Air and Leaf Temperatures and Relative Humidity in a Naturally Ventilated Single-Span Greenhouse with a Fogging System for Cooling and Its Evaporative Cooling Efficiency

Abstract

Dry bulb (or air) temperature, leaf temperature of tomato plant (T_leaf), and relative humidity were measured under various fogging times (t_f) around midday in summer inside a naturally ventilated single-span experimental greenhouse with a fogging system for cooling and with side-wall and roof ventilators which remained open. Then, the evaporative cooling efficiency (η) of the greenhouse, expressed as the ratio of the difference in dry bulb temperature between the outside (T_odb) and the inside (T_idb) to the difference between T_odb and the outside wet bulb temperature (T_owb), was calculated. The fogging system was comprised of a water filter, a water reservoir, a fogging pump, a pressure regulator, and fog-generating nozzles. Fog was applied for periods between 20 and 75 s, separated by intervals of 160 s.
For all t_f, a drop in T_idb increased with increasing vapor pressure deficit of the inside air (VPD_i) at the start of fogging. T_idb reached 6.0°C lower than T_odb by fogging. During fogging, T_idb was more or less the same as T_leaf. During the interval, however, T_idb increased and T_leaf tended to remain unchanged. The increase in VPD_i resulted in decreased (T_leaf-T_idb) . The η was in the range between -30% and 21% when using the average T_idb, and 57% and 80% when using the minimum T_idb. The η compares favorably with a conventional misting system.