COMBINED ROOF AND SIDE VENTILATION WITH MISTING AND AIR BLOWING FOR COOLING TROPICAL GREENHOUSES

Temperature regulation in tunnel greenhouses used for protected culture in tropical Asia mostly depends on natural (static) ventilation. This single mode of greenhouse cooling is inadequate during less windy and peak hot periods, causing to heat up the greenhouse interior to temperatures beyond 35°C, unfavorable for plant growth and development. Therefore individual and combined use of different temper¬ature control strategies, namely enhanced natural ventilation, forced-air exchange and evaporative cooling were tested for their efficacy in a commercial scale greenhouse (without vegetation inside) under humid tropical conditions in Sri Lanka. Enlarged roof vent (up to 36% of the floor area) with net-covered side vent (mesh size: 20 or 40) improved greenhouse ventilation based temperature control, making the mean day¬time temperature in the greenhouse down to the level outside (33°C) and without affecting the internal relative humidity (RH) significantly (63-65%). Forced-air intake was more effective for temperature regulation, compared to exhaust when introduced to naturally ventilated greenhouses. Combined use of forced-air exhaust and intake contributed to maintain RH at a minimum difference with the outside air. Misting could reduce the internal temperature far below the outside (30°C) when operated with improved natural ventilation, subjected to slight increase in RH. With further improvements, forced-air ventilation would be used to bring down the internal RH, increased due to misting in naturally ventilated tunnel greenhouses in the tropical region.