TEMPERATURE GRADIENTS IN FAN-VENTILATED GREENHOUSES

Fans are often used in warm climates to ventilate commercial greenhouses. They are usually mounted at one end of the greenhouse and suck ambient air through openings at the opposite end. As the air moves from one end of the greenhouse to the other, gradients of temperature develop along the structure. These gradients result in an inhomogeneous microclimate for the crop and are, therefore, undesirable for growers. Results are reported from experiments in two fan-ventilated greenhouses. One located in Shimen town of Tongxiang city, Zhejiang province of China and the other in the Besor region in the southern part of Israel. The greenhouse in China had eight-spans with gutters oriented north-south and its area was about 3,100 m². The greenhouse in Israel had two spans with gutters oriented north-south and its area was about 360 m2. Both greenhouses were covered with polyethylene sheets. To reduce solar radiation intensity at crop level, the Chinese greenhouse was shaded by two shading nets: an external one (16% shading) and an internal one (10% shading). No shading was applied to the Israeli greenhouse. Air temperature inside the greenhouses along the airflow direction was measured by copper-constantan thermocouples shielded from direct solar radiation. In addition to air temperature leaf temperature was measured in the Israeli greenhouse using fine thermocouples that were attached to leaves of several plants along the airflow direction. The experimental results show that temperature gradients develop in the air along the airflow direction as well as in the vertical direction. The vertical gradients in the air temperature were much larger than those along the airflow direction. Temperature gradients develop also in the crop along the airflow direction; however they are smaller than those observed in the air. The application of shading nets reduced the temperature gradients in comparison to the case where no nets were applied.