic distribution reveals that strong temperature gradients occur from the windward to leeward area and above the ground. Differences of inside air velocity and temperature with and without the crop vary from 0 to 0.6 ms-1 and 0 to 1.2 K along the horizontal direction at 0.1 m above the ground, respectively. Photosynthetically active radiation (PAR) is symmetrically distributed in the greenhouse.