Vapour Removal From the Greenhouse Using Forced Ventilation When Applying a Thermal Screen

Abstract

The objective of this study is to dimension a system capable of removing water vapour mainly generated by evaporation of the crop when a thermal screen is applied. The humid greenhouse air is replaced by cold dry outside air using an air distribution system. The dry air is injected above the greenhouse floor thereby forcing humid air to pass through the thermal screen. The excess air in the greenhouse will flow away though leaks in the cover. The common procedure to remove moisture from the greenhouse where a thermal screen is applied is by slightly opening the screen. This results in an air exchange of relatively dry air from above the screen and the humid air below the screen. This procedure is difficult to control and causes horizontal temperature differences in the greenhouse. By mechanically controlling the exchange of the greenhouse air and outside air these problems can be resolved. The airflow through the system needed over the year resulting from the evaporation of the crop and the outside conditions is determined using a greenhouse simulation model KASPRO. Based on this result the dimensions of the system are calculated and a control strategy is suggested. The KASPRO calculations also showed that using outside air for vapour removal is more energyefficient as using the air above the thermal screen. The greenhouse climate resulting from using the conventional method of vapour removal as well as with the forced ventilation is investigated using CFD. The three dimensional CFD calculations show that the climate using the forced ventilation system is much more homogenous and the control is more efficient compared to the conventional method of vapour removal. The system proves to be economical since investment costs are low (expected to be around 3 EURO/m 2 maximum) and it ensures a proper wellcontrolled climate under the thermal screen resulting in a higher use of the screen through the year. The system is currently being tested in practise at a Dutch commercial tomato grower. INTRODUCTIONClimate control for horticulture is crucial in order to obtain a high production. Temperature can be well controlled by implementing a heating system and through ventilation. Humidity is also controlled by heating and ventilation. Humidity is difficult to control when the insulation is high. Most greenhouses are insulated by applying a thermal screens that causes less water vapour to condensate on the cover and reduces the air exchange between the greenhouse and the surroundings. Controlling the humidity under the thermal screens is done by slightly opening the screens replacing warm humid air below the screen by cold dry air above the screen. The disadvantages of this measure are that it is not well controllable and it causes horizontal temperature differences. An alternative system has been developed for the humidity control where dry air is blown under the screen by a ventilator. The system is depicted in Figure 1. Three designs of the system, in the figure shown as A1, A2 en B have been studied. In design A the...

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