Energy consumption due to dehumidification of greenhouses under northern latitudes

Transpiration and water uptake play an important role in the growth of horticultural crops such as tomatoes. However, the transpiration rate is affected by the humidity level in the greenhouse. High levels of humidity restrict transpiration and lead to fungal diseases resulting in yield losses. Under northern latitudes, the use of more airtight structures combined with high levels of supplementary lighting increase the humidity level inside the greenhouses. To decrease humidity, growers have to dehumidify by ventilation leading to an increase in energy consumption. However, the literature does not report on the energy consumption needed to dehumidify. To evaluate this energy consumption, we used the GX software system which simulates the heat and mass exchanges in a greenhouse as well as the behaviour of the climate-control strategy. Evapotranspiration, condensation on the cladding and infiltration and ventilation rates were taken into account for the water balance. Based on one year of climatic data, three sets of simulations were made: (1) no dehumidification; (2) dehumidification by on-off ventilation (one air change per hour); and (3) dehumidification by proportional ventilation. Simulation results indicate that for an acceptable level of humidity, within a greenhouse containing a tomato crop (corresponding to a vapour pressure deficit above 0.5 kPa), the energy consumptions with on-off ventilation and with proportional ventilation are, respectively, 12.6 and 18.4% higher than without dehumidification. Proportional ventilation was also more effective than on-off ventilation for humidity control. (C) 1998 Silsoe Research Institute.