The effect on greenhouse microclimate through fan motor control by variable frequency drives

Since large fans are often used in greenhouse ventilation to remove heat and reduce humidity in mild/warm climates, it was of interest to check the performance of fans connected to a variable frequency drive unit (VFD). The present work concentrates on the comparison between ON-OFF and VFD control systems. The study is aimed at finding the effect of each system on the energy consumption, fan performance and greenhouse microclimate. To check the performance of a 48" fan that is controlled by a VFD system, it was installed in a test facility and operated under several rotation speeds. In each speed of rotation, the static pressure on the fan was changed and parameters such as electricity consumption and air-flow rate were measured. In addition, experiments were carried out in a commercial size greenhouse in which pepper was grown. The experiments were done to study the effect of changing the air-flow rate, in the greenhouse, on temperature, humidity and their gradients along the greenhouse and in the vertical direction at the center of the greenhouse. The results show that the VFD system has a better potential than the ON-OFF in reducing the fluctuations in temperature and humidity since with the VFD system the fans operate continuously and change the speed of rotation according to the heat and humidity loads. Reducing the fan speed with the VFD system resulted in a reduction in the air-flow rate and energy consumption, the later being much more significant. As a result of the reduction in the air-flow, with the VFD system, the temperature and humidity gradients in the vertical direction and along the greenhouse increased. Despite the increase in the gradients, visual inspection of the plants suggest that the spatial variability among the plants with respect to height and LAI was smaller with the VFD system than with the ON-OFF one. It appears that VFD control can reduce electricity consumption in comparison to ON-OFF operation by an amount, which dependents on the weather outside.