Temperature and relative humidity changes inside greenhouse

Better growing conditions are achieved in greenhouses by maintaining a higher internal ambient as compared with external ambient temperature. A computer-based control and monitoring system which provides visualization, control and coordination of temperature and humidity in a greenhouse was recently developed. To validate the system performance, a number of experiments were carried out during the autumn of 2003. In this paper, one of the experimental results conducted from 10 to 12 a.m. on December 7, 2003, in the city of Karaj, is presented and discussed. The system was tested for two modes of operation: the uncontrolled mode of operation and the controlled mode. Four sensors, three for temperature measurements and one for relative humidity measurements, were installed inside and outside. During the first hour and a half the system was tested as a data-acquisition system, ie, only data from the sensors were recorded and monitored on the screen with no operation of fans, sprayer and other installed environmental systems in the greenhouse. For the last 20 min of the experiment, inside air temperature was controlled by the system. The result on temperature measurements shows that external ambient temperature, Tout, is always less than the inside temperature. This is attributed to the solar radiation entering the greenhouse through transparent plastic and being trapped there. We also observed fluctuations on temperature profile inside the greenhouse. This is caused by natural conditions such as surface evaporation within the greenhouse, solar radiation, external ambient temperature and rapid weather changes during the time of the experiment. It was also found that the rate of change of temperature increase in the upper part, Tup, ie near plastic cover, is higher than that of the plants height, Tmid. This rise in vertical temperature gradient is partly due to the different amount of solar incident radiation being received at the locations of sensors. This trend proves the effectiveness of our polytube system, a re- circulating fan with an attached perforated polyethylene tube, in guiding the air toward the plant root zone. For the last 20 min of the experiment, the controller was put into action. The overall per- formance of the system in maintaining the temperature within a given range, around the set point, is found to be satisfactory. The time constant of the fan and heater combination was short, about 10 min, in reaching the desired set point temperature.