Mechanically controlled moisture removal from greenhouses

The object of this study was to design and test a system capable of dehumidifying air in a greenhouse when a thermal screen is in use. Dehumidification is required to reduce the risk of fungal diseases and prevent physiological disorders. The most common procedure used to remove moisture from a greenhouse fitted with a thermal screen is to open slightly the thermal screen. This causes an exchange between the relatively dry air above the screen and the humid air below the screen. However, this procedure is difficult to control and it can cause horizontal temperature differences in the greenhouse, which negatively effect crop production. In the dehumidification system proposed here, outside air is exchanged at low level with greenhouse air. This ventilation with cool dry outside air is mechanically controlled using an air distribution system. The dry air is injected near the greenhouse floor thereby forcing humid air to pass through the thermal screen. The excess air in the greenhouse then flows out through leaks in the cover. The airflow required by the system throughout the year, being dependent on the evaporation of the crop and the outside conditions, was determined using a greenhouse-climate simulation model. The model was validated using climate data from the commercial greenhouse where the system was installed. The dimensions of the system were calculated from the results of the model and a control strategy was suggested. The model calculations showed that using outside air for vapour removal is more energy efficient than using air from above the thermal screen. The distribution of climate in the greenhouse using the conventional and proposed methods of vapour removal was investigated using computational fluid dynamics (CFD). The system was tested in a commercial greenhouse and compared to a conventional system at the same location. The performance of the system, as determined by the dynamic simulation model, proved to be efficient and the climate proved to be more homogenous, as was predicted by the CFD calculations.